UPWELLING IN THE EASTERN SUBTROPICAL
NORTH ATLANTIC OCEAN
Tesis Doc o al p esen ada po
Ma ia V. Pas o Mollà
Di igida po el D .
Josep Lluís Peleg í Llopa
y la D a.
Jaime Pal e
P og ama de Doc o ado: Oceanog a ía
Depa amen o de Física. Facul ad de Ciencias del Ma
UNIVERSIDAD DE LAS PALMAS DE GRAN CANARIA
El Di ec o , La Di ec o a, La Doc o anda,
Ba celona, a 11 de Julio de 2011
Abs ac
Coas al upwelling in he eas e n ma gin and o sho e cu l-d i en upwelling in
he sou heas e n ma gin, make he sub opical No heas A lan ic a egion o ma-
jo p ima y p oduc i i y. When examining a b oad zonal a ea, om he coas o
40◦W, we ind ha he upwa d anspo o nu ien s due o o sho e cu l-d i en
upwelling becomes he main con ol on p oduc i i y. Ne e heless, despi e i s
ela i ely small zonal ex ension o abou 100 km, coas al upwelling ex ends i s
impac owa ds he open ocean h ough o sho e Ekman anspo and con e -
gence o he me idional low a Cape Blanc (21◦N). Analysis o hyd og aphic
da a om sp ing 1973 and all 1975 shows an expo om he coas o he open
ocean o 2.9 S o wa e mass and 53 kmol s-1 o ni a e du ing sp ing and 0.6 S
and 3 kmol ni a e s-1 du ing all in he a ea sou h o Cape Blanc.
I is undamen al o imp o e ou unde s anding o he dis ibu ion o he di -
e en wa e masses o he uppe he mocline, as hey ca y he nu ien s ha will
each he eupho ic laye and sus ain p ima y p oduc ion. In he s udy egion,
cen al wa e s o no he n and sou he n o igin mee a he Cape Ve de on al
zone. No he n wa e s a e nu ien -poo and oxygen- ich while local sou he n
wa e s a e nu ien - ich and oxygen-poo . He e, in ense double di usi e mixing
enhances ho izon al hea ans e , hus he on appea s as a smoo h ea u e in
e ms o empe a u e, bu esembles a ba ie in e ms o o he p ope ies such
as sal o nu ien s. The applica ion o an Op imum Mul ipa ame e analysis o
hyd og aphic da a collec ed du ing No embe 2007 and No embe 2008 shows
a sha p on sepa a ing he cen al wa e s o no he n and sou he n o igin. In
con as , a in e media e laye s, he ansi ion be ween Medi e anean Wa e and
An a c ic In e media e Wa e is smoo he .
i
Resumen
El a lo amien o cos e o en el ma gen es e y el a o amien o po o acional del
ien o, o bombeo de Ekman, en el ma gen su es e hacen de es a zona del A -
lan ico No e sub opical una egión de p incipal impo ancia po su al a p oduc-
i idad. Examinando una ex ensi a á ea, desde la cos a has a 40◦O, encon amos
que el anspo e de nu ien es po bombeo de Ekman eje ce un con ol p incipal
en la p oduc i idad. Po o o lado, aunque la escala zonal del a lo amien o cos-
e o es de an solo unos 100 km, su in luencia se ex iende hacia océano abie o
g acias al anspo e de Ekman y a la con e gencia del lujo me idional en Cabo
Blanco (21◦N). El análisis de da os hid og á icos co espondien es a la p ima e a
de 1973 y el o oño de 1975 mues an una expo ación de cos a a océano abie o
de 2.9 S de agua y 53 kmol s-1 de ni a os du an e la p ima e a, y de 0.6 S y 3
kmol de ni a os s-1 du an e el o oño en el á ea al su de Cabo Blanco.
En ende la dis ibución de las di e en es masas de agua en la e moclina su-
pe io es undamen al, ya que anspo an los nu ien es que an a a lo a a la
capa ó ica y man ene una ele ada p oducción p ima ia. En la egión de es u-
dio, aguas cen ales de o igen no e (pob es en nu ien es) y su ( icas en nu ien-
es) se encuen an en la zonal on al de Cabo Ve de. Aquí, mezcla po doble
di usión in ensi ica la ans e encia ho izon al de calo . Como consecuencia, el
en e apa ece como una ue e ba e a en cuan o sal y nu ien es, pe o sua izado
en cuan o a la empe a u a. La aplicación de un Análisis Mul ia amé ico Óp-
imo a un conjun o de da os hid og á icos ecogidos du an e no iemb e de 2007
y no iemb e de 2008 mues a el en e como una ansición ápida en e las dos
masas de agua cen ales. Sin emba go, en las capas in e medias la ansición en e
aguas Medi e ánea y An á ica es más sua e.
ii
Ac onyms
AAIW An a c ic In e media e Wa e
AC Azo es Cu en
CC Cana y Cu en
Chl:C chlo ophyll o Ca bon a io
CORE Common Ocean-Ice Re e ence Expe imen
CSIC Consejo Supe io de In es igaciones Cien í icas
CUC Cana y Upwelling Cu en
CVFZ Cape Ve de on al zone
EBUS Eas e n Bounda y Upwelling Sys ems
ENSO El Niño-Sou he n Oscilla ion
GD Guinea Dome
GFDL Geophysical Fluid Dynamics Labo a o y
IIP Ins i u o de In es igaciones Pesque as
ITCZ In e opical Con e gence Zone
MC Mau i ania Cu en
MOM4 Modula Ocean Model e sion 4
MW Medi e anean Wa e
NACW No h A lan ic Cen al Wa e
NADW No h A lan ic Deep Wa e
iii

NAO No h A lan ic Oscilla ion
NEC No h Equa o ial Cu en
NECC No h Equa o ial Coun e cu en
OMP Op imum Mul ipa ame e
PU Polewa d Unde cu en
SACW Sou h A lan ic Cen al Wa e
SLP Sea Le el P essu e
SSH Sea Su ace Heigh
SST Sea Su ace Tempe a u e
TOPAZ T ace s o Ocean Phy oplank on wi h Allome ic Zooplank on
i
Con en s
Abs ac i
Resumen ii
Ac onyms iii
1 In oduc ion 1
1.1 Backg ound ................................ 1
1.2 Objec i es and hesis ou line . . . . . . . . . . . . . . . . . . . . . . . 15
2 Wa e and nu ien luxes 19
2.1 In oduc ion................................ 20
2.2 Da ase ................................... 25
2.3 Wa e masses ............................... 27
2.3.1 P ope y-p ope y diag ams . . . . . . . . . . . . . . . . . . 27
2.3.2 Ve ical dis ibu ions . . . . . . . . . . . . . . . . . . . . . . . 31
2.3.3 Ho izon al dis ibu ions . . . . . . . . . . . . . . . . . . . . . 35
2.4 Mixing in he Cape Ve de on al sys em . . . . . . . . . . . . . . . . 37
2.4.1 La ge scale ins abili y . . . . . . . . . . . . . . . . . . . . . . 37
2.4.2 Doubledi usion ......................... 39
2.5 Wa e and nu ien luxes . . . . . . . . . . . . . . . . . . . . . . . . 45
2.5.1 Re e encele el .......................... 46
2.5.2 Dynamicheigh .......................... 47
2.5.3 Geos ophic eloci ies and ni a e luxes . . . . . . . . . . . . 49
2.6 Wa e and nu ien balances . . . . . . . . . . . . . . . . . . . . . . . 51
2.6.1 Alongsho e anspo s . . . . . . . . . . . . . . . . . . . . . . 51
2.6.2 C oss-sho e anspo s . . . . . . . . . . . . . . . . . . . . . . 54
2.6.3 Closing he balances . . . . . . . . . . . . . . . . . . . . . . . 55
2.7 Conclusions ................................ 59
3 Me idional changes in wa e p ope ies 63
3.1 In oduc ion................................ 64
3.2 Da ase s .................................. 67
3.3 Spa ial dis ibu ion o wa e p ope ies . . . . . . . . . . . . . . . . 69
3.4 Wa e ypes and op imum mul ipa ame e analysis . . . . . . . . . 76
3.5 Dis ibu ion o wa e masses . . . . . . . . . . . . . . . . . . . . . . . 81
3.6 Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . 85
4 Physical d i e s o in e annual chlo ophyll a iabili y 89
4.1 In oduc ion................................ 90
4.2 Sa elli e da a, model ou pu and me hods . . . . . . . . . . . . . . . 94
4.3 Sa elli e −modelcompa ison...................... 98
4.4 Sea su ace heigh and chlo ophyll . . . . . . . . . . . . . . . . . . . 102
4.5 Mechanisms o chlo ophyll a iabili y . . . . . . . . . . . . . . . . . 106
4.5.1 Nu ien supply..........................108
4.5.2 Coas al upwelling e sus o sho e upwelling . . . . . . . . . 112
4.6 Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . 115
i
5 Conclusions and ou look 119
5.1 Conclusions ................................119
5.2 Fu u e esea ch ..............................123
6 Resumen en español 127
6.1 In oducción y obje i os de la esis . . . . . . . . . . . . . . . . . . . 127
6.1.1 In oducción............................127
6.1.2 Obje i os y esumen de la esis . . . . . . . . . . . . . . . . . 143
6.2 Flujos de masa y nu ien es . . . . . . . . . . . . . . . . . . . . . . . 146
6.3 Cambios me idionales en las p opiedades de las masas de agua . . 151
6.4 Va iabilidad in e anual de la clo o ila . . . . . . . . . . . . . . . . . 159
6.5 Conclusiones y abajos u u os . . . . . . . . . . . . . . . . . . . . . 170
6.5.1 Conclusiones ...........................170
6.5.2 T abajos u u os..........................174
Appendices
A Adiaba ic app oxima ion 179
B Double di usion and he Tu ne angle 181
C MOM4 and TOPAZ 183
C.1 Modula OceanModel..........................183
C.2 TOPAZ...................................184
Re e ences 187
ii
Chap e 1. In oduc ion
¬: ¬: ¬:
Cape Ve de
Cape Blanc
Cape Timi is
Cape Bojado
Cape Juby
Cape Ghi
AC
CUC
WINTER
NECC
MC
CC
NEC
Cana y Islands
Cape Ve de I.
Azo es I.
Madei a
¬: ¬: ¬:
¬1
¬1
¬1
¬1
SUMMER
Cape Ve de
Cape Blanc
Cape Timi is
Cape Bojado
Cape Juby
Cape Ghi
Cana y Islands
Cape Ve de I.
GD
NEC
NEC
MC
NECC
AC
CC/CUC
NECC
Azo es I.
Madei a
Figu e 1.2: Schema ic map o he main su ace cu en s in he eas e n No h A -
lan ic Ocean du ing summe and win e . Blue bands along he coas ma k he
occu ence o in ense upwelling du ing each season; du ing summe he Guinea
Dome (GD) is also shown in blue. The dashed g ay line ma ks he Cape Ve de
on al zone. Cu en labels: Azo es Cu en (AC), Cana y Cu en (CC), Cana y
Upwelling Cu en (CUC), Mau i ania Cu en (MC), No h Equa o ial Cu en
(NEC), No h Equa o ial Coun e cu en (NECC). The Polewa d Unde cu en
lows no h as a subsu ace cu en along he uppe slope, i s co e ypically a
some 200 m dep h, in many ins ances eaching he ocean su ace.
6

Chap e 1. In oduc ion
Figu e 1.3: Tempe a u e-salini y dig am showing wa e mass ypes in he cen al,
in e media e and deep laye s o he A lan ic Ocean, om S e d up e al. (1942).
coas al je and CUC each lowes la i udes as a sou hwa d low along he A ican
coas be ween capes Blanc and Ve de (Láza o e al., 2005), p o iding a connec ion
be ween he no he n and sou he n wa e s (Mi els aed , 1991). The s eng hen-
ing o he ades a hese lowe la i udes cause he NECC o weaken and meande ,
and he MC eaches only sou h o Cape Ve de (Fig. 1.2). The cyclonic low sys em
a ound he Guinea Dome is masked by he wes wa d Ekman d i a he su ace,
bu Siedle e al. (1992) epo ed ha he dome s ill exis in he subsu ace laye s.
Wa e masses
The su ace laye in he eas e n sub opical No h A lan ic (app oxima ely he
uppe 100 m) is cha ac e ized by high salini ies, high dissol ed oxygen and low
nu ien concen a ions (F aga and Man íquez, 1974). A pa icula ea u e o he
No h A lan ic sub opical gy e is he subsu ace salini y maximum (e.g. Figs.
7
Chap e 1. In oduc ion
2.4 and 2.5). Dense high-salini y su ace wa e s, p oduced by he excess e apo a-
ion o e p ecipi a ion, subduc due o Ekman anspo con e gence and win e
con ec ion, and a e ca ied in o he la ge-scale ci cula ion o he sub opical gy e
(De an , 1936; Baue and Siedle , 1988).
The wa e masses o he pe manen he mocline a e he cen al wa e masses
(S e d up e al., 1942). These wa e s o igina e in he sub opical oceans o bo h
hemisphe es, whe e su ace wa e s con e ge and Ekman puming is nega i e,
and sp ead owa ds he equa o ial egions. They a e ecognized in a TS-diag am
by nea ly linea TS- ela ionships (Fig. 1.3). The cen al wa e s ex end om ap-
p oxima ely 100 o 700 m. NACW ha e hei sou ce in he No h A lan ic su -
ace con e gence (subduc ing) zone and each subsu ace wa e s a lowe la i-
udes h ough he he mocline ci cula ion (Sa mien o e al., 1982; Kawase and
Sa mien o, 1985). Only wa e masses o med in he subduc ing zone du ing la e
win e and ea ly sp ing may escape he su ace laye s o be injec ed in he pe -
manen he mocline; du ing he es o he yea , wa e s subduc ed emain in he
mixed-laye as he escape eloci ies caused by Ekman pumping a e less han
he seasonal ad ance o he mixed-laye he mocline. The e ical ex ension o
he NACW in lowe la i udes will be de e mined by he denses win e -ou c op
isopycnal wi hin he subduc ing zone, nea σθ= 27.3 (Kawase and Sa mien o,
1985; Reid, 1994).
Eme y and Meincke (1986) subdi ided he Cen al Wa e o he No h A -
lan ic (NACW) in o Eas e n (ENACW) and a Wes e n (WNACW) ypes. The
di ision e lec ed di e en o ma ion egions, sou h o he Suba c ic F on o
WNACW and sou h o he Iceland-Fa oe F on o ENACW. Con e sely, Tom-
czak and God ey (1994) a gued ha he empe a u e-salini y changes obse ed
8
Chap e 1. In oduc ion
ac oss he ocean basin esul om en i onmen al a iabili y wi hin he o ma-
ion egion. Meanwhile, Eme y and Meincke (1986) iden i ied only one ype o
Sou h A lan ic Cen al Wa e (SACW), o med nea he B azil-Mal inas Con lu-
ence egion o he Sub opical Con e gence. Go don e al. (1992) and Sp in all
and Tomczak (1993) showed ha cen al wa e o med in he Sub opical Con-
e gence o he Indian Ocean was an impo an con ibu o o he he mocline o
he A lan ic Ocean, en e ing he A lan ic basin ia he Agulhas Cu en .
SACW a els h ough he Sou h A lan ic he mocline in o he equa o ial cu -
en sys em and he opical egion o he No h A lan ic. The Cape Ve de on al
zone (CVFZ) cons i u es he bounda y be ween NACW and SACW and co e-
sponds o he sou he n limi o he No h A lan ic he mocline eci cula ion (S amma
and Siedle , 1988; Zenk e al., 1991; A han e al., 1994). The CVFZ s e ches
sou hwes om 20◦N o he coas o A ica o he Cape Ve de Islands, and hen
acqui es a mo e zonal o ien a ion as i p og essi ely di uses ou owa ds he
wes e n side o he basin. Bo h cen al wa e masses occupy he same densi y
ange, wi h NACW being sal ie and wa me han he SACW. As a esul he
on is densi y-compensa ed and is p one o mul i ude o in usions, ilamen s
and lenses (Zenk e al., 1991; Pé ez-Rod íguez e al., 2001; Pas o e al., 2008).
The in e media e laye in he s udy egion, app oxima ely be ween 700 and
1500 m, is occupied by An a c ic In e media e Wa e (AAIW) and Medi e anean
Wa e (MW). AAIW is o med in he Suban a c ic F on and ca ied wi h he sub-
opical gy e o he Sou h A lan ic owa ds he opics (Suga and Talley, 1995).
The AAIW is easily iden i ied by low salini ies wi h a salini y minimum cen-
e ed a abou 800 m dep h. Two pa hways anspo AAIW o he eas e n No h
A lan ic, one is h ough he wes e n bounda y cu en sys em and he Azo es
9
Chap e 1. In oduc ion
Cu en (Kawase and Sa mien o, 1985; Tsuchiya e al., 1992). The second pa h
is h ough he eas e n ma gin along he A ican coas line (Machín and Peleg í,
2009), wi h a maximum no hwa d pene a ion du ing all (Machín e al., 2010).
MW is o med in he Medi e anean Sea and en e s he A lan ic h ough he S ai
o Gib al a ; om he e, i sp eads no h and sou hwa ds, in luencing he whole
No h A lan ic (Wo hing on, 1976). I s cha ac e is ic high salini y and empe a-
u e signa u e is obse ed be ween abou 600 and 1500 m dep h. Bo h in e medi-
a e wa e s mee a abou 32◦N (e.g Fig 3.5), wi h AAIW occupying a dep h ange
sligh ly shallowe han MW. Sou ce wa e p ope ies o in e media e wa e s a e
modi ied by mixing wi h wa e abo e and below, and hei TS p ope ies in he
s udy egion appea as local ex eme TS alues ha de ia e subs an ially om
he alues ound in hei o ma ion egion (Fig. 1.3).
No h A lan ic Deep Wa e (NADW) is ound in he laye below, be ween
2000 and 4000 m app oxima ely. NADW in he eas e n No h A lan ic is o med
mainly by he Iceland Sco lan O e low Wa e , modi ied by Lab ado Sea Wa e
and Lowe Deep Wa e (McCa ney, 1992; Dickson and B own, 1994; an Aken,
2000).
In e annual a iabili y and clima ic modes
In e annual a iabili y in coas al upwelling has mos ly been ela ed o he No h
A lan ic Oscilla ion (NAO) and o El Niño - Sou he n Oscilla ion (ENSO) e en s.
The NAO index is de ined as he anomalous di e ence in sea-le el ai p essu e
be ween he Iceland Low p essu e sys em and he Azo es High p essu e sys em
du ing he win e season (Decembe h ough Ma ch). An inc ease in he index
10
Chap e 1. In oduc ion
implies a s onge Azo es High and inc eased eas e ly low o e NW A ica, lead-
ing o an inc ease in he wind d i en coas al upwelling. Analyzing sa elli e SST
om 1982 o 2001, San os e al. (2005) epo ed a decadal scale shi o upwelling
egime in ensi y om weak upwelling in 1980’s o in ense in he 1990’s, linked o
a shi in he NAO index. Meine s (2007) and Meine s e al. (2010) also ound a
posi i e co ela ion be ween he NAO index and he ade winds when s udying
impac s o clima e a iabili y on black hake dynamics in he No hwes A ican
coas . Speci ically, he NAO index could explain 53% o he a iabili y in he
me idional componen o he wind s ess o he coas o Mau i ania and Senegal
be ween 1960 and 2004.
Roy and Reason (2001) in es iga ed links be ween he Mul i a ia e ENSO
Index and SST anomalies and wind s ess anomalies o No hwes A ica (be-
ween 10◦and 20◦N). Thei wo k showed ha wa m e en s in he Paci ic du ing
all and ea ly win e (El Niño condi ions) lead o a elaxed s a e o he wind-
induced upwelling on he eas e n side o he A lan ic basin and o wa m condi-
ions being obse ed along he coas o Wes A ica du ing la e win e and sp ing.
ENSO e en s also in luence he A lan ic equa o ial cu en sys em and he Guinea
Dome. Láza o e al. (2005) obse ed an in ensi ica ion o he NECC du ing sp ing
1997 and 1998, and o he Guinea Dome du ing summe 1997, associa ed wi h he
p ema u e no hwa d displacemen o he ITCZ epo ed by En ield and Maye
(1997), and coinciding wi h a La Niña e en in he Paci ic.
T ends in coas al upwelling ha e also been iden i ied by a ious au ho s, al-
hough hey a e some imes con adic o y. Bakun (1990) iden i ied a signi ican
inc ease in upwelling- a o able wind s ess in a loca ion o No hwes A ica a
28◦N om 1946 o 1981. The in ensi ied winds in his si e coincided wi h consis-
11

Chap e 1. In oduc ion
en inc eases in o he si es o he wo ld’s majo coas al upwelling sys ems. They
hypo hesized ha , in a global wa ming scena io, as a mosphe ic g eenhouse gas
concen a ions inc ease, he e would be an in ensi ica ion o coas al upwelling
due o an inc ease in he land-sea p essu e g adien . Using p oxy empe a u e
da a de i ed om sedimen a y eco ds ex ending back 2500 yea s, McG ego
e al. (2007) in e ed an anomalous and unp eceden ed inc ease in coas al up-
welling o Cape Ghi du ing he 20 h cen u y. Con e sely, analyzing QuickSca
wind da a om 2000 o 2007, Dema cq (2009) ound a dec easing end in he
me idional componen o he wind s ess o No hwes A ica. Howe e , his
may only e lec in e annual o decadal a iabili y supe imposed on a longe ime
scale end.
P ima y p oduc ion
The coas al upwelling egion o No hwes A ica is one o he ou majo Eas -
e n Bounda y Upwelling Sys ems (EBUS) o he wo ld’s oceans. EBUS display
high p oduc i i y and impo an ishe y yields (Pauly and Ch is ensen, 1995).
The sub opical No heas A lan ic has he la ges ac i e zone o all main EBUS,
de ined as he a ea whe e chlo ophyll concen a ions a e abo e 1 mg m-3 (Ca ,
2002). I is he second mos p oduc i e EBUS, wi h an annual p ima y p oduc-
ion o 0.33 G o Ca bon pe yea , a e he Benguela EBUS in he Sou h A lan ic
(Ca , 2002).
Chlo ophyll concen a ions p o ide an indi ec measu emen o phy oplank-
on abundance and hus nume ous e o s ha e been made o de elop an algo-
i hm o de i e p ima y p oduc ion om emo e sensed chlo ophyll (Beh en eld
12
Chap e 1. In oduc ion
0.03 0.1 0.3 1 3
CHL (mg m )
-3
40
30
20
10
40
30
20
10
-30 -20 -10 -30 -20 -10
SUMMER
WINTER SPRING
FALL
Cape Blanc Cape Blanc
Cape Blanc Cape Blanc
Figu e 1.4: Mean seasonal chlo ophyll concen a ions (mg m-3) o win e (JFM),
sp ing (AMJ), summe (JAS) and all (OND). Black con ou s ma k he 0.2 and 1
mg m-3 chlo ophyll isolines. Da a co esponds o he SeaWiFS sa elli e senso o
he yea s 1998 o 2007.
13
Chap e 1. In oduc ion
and Falkowski, 1997; Ca , 2002; Ma a e al., 2003). Figu e 1.4 shows seasonal
chlo ophyll concen a ions de i ed om 10 yea s o SeaWiFS sa elli e chlo ophyll
da a. The egion be ween 24◦N and he S ai o Gib al a displays weak sea-
sonal a iabili y. Chlo ophyll concen a ions abo e 1 mg m-3 a e con ined o he
shel , despi e he yea -long upwelling occu ence. The weak o sho e ex ension
o chlo ophyll in his egion may be caused by nu ien limi a ion, as he neg-
a i e wind s ess cu l ha de ines he sub opical gy e dep esses he nu icline
(La huiliè e e al., 2008).
The egion be ween 18 and 24◦N also shows a weak seasonali y, bu in his
case high chlo ophyll concen a ions ex end a o sho e all yea ound. This la -
i udinal band includes, as he mos p edominan ea u e, he Cape Blanc gian
ilamen (Gab ic e al., 1993). Du ing summe and all, in ense o sho e ans-
po o Cape Blanc occu s as he esul o con e gence o he sou hwa d CUC
and he polewa d MC. In win e and ea ly sp ing, as upwelling eaches u he
sou h owa ds Cape Ve de, con e gence and o sho e anspo a e educed bu
s ill p esen (Peleg í e al., 2006). Filamen -like s uc u es ha ad ec chlo ophyll
o sho e can also be seen o he majo capes, such as Cape Ghi , whe e p ima y
p oduc ion eaches alues o 5 g C m-2 yea -1 (Ga cía-Muñoz e al., 2005; Peleg í
e al., 2005).
Be ween 18◦N and he S ai o Gib al a mesoscale ea u es, like ilamen s
and eddies, and wa e p ocesses con ibu e o he dynamical and biochemical
a iabili y in he egion, bu hey a e no discussed he e. An ex ensi e analysis o
mesoscale p ocesses is gi en by Ba on e al. (1998) and ci e Ba on1998a and an
excellen e iew on p opaga ing wa es is p o ided by Hagen (2001).
The egion sou h o 18◦N p esen s a la ge o sho e ex ension o chlo ophyll
14
Chap e 1. In oduc ion
du ing win e and sp ing, when coas al upwelling b ing nu ien ich wa e s o
he su ace. Du ing summe , he s eng hening o he NECC and he o sho e
displacemen o he posi i e Ekman pumping a ea upli s he uppe laye s o he
he mocline, and by all high p ima y p oduc ion can be obse ed in he GD a ea
(10◦N 22◦W app oxima ely). Peleg í e al. (2006) p opose a mechanism by which
he subsu ace laye s in he GD main ain a high nu ien concen a ion le el. The
GD de elops du ing summe and all and nu ien s ha each he eupho ic laye
a e u ilized. Du ing win e , upwelling- a o able ade winds ex end sou h o
Cape Blanc, and he GD elaxes. The e ical cell associa ed o coas al upwelling
needs a supply o subsu ace wa e s om he in e io ocean, eplenishing he
nu ien le els o he GD egion.
1.2 Objec i es and hesis ou line
This Ph.D. hesis con ains h ee cen al chap e s in scien i ic a icle o ma , p e-
ceded by a gene al in oduc ion and ended by gene al conclusions. A spanish
summa y o he hesis ollows, which includes he objec i es, me hodology, main
indings and conclusions.
A wide ange o da a sou ces ha e been examined. His o ical da a ha e been
eanalyzed. We also examine newly acqui ed hyd og aphic da a and a ailable e-
mo e sensed p ope ies such as chlo ophyll concen a ions and sea su ace heigh .
Finally, an ocean gene al ci cula ion model coupled o a s a e o he a biogeo-
chemis y model p o ides insigh in o physical mechanisms d i ing a iabili y
in biochemical p ope ies.
15
Chap e 2. Wa e and nu ien luxes
The s eady-s a e connec ion be ween he o sho e and coas al-upwelling on s is
con olled by h ee main ac o s: he loca ion o he Cape Ve de on al zone, he
in ensi y and la i udinal ex ension o coas al upwelling, and he size and loca ion
o he Guinea Dome open-ocean upwelling a ea.
One o he main ea u es in his egion al eady desc ibed in he In oduc-
ion is he Cape Ve de on al sys em. I s e ches sou hwes om Cape Blanc
o he Cape Ve de Islands, and e ec i ely sepa a es ela i ely new (sal y, wa m,
nu ien -poo , and oxygen- ich) NACW om he olde ( esh, cold, nu ien - ich,
and oxygen-poo ) SACW. Running me idionally along he coas we ind an ad-
di ional on be ween he cold upwelling bel and he wa me o sho e wa e s.
Sou h o he Cape Ve de on al zone we ind a cyclonic ci cula ion a ound he
Guinea Dome. The dome and associa ed ci cula ion mo e o sho e, owa ds
he cen al A lan ic, du ing summe ; in win e Ekman pumping in ensi ies and
mo es eas (Nykjae and Van Camp, 1994), me ging wi h he coas al upwelling
zone.
When conside ing he main con ols on he s eady-s a e dynamics, i is im-
po an o keep in mind ha he mean ield also expe iences emo ely- o ced
in e annual and in e decadal a ia ions. A i (1985) used local da a a 20◦N o
sugges ha upwelling in ensi ied om he 60’s o he 70’s. Roy (1991) also p e-
sen ed esul s suppo ing ha he 70’s was a pe iod o ela i e in ense upwelling
which dec eased in he 80’s. Mo e ecen ly sa elli e-de i ed sea su ace empe a-
u e (SST) measu emen s ha e shown ha he 80’s was a pe iod o li le in e an-
nual a iabili y o he upwelling index (di e ence in SST be ween he coas and
he open ocean) o No hwes A ica, wi h oscilla ions o a ew en hs o a de-
g ee (Nykjae and Van Camp, 1994; He nández-Gue a and Nykjae , 1997), bu
22

Chap e 2. Wa e and nu ien luxes
ha he e exis ed a majo shi by he end o he 80Õs in o he mid 90Õs, wi h a
change in he upwelling index g ea e han one deg ee (San os e al., 2005). These
changes a e qui e impo an as compa ed wi h he mean upwelling indexes o
he 18 o 26◦N band, which ange be ween 1 and 2◦C (Nykjae and Van Camp,
1994; He nández-Gue a and Nykjae , 1997; San os e al., 2005). The in e annual
oscilla ions in he upwelling index appea o be associa ed o indi idual No h
A lan ic Oscilla ion (NAO) e en s while he in e decadal a ia ions may be e-
la ed o sus ained NAO e en s du ing se e al consecu i e yea s (San os e al.,
2005). Ou pe iod o in e es (Ma ch-Ap il 1973 and Oc obe -No embe 1975)
had mode a e h ee-mon h a e aged NAO indexes (Clima e P edic ion Cen e ,
h p://www.cpc.ncep.noaa.go ), which a e cha ac e is ic o he mild in ensi i-
ca ion obse ed du ing he 70’s.
The abo e b ie desc ip ion o he main ci cula ion pa e ns immedia ely poin s
a se e al key issues ha con ol he luxes and la ge-scale pa e ns o cen al wa-
e s along No hwes A ica. The Cape Ve de on al sys em is cha ac e ized by
sha p mesoscala in usions o bo h NACW and SACW (named in e lea ing a -
e Ba on and Hughes, 1982), how e ec i e is i as a ba ie be ween no he n
and sou he n wa e s? The coas al upwelling on s e ches me idionally un-
il he Cape Ve de on , beyond in win e , is he e a connec ion be ween hese
wo on al sys ems? The coas al ansi ion zone is cha ac e ized by e ical up-
welling cells and ubiqui ous ilamen s ha e ec i ely ans e nu ien - ich wa-
e s in o he nu ien -deple ed su ace wa e s o he sub opical gy e, whe e and
a wha a es hese mass and nu ien exchanges ake place?
In his wo k we ha e eco e ed he da a o ou his o ical c uises o p oduce
wo la ge-scale synop ic da a se s. These da a se s a e ex ensi ely analyzed o
23
Chap e 2. Wa e and nu ien luxes
24W 22W 20W 18W 16W 14W
16N
18N
20N
22N
24N
26N
ATLOR II-III
Ma ch-Ap il 1973
A lo II
Ma ch
A lo III
Ap il
C. VERDE
BOJADOR
C. BLANC
50 m
N
S
L
Sp ing
50 m
0
24W 22W 20W 18W 16W 14W
16N
18N
20N
22N
24N
26N
C.VERDE
BOJADOR
C.BLANC
A lo VI
Oc obe
A lo VII
No embe
N
S
L
500 m
50
m
Fall
Figu e 2.1: Hyd og aphic s a ions and selec ed sec ions o he sp ing (le ) and
all ( igh )da a se s. The legend o he sec ions is as ollows: N, no h; S, sou h;
L, along-slope.
in es iga e he abo e ques ions, so ha we can imp o e ou unde s anding and
quan i ica ion o he wa e -mass and nu ien connec ions in he cen al wa e s all
along No hwes A ica. In sec ion 2.2 we b ie ly desc ibe he o iginal da a, and
how i has been assembled o p oduce sp ing and all da a se s, and in sec ion 2.3
we plo he da a o show he hyd og aphic condi ions in he a ea du ing hese wo
seasons. In sec ion 2.4 we explo e wha p ocesses a e esponsible o enhancing
mixing a he on al sys em, in sec ion 2.5 we use he da a o in e he su ace
and subsu ace dynamic ields in he egion, and in sec ion 2.6 we compu e he
along and c oss-sho e mass and nu ien anspo s.
24
Chap e 2. Wa e and nu ien luxes
2.2 Da a se
The da a used in his wo k we e collec ed by IIP as pa o ou di e en hyd o-
g aphic c uises: A lo II (Ma ch 1973), A lo III (Ap il 1973), A lo VI (Oc obe
1975), and A lo VII (No embe 1975). These c uises s e ched along he A ican
coas line, be ween 16.5 and 26.1◦N (Fig. 2.1) , and he da a we e compiled and
published in C uzado and Man íquez (1974); F aga and Man íquez (1974); Man-
íquez and Rucabado (1976); Man íquez and F aga (1978). C uises A lo III and
A lo VI co e ed he no he n con inen al shel and uppe slope, limi ed by Poin
Du n o d (23.6◦N), Cape Bojado (26.1◦N), he coas and he 500 m isoba h. In
hese no he n c uises he s a ion spacing was abou 30 km, and samples we e
aken down o 500 m (o o he bo om i shallowe ) a s anda d dep hs. C uises
A lo II and A lo VII an om 17◦N/16.5◦N (A lo II/VII) o Cape Blanc (21◦N)
in he egion om he con inen al slope o he 22◦W me idian, and had se e al
addi ional s a ions as a no h as 23◦N. In he sou he n c uises he mean dis-
ance be ween s a ions was app oxima ely 80 km, and samples we e aken down
o 1000 m (o o he bo om), also a s anda d dep hs. Du ing all c uises samples
we e aken wi h ei he Niskin o Hyd o-Bios bo les. Tempe a u e was ob ained
using The moschneide We heim/Main p o ec ed and unp o ec ed he mome-
e s, co ec ed using Hidaka’s equa ion (Key e, 1965). Conduc i i y was ob ained
using a Hy ech 6220 induc ion salinome e , and salini y was calcula ed using
UNESCO’s (1966) equa ions. Dissol ed oxygen was de e mined using he Win-
kle me hod (S ickland and Pa sons, 1968), and nu ien s we e measu ed using a
Technicon au oanalyze . O he de ails on ins umen a ion and some p elimina y
analysis o hese da a se s may be ound in Balles e e al. (1972) (A lo II and A -
25
Chap e 2. Wa e and nu ien luxes
lo III) and Man íquez and Rucabado (1976) (A lo VI and A lo VII). Due o hei
loca ion and da es, he ou c uises may be combined in o wo da a se s (each
wi h abou 80 hyd og aphic s a ions) wi h simila egional co e age, oughly
om no h o Cape Ve de (17◦N) o Cape Bojado (26◦N). The i s da a se com-
p ises A lo II and A lo III, and will he ea e be e e ed as sp ing (Ma ch-Ap il
1973). The second da a se includes A lo VI and A lo VII, and will he e o h be
e e ed as all (Oc obe -No embe 1975). These da a se s allow he in e compa -
ison o wo synop ic si ua ions o opposi e seasons. One limi a ion o such an
in eg a ed s udy is he dis inc zonal co e age o he da a, as he o iginal no he n
and sou he n c uises we e designed o s udy he upwelling ansi ion zone no h
o Cape Blanc and he Cape Ve de on al sys em, espec i ely. Ne e heless, all
c oss-sho e sec ions ex end well beyond he con inen al slope, so ha we may
expec hey ha e good co e age o he coas al upwelling je and adjacen in e io
ocean. Figu e 2.1 shows he loca ion o all hyd og aphic s a ions, as well as h ee
selec ed sec ions ha will be used o illus a e he e ical dis ibu ion o hyd o-
g aphic and chemical p ope ies. The no he n sec ion (N, abou 25.5◦N) ex ends
om he shel o he middle slope (100 km long), he sou he n sec ion (S, 21◦N)
s e ches some 500 km o sho e om he con inen al slope, and he along-sho e
sec ion (L, om 16 o 26◦N) uns some 1200 km oughly along he 500 m isoba h.
26
Chap e 2. Wa e and nu ien luxes
2.3 Wa e masses
2.3.1 P ope y-p ope y diag ams
Figu e 2.2 illus a es he po en ial empe a u e-salini y (θ-S) ela ionships du -
ing bo h seasons, as ob ained using all a ailable s a ions (he ea e , when alking
abou empe a u e we will ac ually e e o po en ial empe a u e, calcula ed us-
ing he su ace as he e e ence le el). In his igu e, as well as in Figu e 2.3 , we
ha e included s aigh lines ha de ine NACW and SACW (Tomczak, 1981).
The hyd og aphic s a ions may be di ided in h ee g oups, ollowing a No h-
o-Sou h θ-S ansi ion om NACW o SACW: no he n, on al, and sou he n.
No he n s a ions, wi h NACW cha ac e is ics, a e ound no h o Cape Blanc in
bo h da a se s (Fig. 2.2 , le ). The e is also one s a ion in each se a he la i ude o
Cape Blanc (20◦N, 500 km o sho e in sp ing; 21◦N, 300 km o sho e in all) ha
displays NACW cha ac e is ics. F on al s a ions, wi h ansi ional cha ac e is ics
be ween NACW and SACW, a e loca ed o Cape Blanc all he way om he coas
o deep wa e s, and ye u he sou h in deep wa e s (Fig. 2.2 , cen e ). Sou h-
e n s a ions, wi h SACW o igin, a e ound only sou h o Cape Blanc. These a e
mainly loca ed o e he con inen al slope, p obably e lec ing he in luence o he
polewa d unde cu en , al hough in sp ing hey each u he o sho e and wo
slope s a ions display mixed cha ac e is ics a se e al dep hs (Fig. 2.2 , igh ). The
all da a (Fig. 2.2 , bo om) shows high dispe sion nea he sea su ace because o
he p esence o he seasonal mixed laye . Du ing his season he e a e also some
low salini y nea -su ace alues ha co espond o he h ee sou he nmos s a-
ions (loca ed be ween la i udes 16.5 and 17◦N, and 75 o 200 km o sho e), which
27

Chap e 2. Wa e and nu ien luxes
Figu e 2.2: Sp ing ( op) and all (bo om) po en ial empe a u e-salini y diag ams
displaying NACW (le ), SACW ( igh ), and in e media e (cen e ) cha ac e is ics.
The lines co espond o he wo cen al wa e masses, as explained in he ex .
The inse s indica e he loca ion o he s a ions o each diag am.
28
Chap e 2. Wa e and nu ien luxes
Figu e 2.3: Sp ing ( op) and all (bo om) p ope y-p ope y plo s. Do s, iangles
and c osses co espond, espec i ely, o s a ions displaying NACW, SACW, and
in e media e cha ac e is ics in he empe a u e-salini y diag ams. The solid lines
co espond o he wo cen al wa e masses, as explained in he ex . In he igh
panel he do ed line has he slope o he oxygen-ni ogen s oichiome ic Red ield
a io.
29
Chap e 2. Wa e and nu ien luxes
likely espond o he in luence o esh wa e om he Senegal Ri e discha ge a
16◦N.
In Figu e 2.3 we ha e used exac ly he same g oups (no he n, on al and
sou he n wa e s, indica ed wi h do s, c osses and iangles, espec i ely) as in
Figu e 2.2 o illus a e he dis ibu ion o po en ial empe a u e, dissol ed oxy-
gen concen a ion (O2) and ni a e concen a ion (NO3) as a unc ion o salini y
(S), and o O2as a unc ion o NO3. In his igu e we ha e used all a ailable
s a ions bu exclude he da a poin s o he op 50 m, as he uppe -mixed laye
displays su ace wa m-wa e cha ac e is ics (F aga and Man íquez, 1974) and is
bo h nu ien -exhaus ed as a esul o biological ac i i y and oxygen- ich h ough
con ac wi h he a mosphe e (Lamb, 1984). On hese plo s we ha e again d awn
s aigh lines ha de ine he NACW and SACW. As sou ce wa e alues o O2
a e no included in Tomczak (1981), we ha e ob ained hese om he oxygen con-
cen a ion alues o hose s a ions ha closely ma ch each wa e -mass line in he
θ-S diag am. The p ocedu e consis s in making a linea eg ession o he salini y-
oxygen alues, om which we hen ob ain he oxygen concen a ions ha co e-
spond o he ex eme salini y alues de ined by Tomczak (1981). The O2 alues
ob ained in his manne a e compa able wi h esul s om Klein and Tomczak
(1994), and he p ocedu e, when epea ed o NO3, shows good ag eemen wi h
he wa e ype de ini ions in Tomczak (1981). The s aigh lines in Figu e 2.3
co espond o he wa e ype alues ob ained wi h he abo e p ocedu e o O2,
and Tomczak’s (1981) alues o he o he pa ame e s. Table 2.1 summa izes he
p ope y alues ha cha ac e ize he no he n and sou he n cen al sou ce wa e
ypes.
The le panel o Figu e 2.3 (θ-S diag am) shows ha wa e cha ac e is ics
30
Chap e 2. Wa e and nu ien luxes
change a he smoo hly be ween no he n (NACW) and sou he n (SACW) ends.
The cen al panels (O2-S and NO3-S diag ams) o Figu e 2.3 also illus a e ha
mos da a poin s lie be ween he NACW and SACW lines bu he on al do-
main appea s o ha e mo e spa se co e age, wi h ansi ion s a ions (as de e -
mined om he θ-S diag am) now p e e en ially shi ing owa ds ei he wa e
mass. We will come back o his issue in he nex sec ion, whe e we p opose ha
his ea u e is he esul o in ense double di usion in he Cape Ve de on al sys-
em (Zenk e al., 1991). The igh (O2-NO3diag am) panel in Figu e 2.3 shows
he lines o he wo wa e masses almos as i placed one a e he o he . The
eason is ha oxygen and ni a e a e no ully independen a iables, as ni a e
inc eases/diminishes h ough oxida ion/pho osyn hesis. The wo lines, how-
e e , do no ha e he same slope as he wo wa e masses ha e e y di e en
ages (Poole and Tomczak, 1999) , he slope o he NACW being simila o ha
expec ed om he oxygen-ni a e s oichiome ic Red ield a io (Takahashi e al.,
1985). Finally, i is wo h men ioning ha he diag ams in ol ing ei he dissol ed
oxygen o nu ien s display subs an ial sca e a ound hose lines ha cha ac e -
ize hese wa e masses. This is p obably he esul o enhanced p ima y p oduc-
ion in he upwelling egion.
2.3.2 Ve ical dis ibu ions
In Figu es 2.4 and 2.5 we p esen he sp ing and all dis ibu ions o T, S, σθ,
O2, and NO3, down o 400 m, along sec ions N, S, and L (Fig. 2.3 ). Sec ion N
co esponds o pu e NACW. Du ing bo h seasons he T and NO3isople hs aise
owa ds he coas , down o 100 m du ing all and o 200 m in sp ing. The S and O2
31
Chap e 2. Wa e and nu ien luxes
C. Blanc C. Blanc
C. Blanc C. Blanc C. Blanc
C. Blanc C. Blanc
cyclonic
an icyclonic
25ºW 20ºW 15ºW
15ºN
20ºN
25ºN
25ºW 25ºW 25ºW
25ºW 20ºW 15ºW
15ºN
20ºN
25ºN
20ºW 20ºW 20ºW 15ºW 15ºW 15ºW
25ºW 20ºW 15ºW 25ºW 20ºW 15ºW
Figu e 2.8: Sequence o i e-day mean SST images spanning om Janua y 11 o
31, 2003 ( op) and om Janua y 21 o Feb ua y 5, 2004 (bo om).
38

Chap e 2. Wa e and nu ien luxes
Blanc, some hing cha ac e is ic o win e mon hs (Peleg í e al., 2006). These im-
ages indica e he p esence o mesoscala s uc u es (cyclonic/an icyclonic wi h
cold/wa m co es) along he coas al upwelling on , no h and sou h o Cape
Blanc, and sugges hey a e ad ec ed along his upwelling on un il eaching
he Cape Ve de on al sys em.
Con e gence o sou he n and no he n wa e s a he on al zone p oduces
o sho e expo o he ela i ely cold upwelling wa e s. The o sho e ad ec ion
o hese upwelled wa e s esul s in he de elopmen o a gian ilamen o ela-
i ely cool wa e s, clea ly isible in he SST images (Gab ic e al., 1993). As he
Cape Ve de on is densi y-compensa ing he in e lea ing o wa e masses a his
on al egion may ei he a ise om he kine ic ene gy o he low con e ging o
Cape Blanc, o simply e lec he o sho e ad ec ion o he mesoscala s uc u es
de eloped along he upwelling on , bo h no h and sou h o Cape Blanc. These
mesoscale meande s and o ices would pene a e o dep hs cha ac e is ic o he
ba oclinic coas al je (abou 200 m), which is ypically abou hal he e ical scale
o mos ea u es in he Cape Ve de on (some 400 m, Figu es 2.4 and 2.5 ), so ha
hey ce ainly can only explain pa o he on al a iabili y. In ei he case, he
ul ima e o igin o in e lea ing would be wind o cing o e he bounda y egion
in he eas e n sub opical and opical gy es.
2.4.2 Double di usion
The uns able e ical dis ibu ion o sal in he A lan ic makes i s uppe ocean a-
o able o double di usion in he o m o sal inge s (Klein and Tomczak, 1994;
S Lau en and Schmi , 1999; You, 2002). This si ua ion is enhanced a he on al
39
Chap e 2. Wa e and nu ien luxes
Ocean Da a Vie w
20
40
60
80
300
200
100
0
75
90
45
75
6
0
Ocean Da a Vie w
20
40
60
80
300
200
100
0
45
45
60
Ocean Da a View
100
200
300
400
45
45
45
45
75
60
60
Ocean Da a View
100
200
300
400
75
75
45
45
45
45
60
60
Ocean Da a View
0
200
400
600
800
1000
45
75
45
45
75
60
60
60
60
75
75
75
Ocean Da a View
0
200
400
600
800
1000
1200
45
45
60
60
60
60
Dis ance [km]
Dep h [m]
A ica
A ica
No h
Sou h
NSL
Figu e 2.9: Sp ing ( op) and all (bo om) dis ibu ions o he Tu ne angle (be-
ween 45◦and 90◦) on sec ions N (le ), S (cen e ), and L ( igh ). No ice he change
in ho izon al scale be ween adjacen columns (scale hal es changing column o
he igh ).
sys em, as sal y NACW o e lies ela i ely esh SACW, so we wonde i double
di usion may be esponsible o he ela i ely smoo h dis ibu ion o empe a-
u e as compa ed wi h o he hyd og aphic p ope ies such as salini y and nu i-
en s. In o de o explo e his idea we ha e compu ed he e ical and ho izon al
dis ibu ions o he Tu ne angle (see Appendix B o mo e de ails on he Tu ne
angle). Fo he compu a ion o he Tu ne angle we ha e used 50 m as he e ical
di e ence in e al (be ween 0 and 300 m) o ob ain he mean s a e o double di -
usion, simila o o he s udies o he A lan ic Ocean (Schmi , 1990; You, 2002).
Sal inge ing may ake place when his angle is be ween 45◦and 90◦, wi h mode -
a e inge ing occu ing o angles be ween 60◦and 75◦, and s ong inge ing (s- )
p e ailing o angles be ween 75◦and 90◦.
Figu e 2.9 p esen s he e ical dis ibu ion o he Tu ne angle o he same
e ical sec ions as in Figu es 2.4 and 2.5 . The dis ibu ion clea ly sugges s ha s-
is associa ed wi h he on al sys em. Sec ion S displays s- angles in he o sho e
40
Chap e 2. Wa e and nu ien luxes
Figu e 2.10: Sp ing (le ) and all ( igh ) dis ibu ion o he Tu ne angle a 225 m.
a ea du ing sp ing, and a he wo ends o he sec ion du ing all. Sec ion L
shows s- angles in he no he n hal , below 100 m du ing sp ing and below 200
m du ing all. The coas al upwelling band in he no he n a ea, cha ac e ized
by Sec ion N, also has high Tu ne angles be ween 100 and 200 m dep h. The
eason o his is he p esence o he subsu ace salini y maximum, which makes
his dep h ange p one o double di usion. This may ac ually be an e ec i e
mechanism o diapycnal mixing, helping o main ain he e ical ci cula ion cell
in his upwelling zone.
Figu e 2.10 shows he ho izon al dis ibu ion o he Tu ne angle a 225 m,
which is close o he dep h le el (200 m) chosen o he ho izon al dis ibu ions
o p ope ies (Figs. 2.6 and 2.7 ) and has good co e age o he on al sys em. We
could ac ually choose any ho izon be ween 100 and 300 m and he esul s would
no change oo d as ically. Du ing sp ing Tu ne angles abo e 60◦, and e en
abo e 75◦, a e ound a o Cape Blanc and in he sou he nmos a eas. Du ing
all, on he o he hand, mos o he on al sys em o Cape Blanc displays high
41
Chap e 2. Wa e and nu ien luxes
Tu ne angles. Again he e we may app ecia e ha he no he n coas al upwelling
band is cha ac e ized by Tu ne angles g ea e han 60◦.
Le us p opose a simple mechanism ha would a o he epipycnal (along-
isopycnal o , because o he densi y compensa ing cha ac e o he on , nea -
ho izon al) smoo hing o he empe a u e ield in on al egions, wi h wa m
saline wa e s o e lying cool esh wa e s (Figu e 2.11). In his igu e we only
illus a e he he mal ield, al hough he ini ial iso he ms and isohalines a e co-
inciden . Suppose he cen al po ion o he on ( he mos uns able one) un-
de goes in ense inge ing, so ha downwa d (downg adien ) sal and hea luxes
ake place (Ruddick and Ga ge , 2003). The hea lux dec eases mo e apidly
wi h dep h han he sal lux, as hea di uses epipycnally (g ay a ows in Fig.
2.11), away om he on al egion. In he wa m-wa e side o he on al e-
gion his lux would oppose he epipycnal down-g adien hea lux (ho izon al
black a ows in Figs. 2.11 a and 2.11 c), while in he cold-wa e side he wo
luxes would add up. As a esul he empe a u e g adien s in he wa m-wa e
side would emain essen ially unchanged bu hose in he cold-wa e side would
dec ease subs an ially.
The enhancemen o ho izon al hea di usion in he on al sys em would
conceal he o iginal wa e masses in θ-S diag ams, in opposi ion o hei un-
masked p esence in o he p ope y-p ope y plo s (such as NO3-S whe e he cha -
ac e is ics o a wa e pa cel emain close o hose o he o iginal wa e masses).
The p ope y-p ope y plo s in Figu e 2.3 con i m he p esence o NACW in he
su ace laye s o se e al slope s a ions sou h o Cape Blanc, some hing no isible
in he mo e di usi e θ-S dis ibu ions (Figs. 2.2 and 2.3 ).
In o de o illus a e he abo e ideas we ha e p epa ed Figu e 2.12 , which
42
Chap e 2. Wa e and nu ien luxes
z c oss- on al coo dina e, x
z = cons , o
ρ
cons
a
≅
b
T
S
T, S
z
T
1
T
4
> T
3
T
5
> T
4
T
2
> T
1
T
3
> T
2
d-g
s-
s-
x
c
d-g
T
Figu e 2.11: (a) Schema ics o on al egion wi h ho izon al isopycnals ( hin do -
ed lines) and sloping iso he ms ( hick lines). The ini ial (do ed) and inal (solid)
empe a u e dis ibu ions a e shown. (b) Sal (S) and hea (T) e ical luxes in
he on al egion, illus a ing he p esence o e ical hea con e gence. (c) Ini-
ial (do ed) and inal (solid) empe a u e dis ibu ion on isopycnal illus a ing
s ong- inge ing (s- ) and o dina y down-g adien (d-g) hea luxes.
43

Chap e 2. Wa e and nu ien luxes
Figu e 2.12: Sp ing ( op) and all (bo om) ni a e-salini y (le ) and po en ial
empe a u e-salini y ( igh ) diag ams. Only da a poin s wi h in e media e wa-
e cha ac e is ics a e included. Squa es, iangles and as e isks co espond o
da a poin s wi h σθapp oxima ely gi en by 26.5, 27.0 and 27.5, espec i ely (as
explained in he ex ), and all o he poin s a e shown as do s.
44
Chap e 2. Wa e and nu ien luxes
shows he NO3-S and θ-S diag ams du ing sp ing and all, bu now including
only he on al s a ions (acco ding o hei dis ibu ion in he θ-S diag am). He e
we ha e no included measu emen s a dep hs less han 50 m, in o de o emo e
he e ec s o he su ace mixed laye . In his igu e we di e en ia e he da a poin s
wi h densi ies close o σθ= 26.5, 27.0 and 27.5 (σθ= 26.5 ac ually co esponds o all
da a poin s 26.45 <σθ<26.55, and simila ly o he o he wo alues). In o de o
illus a e how p ope ies change along a densi y su ace, we i a eg ession line
o each densi y alue. A he mesoscale he di e ences be ween bo h diag ams
can only a ise om di e en di usi e hea beha io . The igu es illus a e ha in
he NO3-S diag am he da a poin s emain close o he wo o iginal wa e masses
while in he θ-S diag ams hey a e dis ibu ed a he e enly along he s aigh
lines. The lack o in e media e ni a e and salini y alues indica es ha hese
p ope ies unde go much less epipycnal di usion han empe a u e.
2.5 Wa e and nu ien luxes
The e ical upwelling cell d i es nu ien - ich subsu ace wa e s o he ba o-
clinic zone, whe e hey become accommoda ed by along-sho e wa e and nu i-
en luxes all he way o he Cape Ve de on . In his manne he upwelling je , i
deep enough, may play a decisi e ole in he along-sho e ad ec ion o he no h-
e n wa e -masses. The Cape Ve de on al sys em is usually hough o be he
sou he n limi eached by he no he n wa e s. This sys em mo es seasonally
se e al deg ees (S amma and Siedle , 1988), bu a mosphe ic o cing has a much
g ea e la i udinal oscilla ion, esul ing in la ge changes in he SACW geos ophic
45
Chap e 2. Wa e and nu ien luxes
Figu e 2.13: Geos ophic eloci ies ac oss sec ion S in all, ela i e o (a) 200, (b)
300, (c) 400, and (d) 500 m.
luxes sou h o he Cape Ve de on . In his sec ion we explo e he cen al wa e
and nu ien pa hs in he egion.
2.5.1 Re e ence le el
In o de o calcula e he geos ophic eloci y ield we need o selec a p ope
e e ence le el. Fo ou da a se s, whe e many s a ions a e loca ed in ela i ely
shallow wa e s, we ha e o comp omise be ween deepness, so ha he e e ence
eloci ies a e indeed small, and shallowness, so ha ou sec ions each down o
he selec ed e e ence le el. Once a e e ence le el is chosen, we may ex end i
46
Chap e 2. Wa e and nu ien luxes
o shallowe s a ions o e he slope and shel , h ough he me hod desc ibed in
Csanady (1979). The main limi a ion o his me hod is ha i equi es he bo om
densi ies o assu e a ze o bo om alongsho e eloci y, which need o be in e po-
la ed om he a ailable alues.
In Figu e 2.13 we illus a e, as an example, he geos ophic eloci ies in sec-
ion S du ing all, wi h ou di e en e e ence le els (200, 300, 400, and 500 m).
The e a e signi ican di e ences be ween he eloci y ield calcula ed using he
200 m and 300 m e e ence le els. The di e ences, howe e , dec ease g ea ly be-
ween 300 m and he deepe e e ence le els. This si ua ion epea s i sel o mos
ansec s (no shown), o which eason we ha e selec ed he 300 m as an app o-
p ia e e e ence le el. Howe e , use o such a shallow e e ence le el o e he
slope may lead o unde es ima es o he polewa d unde cu en .
2.5.2 Dynamic heigh
Figu e 2.14 shows dis ibu ions o dynamic heigh ( e e enced o 300 m) a he
sea su ace and 100 m, o bo h he sp ing and all da a se s. F om he dynamic
heigh di e ences we may in e he g oss anspo pa e ns (pe uni dep h).
These ha e been ske ched o e he dynamic heigh dis ibu ion o p o ide an
o e iew o he nea su ace egional ci cula ion. Each a ow co esponds o a
lux o 3.7 ×103m2s−1o , equi alen ly, o a wa e anspo o 0.37 S o e a 100
m deep wa e -column.
The low pa e ns a e a he complex, simila o hose sugges ed by he se-
quences o SST images in Figu e 2.8 . A ecu en pa e n o bo h seasons is he
o sho e-insho e cyclonic eci cula ion ha occupies he whole egion be ween
47
Chap e 2. Wa e and nu ien luxes
anspo s ake place o Cape Blanc, 1.0 S o wa e -mass and 14 kmol s-1 o
ni a e. In he sou he n a ea he anspo s e e sign, hei maximum alue
being 1.8 S and 38 kmol s-1. Du ing all we ind al e na ion o no hwa d and
sou hwa d anspo s in bo h he no he n and sou he n a eas. Despi e he sou h-
wa d wa e -mass anspo p edomina es o e mos no he n sec ions we ind
ha he mean ni a e anspo is e y close o ze o, his being possible as no h-
wa d low usually akes place associa ed o he polewa d unde cu en , whe e
NO3is g ea e han nea -su ace. Only in he sou he nmos h ee sec ions we
ind p edominan no hwa d mass and nu ien anspo s, wi h maximum al-
ues o 0.5 S and 9 kmol s-1 o 19◦N. The p esence o no hwa d anspo in his
sou he n egion du ing bo h seasons (subs an ially g ea e in sp ing) e lec s he
in ensi ica ion o he polewa d unde cu en .
2.6.2 C oss-sho e anspo s
Along-sho e con e gence/di e gence, as calcula ed om he di e ence in along-
sho e anspo h ough consecu i e no mal- o-sho e sec ions, aduces in c oss-
sho e expo /impo . In Fig. 2.18 ( igh ) we p esen hese alues o he no he n
and sou he n a eas o bo h da a se s. Du ing bo h seasons he no he n a ea expe-
iences li le ne wa e -mass and nu ien exchange be ween he bounda y egion
and he deep ocean. The sou he n a ea expo s subs an ial amoun s o wa e -
mass and ni a e du ing sp ing, wi h maximum alues o 1.7 S o wa e -mass
and 27 kmol s-1 jus sou h o Cape Blanc (20.5◦N). Du ing all he sou he n a ea
shows a apid la i udinal ansi ion om subs an ial expo a 20.5◦N (0.7 S o
wa e -mass and 7 kmol s-1) o impo a 18.5◦N (0.4 S o wa e -mass and 5 kmol
54

Chap e 2. Wa e and nu ien luxes
s-1), and he ne exchange is, once again, qui e small.
Despi e we lack good s a ion co e age o he cen al a ea, we may ob ain he
wa e mass and nu ien con e gence/di e gence o each a ea by simply con-
side ing he along-sho e anspo s in ou c oss-sho e sec ions, hose loca ed a
he la i udinal ends o he h ee a eas (18◦N, 21◦N, 23.5◦N and 26◦N). Fo sec-
ions lanked o he eas by he A ican coas line, as hose in he no he n and ( o
a lesse deg ee) cen al a eas, along-sho e con e gence/di e gence aduces in
c oss-sho e expo /impo (ac oss sec ion L). In he sou he n a ea, howe e , ou
c oss-sho e sec ions do no qui e each he slope so he e may also be exchange
wi h he con inen al slope and shel .
Du ing sp ing he no he n a ea has small con e gence/di e gence alues
while he cen al a ea is cha ac e ized by mode a e wa e and nu ien di e -
gence (Table 2.2), o impo om he deep ocean o he coas al ansi ion zone
(0.8 S and 14 kmol s-1). The sou he n a ea, on he o he hand is cha ac e ized
by e y in ense wa e and ni a e con e gence, 2.8 S and 53 kmol s-1, o expo
o he deep ocean. Du ing all he e is again li le along-sho e di e gence in he
no he n a ea. The cen al a ea, howe e , has mode a e wa e -mass and nu ien
con e gence. As in sp ing, he sou he n a ea is cha ac e ized by expo o he
deep ocean, bu hese alues a e one o de o magni ude smalle (Table 2.2).
2.6.3 Closing he balances
C oss-sho e exchange may be accommoda ed h ough bo h he geos ophic and
Ekman con ibu ions. The geos ophic c oss-sho e wa e -mass and ni a e ans-
po s a e calcula ed in he no he n and sou he n a eas by in eg a ing he geos ophic
55
Chap e 2. Wa e and nu ien luxes
Table 2.2: Wa e (S ) and ni a e (kmol s−1) anspo s, in eg a ed o each o he
h ee a eas, o bo h he sp ing and all c uises
Wa e Mass Ni a e
Sp ing/Fall Sp ing/Fall
No h GT 0.52 / 0.44 2.7 / 2.9
C/D-0.09 / 0.17 0.5 / 1.9
Cen al C/D0.77 / -0.17 13.5 / -2.6
Sou h
GTL1.15 / -0.22 19.2 / -7.9
GTO-1.63 / -1.10 -31.1 / -12.5
GTO−GTL-2.78 / -0.88 -50.3 / -4.5
C/D-2.84 / -0.56 -52.8 / -3.0
GT s ands o c oss-sho e geos ophic anspo (nega i e o sho e) h ough he along-
sho e sec ions. C/D s ands o con e gence/di e gence (nega i e/posi i e), ha esul
in c oss-sho e anspo (nega i e o sho e), calcula ed as he di e ence in geos ophic
along-sho e anspo be ween adjacen c oss-sho e sec ions. Sou h o Cape Blanc we
p esen he geos ophic anspo ac oss bo h sec ions L (GTL) and O (GTO).
eloci y and ni a e luxes h ough he me idional sec ions delimi ing hese a -
eas: sec ion Lnin he no he n a ea and sec ions Lsand O in he sou he n a ea
( he la e is an open-ocean along-sho e sec ion, oughly pa allel o sec ion Ls,
ha s e ches some 350 km om 18 o 21◦N, Fig. 2.7 ). Table 2.2 compa es hese
e ically and spa ially in eg a ed geos ophic c oss-sho e exchanges, wi h he
abo e along-sho e con e gence/di e gence alues. The ag eemen is easonably
good, wi h di e ences in olume and ni a e anspo s being ypically abou 0.5
S and 2 kmol s-1. This gi es suppo o ou choice o e e ence le el, and g an s
con idence o ou es ima es o impo -expo in he cen al a ea, calcula ed solely
om he along-sho e con e gence/di e gence.
The obse ed di e ences be ween along-sho e con e gence/di e gence and
c osssho e geos ophic anspo in he no he n and sou he n a eas may pa -
56
Chap e 2. Wa e and nu ien luxes
Table 2.3: Mean (and s anda d de ia ion) ni a e concen a ion (mmol m−3) in
he uppe 50 m o he wa e column, a e aged pe a ea, du ing sp ing and all
Sp ing Fall
No h 0.47 (±0.43) 0.81 (±1.01)
Cen al 7.25 (±2.29) 7.20 (±4.23)
Sou h 3.82 (±2.99) 2.55 (±2.74)
ially be jus i ied because o he Ekman con ibu ion. We do no ha e ac ual wind
alues o he egion du ing he c uises bu , as a e e ence, we may use he mean
mon hly Ekman anspo s pe uni along-sho e dis ance epo ed by Nykjae
and Van Camp (1994). As mos Ekman anspo akes place in he su ace mixed-
laye we calcula e he associa ed ni a e anspo mul iplying wa e anspo by
mean ni a e concen a ion o he su ace mixed-laye . Fo his calcula ion we
use, as a g oss es ima e, he mean concen a ion alues in he uppe mos 50 m
o each a ea (Table 2.3).
The wind-induced anspo is indeed o he o de o he obse ed di e ences
in wa e anspo (Table 2.4 ). This anspo is g ea e in sp ing, he sp ing- all
di e ence being g ea es in he cen al and sou he n a eas. Du ing sp ing he
wa e -mass Ekman anspo in he sou he n a ea is much smalle han in he
no he n a ea, bu he nu ien Ekman anspo is almos he same because o
he p esence o nu ien - ich su ace wa e s.
Gi en he app oxima ions in ol ed (shallow e e ence le el, sec ions o lim-
i ed leng h, and mean a he han synop ic winds) he wa e -mass balance closes
easonably. No e, o example, ha an e o in e e ence eloci y o 0.01 m s-1
o a sec ion 100 km long and 300 m deep co esponds o 0.3 S . Fu he , he
wind-induced anspo has a quad a ic dependence on he su ace winds so ha
57
Chap e 2. Wa e and nu ien luxes
Table 2.4: Es ima es o Ekman wa e (S ) and ni a e (kmol s−1) anspo s in e-
g a ed o each a ea, o bo h he sp ing and all c uises (sign con en ion as in
Table 2.2)
Wa e Mass Ni a e
Sp ing/Fall Sp ing/Fall
No h CD −GT -0.61 /-0.27 -2.2 / -1.0
mean wind -0.32 / -0.15 -0.1 / -0.1
Cen al mean wind -0.51 / -0.24 -3.7 / -1.7
Sou h C/D−GTO+GTL-0.06 / 0.32 -2.4 / 1.5
mean wind -0.43 / -0.09 -1.6 / -0.2
These alues a e ob ained bo h as he di e ence be ween along-sho e con e -
gence/di e gence and c oss-sho e geos ophic anspo s, C/D - GT, o om he sp ing
and all mean Ekman anspo s pe uni leng h in he egion, mean wind (Nykjae and
Van Camp, 1994). In he la e case ni a e anspo is ob ained mul iplying he wa e
anspo be ween adjacen s a ions by he mean ni a e concen a ion in he uppe 50 m
o he wa e column and in eg a ing along-sho e.
doubling he wind causes he anspo o quad uple, i.e. he use o a s anda d
o mula (Wu, 1980) in ou egion shows ha an inc ease in he wind speed om
4 o 8 m s-1 causes an inc ease in anspo om 0.6 o 2.4 S . On he o he hand,
ni a e shows some g ea e unbalances possibly as a esul o nu ien u iliza ion
wi hin he same upwelling egion. This is also e lec ed in he s anda d de ia-
ions in op 50 m in Table 2.3.
The ai ag eemen is due o se e al condi ions ha a e speci ic o ou s udy
egion. Fi s , he coas al upwelling je is he p edominan eloci y signal in he
egion, which does no ex end e ically beyond some 200 m. Thus, he c oss-
sho e e ical upwelling cell ha connec s he in e io ocean and he coas al-je
zone does no each g ea e dep hs. Second, his coas al je does no ex end o -
sho e beyond he in e nal adius o de o ma ion (o abou 100 km, equal o less
58
Chap e 2. Wa e and nu ien luxes
han ou no mal- o-sho e s a ions), so ha i is jus i ied o speak abou a Cana y
Upwelling Cu en (Peleg í e al., 2005, 2006). And hi d, he Ekman con ibu-
ion is ela i ely small as compa ed wi h along-sho e con e gence/di e gence,
al hough i may be impo an in he nu ien balance o he sou he n a ea as a
esul o he high su ace-nu ien concen a ions he ein.
2.7 Conclusions
We ha e combined da a om ou hyd og aphic c uises du ing he 1970s in o -
de o p oduce wo seasonal da a se s (sp ing 1973 and all 1975) wi h me idional
co e age o No hwes A ica om 17 o 26◦N. The egion is di ided in h ee a -
eas: sou he n (18-21◦N), cen al (21-23.5◦N), and no he n (23.5-26◦N). The da a
se s ha e been used wi h h ee main pu poses: o desc ibe he hyd og aphy o
he zone, o examine mixing p ocesses a he on al sys em, and o es ima e wa-
e mass and nu ien expo /impo be ween he deep ocean and he coas al
ansi ion zone. The whole egion is cha ac e ized by shallow (down o no mo e
han 200 m) coas al upwelling and by he p esence o a a he ab up ansi ion
om No h A lan ic o Sou h A lan ic Cen al Wa e s (NACW o SACW) in he
Cape Ve de on al sys em. The on al posi ion, de ined as he in e sec ion o
he S = 36.0 and σθ= 26.5 su aces, is loca ed oughly o Cape Blanc bu mo es
no h/sou h du ing all/sp ing. The analysis o wo sequences o SST images
sugges s ha on al in e lea ing may pa ly o igina e as mesoscale ea u es in
he coas al upwelling on , which con e ge o Cape Blanc and become ans-
po ed wes wa ds.
59

Chap e 2. Wa e and nu ien luxes
P ope y-p ope y diag ams wi h s a ions nea by he Cape Ve de on al sys-
em illus a e ha i e ec i ely beha es as a ba ie o all p ope ies bu hea .
We ha e examined hese diag ams o ou p ope ies (po en ial empe a u e,
salini y, ni a e, and dissol ed oxygen) and ha e ound ha in all diag ams (ex-
cep hose using empe a u e) jus a ew da a poin s ha e p ope ies in e me-
dia e be ween NACW and SACW, ins ead mos poin s emain g ouped a ound
he o iginal wa e masses. The empe a u e is a s iking excep ion, as i s dis-
ibu ion in e ical sec ions and in he θ-S diag ams appea s as i he di usion
coe icien o hea was much g ea e han o o he p ope ies. The spa ial dis-
ibu ion o he Tu ne angle shows ha he on al sys em is p one o he exis-
ence o double-di usion in he o m o sal inge ing. We p opose ha double-
di usion is esponsible o enhanced ho izon al hea di usion ha esul s in a
ela i ely smoo h dis ibu ion o empe a u e ac oss he on , as compa ed wi h
o he p ope ies.
The densi y ield is used o es ima e he nea -su ace geos ophic low pa -
e ns in he egion. The e e ence dep h used o hese calcula ions is 300 m,
which a simple sensi i i y analysis sugges s can cap u e he majo elemen s o
he nea -su ace low. Du ing bo h seasons he low be ween Cape Blanc and
Cape Bojado is cyclonic. Wes and sou h o Cape Blanc he low pa e n changes
g ea ly be ween seasons. Wes o Cape Blanc he low is weak and di ec ed sou h-
sou hwes du ing sp ing and al e na es in di ec ion du ing all, while be ween
Cape Blanc and Cape Ve de he sp ing low is along-sho e o he no h and weak-
ens in all.
Nine c oss-sho e sec ions in he coas al ansi ion zone a e used o es ima e he
along-sho e wa e and nu ien anspo s. F om hese alues we hen calcula e
60
Chap e 2. Wa e and nu ien luxes
he along-sho e con e gence/di e gence o , equi alen ly, he exchange be ween
he upwelling egion and he deep ocean in h ee a eas (no he n; 23.5−26◦N;
cen al: 21−23.5◦N; sou he n: 18−21◦N). These alues a e inally explained in
e ms o c oss-sho e geos ophic and wind-induced con ibu ions, he o me cal-
cula ed using along-sho e sec ions and he la e es ima ed wi h mon hly mean-
winds. The o e all pic u e is ha p esen ed by he geos ophic ields in Figu e
2.14, modi ied by su ace Ekman anspo associa ed o he seasonal a ying pa -
e n o no heas e ly winds. The no he n a ea, and he cen al a ea du ing all,
expe ience geos ophic impo and wind-induced expo ha add up o small ne
exchange wi h he deep ocean, each no mo e han 0.5 S and 3 kmol s-1. The cen-
al a ea du ing sp ing impo s wa e and nu ien s, as geos ophic in low dom-
ina es o e wind-induced ou low, o abou 0.8 S and 14 kmol s-1. The sou he n
a ea expo s 2.9 S and 53 kmol s-1 o he deep ocean du ing sp ing, and 0.6 S
and 3 kmol s-1 o he deep ocean du ing all.
61
Chap e 3
Me idional changes in wa e mass
dis ibu ions∗
Abs ac
An Op imum Mul ipa ame e Analysis is applied o a da a se in he eas e n bounda y
o he No h A lan ic sub opical gy e, ga he ed du ing he mon h o No embe o wo
consecu i e yea s and spanning om 16 o 36◦N. This da a se co e s o e 20◦o la i ude
wi h good me idional and zonal esolu ion o e he whole coas al ansi ion zone. The
con ibu ion om six wa e ypes in he dep h ange be ween 100 and 2000 m is sol ed.
In he 100 o 700 m dep h ange he cen al wa e s o sou he n and no he n o igin mee
ab up ly a he Cape Ve de F on al Zone. This on adi ionally has been epo ed
o s e ch om Cape Blanc, a abou 21.5◦N, un il he Cape Ve de Islands, bu in ou
case i pene a es as a as 24◦N o e he con inen al slope. Sou h o his la i ude we
ac ually ind a less saline and mo e oxygena ed a ie y o sou he n cen al wa e , which
∗submi ed o Ciencias Ma inas as M.V. Pas o , J. Peña-Izquie do, J.L. Peleg í, and A. Ma e o-
Díaz. Me idional changes in wa e p ope ies o NW A ica du ing No embe 2007/2008.
63
Chap e 3. Me idional changes in wa e p ope ies
La i ude [ºN]
Dis ance [km]
Dep h [m]
Dep h [m]
Figu e 3.2: Dis ibu ion o salini y in colo scale and dissol ed oxygen as black
isolines along sec ions (a) Me idional, (b) F on , (c) Sou h, and (d) No h. Whi e
isolines ep esen hose isopycnals, σθ= 26.46 and 27.14, delimi ing he cen al
wa e mass laye .
70

Chap e 3. Me idional changes in wa e p ope ies
Dep h [m]
Dep h [m]
La i ude [ºN]
Dis ance [km]
Figu e 3.3: Dis ibu ion o phospha es in colo scale and silica es as black isolines
along sec ions (a) Me idional, (b) F on , (c) Sou h, and (d) No h. Whi e isolines
ep esen hose isopycnals, σθ= 26.46 and 27.14, delimi ing he cen al wa e mass
laye .
71
Chap e 3. Me idional changes in wa e p ope ies
wi h AAIW inding i s no he nmos expansion in la e all and MW s e ching o
he sou h du ing win e (Machín and Peleg í, 2009; Machín e al., 2010). Fu he
deep we ind wa e s o no he n o igin, he e gene ically g ouped unde he No h
A lan ic Deep Wa e s (NADW) denomina ion.
The op laye , om he su ace down o he po en ial densi y le el σθ= 26.46
o app oxima ely he uppe 100 m o he wa e column, displays highly a iable
θ-S cha ac e is ics as he esul o coas al upwelling and also due o he p esence
o mesoscala a iabili y in he coas al ansi ion zone. The mesoscale ea u es a e
hemsel es he esul o ins abili ies in he coas al upwelling je (e.g. Peleg í e al.,
2005; Pas o e al., 2008) as well as he dis u bance o he CC low by he islands,
p ominen ly he Cana y a chipelago (Sang à e al., 2005, 2007, 2009; Machín e al.,
2006). In gene al his su ace laye has ela i ely high salini y and dissol ed oxy-
gen alues and is deple ed in nu ien s (Figs. 3.2 and 3.3 ). A pa icula ea u e o
he eas e n No h A lan ic sub opical gy e is he subsu ace salini y maximum
be ween 50 and 100 m (Figu e 3.4), which is o med due o an excess o e apo a-
ion o e p ecipi a ion. I sinks down o he co esponding densi y le els due o
Ekman anspo and win e e ical con ec ion. Th oughou sp ing and summe
he e is su ace capping o his salini y anomaly, which hen sp eads ho izon ally
o e long dis ances (Baue and Siedle , 1988).
Immedia ely below and down o some 600 m (26.46 <σθ<27.14) we ind he
wo dominan cen al wa e s o ou domain, NACW and SACW. The con luence
o NACW and SACW occu s a he CVFZ, which is loca ed in he No h A lan ic
because o he yea -long no he n posi ion o he In e opical Con e gence Zone
(ITCZ, o he Ea h’s he mal equa o ). The Cape Ve de on al sys em is cha ac-
e ized by an ab up ansi ion in empe a u e, salini y, dissol ed ino ganic nu-
72
Chap e 3. Me idional changes in wa e p ope ies
35 35.5 36 36.5 37
2000
1500
1000
500
0
Salini y [psu]
Dep h [m]
5 10 15 20 25
θ [ºC]
0 1 2 3 4 5
Oxygen [ml/l]
0 0.5 1 1.5 2
2000
1500
1000
500
0
PO4
[μmol/l]
Dep h [m]
0 5 10 15 20 25
SiO4
[μmol/l]
x
E5 NACW, MW
E41 In e lea ing
E54 SACW*, AAIW
E70 SACW, AAIW
Figu e 3.4: Ve ical dis ibu ion o po en ial empe a u e θ, salini y S, phospha e
PO4, Silica e SiO4, and dissol ed oxygen O2, a selec ed s a ions displaying di -
e en wa e cha ac e is ics. The do s and c osses no only se e o iden i y he
di e en s a ions, hey also indica e he sampling dep hs o ino ganic nu ien s.
The loca ion o s a ions E5, E41, E54 and E70 is shown in Figu e 3.1.
73
Chap e 3. Me idional changes in wa e p ope ies
ien s and dissol ed oxygen. These p ope ies a e subs an ially di e en as a e-
sul o hei di e en o igin, he o me being ela i ely young wa e s ( he e o e
well oxygena ed and nu ien poo ) o med a he no he n edge o he ela i ely
saline and wa m No h A lan ic sub opical basin while he la e being much
olde wa e s o a mo e emo e o igin ( he sub opical con e gence o he Sou h
A lan ic).
The CVFZ has been said o s e ch sou hwes om nea Cape Blanc owa ds
he Cape Ve de Islands (see inse in Fig. 3.1, Zenk e al., 1991; Pas o e al., 2008).
In ou obse a ions, howe e , he on appea s o begin signi ican ly no h o
Cape Blanc, a abou 24◦N (Figs. 3.2a, 3.3a). Along he CVFZ he e is in e -
lea ing, o in usions o NACW and SACW a di e en longi udes and dep hs
(Zenk e al., 1991), a o ed by he densi y-compensa ing cha ac e o he empe -
a u e and salini y ields. This in e lea ing is clea ly obse ed in Figu es 3.2b and
3.3b, whe e high salini y-oxygen and low nu ien concen a ions, cha ac e is ic
o NACW, al e na e wi h low salini y-oxygen and high nu ien SACW concen-
a ions.
A ema kable ea u e o he SACW a e he ela i ely low dissol ed oxygen
alues, which espond o he high p ima y p oduc ion in he su ace laye s (a e
upwelling in coas al wa e s and wi hin he GD) combined wi h he long eci -
cula ion imes in his eas e n basin, p ecisely a ound he GD. This is sha p clea
in he e ical p o iles o hose s a ions wi hin (E41) and sou h (E54 and E70) he
CVFZ, whe e we ind ha SACW a e cha ac e ized by a minimum in oxygen be-
ween abou 100 and 500 m. A hese dep hs nu ien s a e high, al hough hei
maximum co esponds o in e media e wa e s (see below). An in e es ing ea u e
is appa en in s a ion E54, loca ed o e he slope along ansec Sou h. Down o
74
Chap e 3. Me idional changes in wa e p ope ies
abou 400 m his s a ion shows ela i ely high dissol ed oxygen concen a ions
and i is subs an ially less saline han he su ounding SACW, sugges ing a less
dilu ed sou he n a ie y. We will e u n o his issue in he ollowing sec ions.
Below he cen al s a um we ind he in e media e laye s, he e again o med
by wa e s o sou he n (AAIW) and no he n (MW) o igin. In gene al, hose s a-
ions nea he S ai o Gib al a show a p onounced salini y maximum be ween
abou 1000 and 1500 m, associa ed o he p esence o MW (Fig. 3.2a,d). The
high salini y, low nu ien alues obse ed 200 km o sho e in he no he n sec-
ion (Figs. 3.2d and 3.3d) a e indica i e o a p e e en ial o sho e pa h o MW.
The ansi ion in his s a um appea s o be mo e p og essi e han in he o e lay-
ing cen al wa e s, al hough he e a e ins ances o isola ed highly-saline Medi e -
anean lenses in he no he n end o he domain. Such an ins ance was ound in
one o he o sho e s a ions du ing he CANOA07 c uise, jus sou hwes o sec-
ion No h (no shown). The 1200 m salini y maximum (S = 36.5) in s a ion E5 is
he clea signa u e o MW in he no he n s a ions (Fig. 3.4), ye no app oaching
he S = 38.4 Medi e anean ou low salini y alues.
As we p og ess sou h, he in e media e salini y maximum ge s e oded and
ins ead he AAIW cha ac e is ics domina e. As in he cen al s a um, hose wa-
e s o sou he n o igin a e ela i ely esh, cool, and nu ien /oxygen ich/poo
as compa ed wi h hose o no he n o igin. This again e lec s he ela i ely long
esidence ime o he sou he n wa e s combined wi h he in luence o he wa m
and sal y Medi e anean ou low. AAIW appea s o p opaga e no h close o
he slope, eaching a leas un il he Cana y Islands, and u he n h in some
ins ances (Machín and Peleg í, 2009; Machín e al., 2010). The low oxygen and
high nu ien alues in he sou he nmos s a ions a e likely a esul o he long
75

Chap e 3. Me idional changes in wa e p ope ies
pa h o hese wa e s be o e eaching he eas e n No h A lan ic sub opical gy e.
Howe e , i is ema kable ha he absolu e oxygen minimum occu s wi hin cen-
al wa e s (200 o 400 m) while he absolu e nu ien maximum is ound much
deepe (800 o 1200 m). The di e en e ical ex ension o he low-oxygen and
high-nu ien laye s is an imp in o he high oxygen concen a ion o his wa e s
a o igin, i.e. despi e hei long ci cula ion ime and in ense emine aliza ion hey
e ain ela i ely high oxygen alues.
Wi hin he deepe laye s we ind NADW a all la i udes, wi h me idional
changes subs an ially smalle han hose wi hin he in e media e and uppe lay-
e s. Ne e heless, Figs. 3.2a and 3.3a sugges ha he Cana y Islands beha e
as an obs acle o he p opaga ion o hese deep wa e s, as he e a e signi ican
changes in salini y, nu ien s and oxygen a dep h ac oss he a chipielago.
3.4 Wa e ypes and op imum mul ipa ame e analy-
sis
Op imum mul ipa ame e (OMP) analysis is a ool o analyse he wa e mass
mix u e in a wa e sample. The me hod calcula es he con ibu ions om he
o iginal wa e masses, called sou ce wa e masses o wa e ypes, o he wa e
sample. The wa e ype con ibu ions o each da a poin a e ob ained by inding
he bes linea mixing combina ion in a mul i-pa ame e space (e.g. empe a u e,
salini y, oxygen and ino ganic nu ien s) ha leads o he obse ed alues. This
is done by minimizing he esiduals be ween p edic ions and obse a ions, in a
non-nega i e leas -squa es sense (Mackas e al., 1987; Tomczak and La ge, 1989).
76
Chap e 3. Me idional changes in wa e p ope ies
The solu ion o he OMP analysis includes wo physical cons ain s: he con i-
bu ions om all sou ces add up o one (mass conse a ion), and all con ibu ions
mus be non-nega i e.
In his wo k we use empe a u e (T), salini y (S), phospha e (PO4), silica e
(SiO4) and oxygen (O2) o esol e he ollowing linea sys em o mixing equa-
ions:
∑
i
xiθi=θobs +Rθ(3.1)
∑
i
xiSi=Sobs +RS(3.2)
∑
i
xi(PO4)i= (PO4)obs +RPO4(3.3)
∑
i
xi(SiO4)i= (SiO4)obs +RSiO4(3.4)
∑
i
xi(O2)i= (O2)obs +RO2(3.5)
∑
i
xi=1 (3.6)
whe e θi,Si,(PO4)i,(SiO4)i,(O2)ia e he alues o each sou ce wa e mass and
θobs,Sobs,(PO4)obs,(SiO4)obs,(O2)obs a e he obse ed alues, he las equa ion be-
ing o mass conse a ion.
Be o e esol ing he sys em, he wa e ype ma ix is no malized o commen-
su a e he di e en a iables, and di e en weigh s a e applied o each a iable.
The weigh s a e calcula ed ollowing Tomczak and La ge (1989),
Wj=σ2
j
δjmax
(3.7)
77
Chap e 3. Me idional changes in wa e p ope ies
Table 3.1: Sou ce wa e mass alues used in he OMP analysis o po en ial em-
pe a u e θ(◦C), salini y S, phospha e PO4(µmol/l), Silica e SiO4(µmol/l), and
dissol ed oxygen O2(ml/l)
θS PO4SiO4O2
NACWU18.65 36.76 0.25 0.36 4.79
NACWL11.00 35.47 1.05 5.65 4.26
SACWU15.25 35.70 1.41 6.92 1.51
SACWL9.70 35.18 1.94 14.08 1.64
SACW∗12.08 35.27 1.62 9.31 1.21
AAIW 6.50 34.90 2.02 22.55 2.73
MW 11.74 36.50 0.67 7.20 4.42
NADW 2.50 34.94 1.40 34.80 5.71
Weigh 1 0.86 0.21 0.14 0.15
whe e σjis he s anda d de ia ion o he wa e ype ma ix o a iable j, a mea-
su e o he abili y o a iable j o esol e di e ences in wa e mass con en ; and
δjmax is a measu e o he en i onmen al a iabili y o he a iable j ha cha ac-
e izes he wa e ype, he e es ima ed as he la ges a iance in a iable j o he
de ini ion o any wa e ype. Weigh s a e hen no malized o empe a u e, i.e. a
weigh o one is gi en o empe a u e and alues less han one o he o he a i-
ables, and a weigh o 10 is assigned o he las equa ion in o de o emphasize
mass conse a ion (Table 3.1).
An essen ial componen o OMP is he de ini ion o he wa e ype ma ix.
We ollow he app oach o de ining alues in he icini y o he s udy egion
a he han a he emo e a eas o wa e mass o ma ion. In his way we min-
imize emine aliza ion e ec s on ino ganic nu ien s and dissol ed oxygen, so
ha we may assume ha phospha es, silica es and oxygen a e app oxima ely in-
dependen and conse a i e. The alues used a e gi en in Table 3.1. The OMP
78
Chap e 3. Me idional changes in wa e p ope ies
analysis is applied o da a in he densi y ange 26.46 <σθ<27.82. Da a wi h
densi y less han he uppe limi ha e been excluded as hei p ope ies may be
al e ed by a mosphe ic and biogeochemical p ocesses.
Six sou ce wa e masses may be disce ned in he s udy a ea. The uppe pa
o he wa e column, down o σθ= 27.14, is domina ed by he cen al wa e masses
o no he n (NACW) and sou he n o igin (SACW). He e we ha e used hose θ-
S cha ac e is ics de ined by Tomczak (1981) using hyd og aphic da a om he
egion 20◦ o 26◦N. Due o hei o ma ion p ocess, he θ-S ela ionship o cen-
al wa e s is de ined by a s aigh line (Mamaye , 1975). The e o e, in o de o
cha ac e ize a cen al wa e mass in he densi y ange 26.46 −27.14 we equi e
wo sou ce wa e ypes, i.e. o each wa e mass we need uppe and lowe end
membe s (NACWU, NACWL, SACWU, SACWL).
An addi ional wa e ype o sou he n o igin is de ec ed in he sou he nmos
pa o he s udy egion, on s a ions along he con inen al slope abo e 200 m
dep h, in wa e s ha ing a ela i e salini y and empe a u e minimum and oxygen
maximum. This a ie y, he e named SACW∗, has been p e iously iden i ied as
a egional SACW a ie y om he opical egion (F aga and Man íquez, 1974;
Voi u iez and Chuchla, 1978; Man iquez and F aga, 1982). The θ-S cha ac e is ics
o SACW∗ha e been de ined using hose sou he n s a ions ha display a salini y
minimum in he cen al wa e mass laye .
Two wa e masses a e ound a in e media e laye s: MW and AAIW. The θ-
S alues o MW ha e been de i ed om he WOCE hyd og aphic clima ology
(Gou e ski and Kol e mann, 2004), by sea ching he empe a u e and salini y al-
ues which co espond o he salini y maximum in an in e media e laye wi hin a
egion bounded by la i udes 12 and 49◦N and om he coas o 1500 km o sho e.
79
Chap e 3. Me idional changes in wa e p ope ies
leas as a no h as 20◦N in summe and weakening and only eaching 17◦N
du ing win e (Pe e son and S amma, 1991; Siedle e al., 1992; Láza o e al.,
2005). He e we ha e used he dilu ed egional a ie y, SACW, o de ine hose
wa e s sou h o he on (Tomczak, 1981, 1984; Klein and Tomczak, 1994). In his
manne he on shows up sha ply in he changes o wa e mass composi ion,
om no he n o sou he n cha ac e is ics. I we had chosen his second a ie y,
SACW∗, o de ine he dominan wa e masses in his egion (e.g. Man iquez and
F aga, 1982) hen he on would ha e appea ed as a less ab up con as be ween
no he n and sou he n wa e pe cen ages.
Ou wo k has also shown he complexi y o he coas al ansi ion zone, pa -
icula ly o e he con inen al slope. Du ing ou measu emen s he e was signi -
ican coas al upwelling as a sou h as 16◦N bu he wa e mass dis ibu ion did
no show a sou hwa d pene a ion o NACW h ough a coas al upwelling je . In-
s ead, he CVFZ was mainly loca ed no h o an imagina y line be ween Cape
Blanc and he Cape Ve de Islands, pa icula ly along he con inen al slope whe e
i s e ched as a as 24◦N. This is likely he esul o he pene a ion o sou he n
wa e s as a esul o he PUC, which appea s o be an impo an phenomenon in
he a ea. Ac ually, i is possible ha he PUC accommoda es he SACW∗ a ie y
and ans e s i along he slope beyond he CVFZ.
Below 600 m and as deep as 1500 m we ind wo in e media e wa e masses,
again o no he n (MW) and sou he n (AAIW) o igin. The ansi ion be ween
MW and AAIW in he in e media e le els is much less ab up han in he cen-
al wa e laye and occu s no h o he Cana y Islands. In ou obse a ions
MW a e cen e ed nea 1200 m, sligh ly deepe han AAIW, which a e cen e ed
a dep hs o abou 1000 m. The obse ed la ge me idional pene a ion o AAIW
86

Chap e 3. Me idional changes in wa e p ope ies
akes place because ou c uises we e done in No embe : Machín and Peleg í
(2009) and Machín e al. (2010) ha e epo ed a s ong seasonal signal in he no h-
wa d pene a ion o AAIW, which su passes he Cana y A chipelago and inds
i s no he nmos ex ension du ing la e all. No h o he Cana y Islands he e a e
Medi e anean Eddies (Meddies, we ac ually ound one in he CANOA07 c uise),
which closely co espond o he undilu ed MW ype.
Below somewhe e be ween 1300 m, in he sou he nmos s a ions, and 1700 m,
in he no he nmos s a ions, we ind he 50% NADW con ou , which de ines he
p edominance o his deep wa e mass. This con ou deepens smoo hly wi h la i-
ude excep a he loca ion o he Cana y Islands whe e i suddenly plunges some
100 m, sugges ing ha he deep wa e s eaching bo h sides o he a chipelago
ha e ollowed signi ican ly di e en pa hs.
87
Chap e 4
Assessing physical d i e s o
in e annual chlo ophyll a iabili y∗
Abs ac
In e annual chlo ophyll a iabili y and i s d i ing mechanisms a e e alua ed in he
eas e n sub opical No h A lan ic, om 10 o 24◦N, and om he coas line o 40◦W.
Nine yea s o SeaWiFS da a a e compa ed wi h he ou pu om an ocean gene al ci -
cula ion model coupled o a s a e o he a biogeochemis y model. The model’s skill
a simula ing seasonal and in e annual chlo ophyll a iabili y in he s udy egion du ing
he SeaWiFS e a is es ablished. We hen assess he d i e s o chlo ophyll a iabili y in
he model du ing he las hal o he wen ie h cen u y by looking a changes in nu ien
supply. A weak posi i e co ela ion be ween chlo ophyll concen a ions and s a i ica ion
in he sunli laye ( op 80 m) is ound ( =0.13). To e alua e he d i ing mechanisms, a
nu ien budge is calcula ed in he s udy egion wi hin he eupho ic laye . We ind ha
∗in p epa a ion as M.V. Pas o , J.B. Pal e , J.L. Peleg í and J.P. Dunne, 2011. Beyond s a i ica-
ion: assessing physical d i e s o in e annual chlo ophyll a iabili y in he eas e n sub opical
No h A lan ic.
89
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
di usi e mixing o nu ien s o he eupho ic zone only explains 9% o he a iabili y in
chlo ophyll concen a ions, while ad ec i e nu ien luxes exe a mo e impo an con ol
on chlo ophyll a iabili y. In pa icula , e ical ad ec ion has he s onges co ela ion
and has also he la ges luxes and anomalies. Two p ocesses a e p ima ily esponsible
o he upwa d lux o nu ien s o he eupho ic laye ha sus ain a chlo ophyll esponse.
One is coas al upwelling, de e mined by he alongsho e componen o he wind s ess. I
occu s in a na ow me idional band (abou 100 km wide) adjacen o coas , bu u he
in luences he o sho e domain h ough ho izon al Ekman anspo and con e gence o
he me idional low. O sho e upwelling d i en by posi i e wind-s ess cu l du ing all
p o ides he second nu ien sou ce. The cu l-d i en e ical eloci ies a e smalle han
he coas al upwelling bu ex end o e a b oade a ea. We ind ha a iabili y in he
posi ion o he bounda y sepa a ing he upwelling and downwelling domains, mainly du -
ing win e mon hs, is c i ically impo an in se ing he o sho e ex ension o he high
chlo ophyll egion in he eas e n sub opical No h A lan ic.
4.1 In oduc ion
The eas e n sub opical No h A lan ic accommoda es one o he mos impo an
eas e n bounda y cu en ecosys ems in e ms o o al annual p ima y p oduc-
ion (Ca , 2002; Ca and Kea ns, 2003). Ele a ed p ima y p oduc ion sus ained
by nu ien - ich subsu ace wa e s upwelled in eas e n bounda y egions sup-
po high ish ca ches. Al oge he , he ou majo coas al upwelling ecosys ems
accoun o 20% o global ma ine ish ca ch despi e occupying abou 1% o he
global ocean (FAO, 2009). Unde s anding how p oduc i i y in hese a eas may
a y in a changing and a iable clima e has impo an ecological and socioeco-
90
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
a
40
30
20
10
-60 -40 -20
40
30
20
10
40
30
20
10
-60 -40 -20
-0.5 0 0.5
Chlo ophyll di e ence
(mg m )
-3
a
c
b
d
e
Sa elli e Model
win e - summe
Feb 2000 - Feb 1998(1959_1968) - (1977_1986)
Figu e 4.1: Maps illus a ing chlo ophyll (mg/m3) a iabili y a h ee imescales.
Seasonal a iabili y (a,b) is shown as win e minus summe chlo ophyll concen-
a ions. In e annual a iabili y (c,d) is shown as Feb ua y 2000 (Feb ua y wi h
maximum sa elli e a e aged chlo ophyll) minus Feb ua y 1998 (Feb ua y wi h
minimum sa elli e a e aged chlo ophyll). In e decadal a iabili y (e) is shown
as chlo ophyll a e aged du ing he decade 1959 o 1968 minus he a e age om
1977 o 1986. Le panels co espond o SeaWiFS da a, igh o model ou pu .
Da k g ey con ou shows he 0.2 mg/m3isoline o chlo ophyll o win e , Feb u-
a y 2000 and he pe iod 1959 o 1968. Ligh g ey con ou shows he same isoline
o summe , Feb ua y 1998 and he pe iod 1977 o 1986.
91

Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
nomic implica ions gi en he expec ed inc ease in ish demand (Delgado e al.,
2003).
The eas e n sub opical No h A lan ic is he mos spa ial and empo ally a i-
able o he ou majo eas e n bounda y cu en ecosys ems (Ca , 2002). The
basin’s eas e n limb shows one o he s onges zonal g adien s o chlo ophyll
in he wo ld’s ocean (e.g. Fig. 2 in McClain, 2009). The la ge-scale wind-s ess
ield d i es downwelling in he sub opical gy e, which esul s in a deep pycn-
ocline and nu icline, and consequen low su ace chlo ophyll (<0.07 mg/m3).
Along he basin’s eas e n ma gin, he wind o cing causes di e gence o he ho -
izon al ocean cu en s as a esul o bo h Ekman anspo along he coas and
o sho e cu l-d i en Ekman pumping (McClain and Fi es one, 1993). The up-
wa d eloci ies induced by his coas al Ekman anspo and Ekman pumping
b ing nu ien - ich wa e s o he eupho ic laye , h ough a combina ion o along-
isopycnal and diapycnal ans e , sus aining high chlo ophyll concen a ions a
he basin’s eas e n ma gin (Peleg í e al., 2006). In a wa ming en i onmen , con-
di ions in he sub opical gy e and in he coas al upwelling egion may e ol e
di e en ly. Coas al upwelling a es ha e been p ojec ed o inc ease due o an
inc eased land-sea p essu e g adien (Bakun, 1990; Bakun e al., 2010). On he
o he hand, inc eased s a i ica ion in oligo ophic gy es (a possible consequence
o wa ming) is hough o educe upwa d mixing o nu ien s and dec ease pho-
osyn hesis (McClain e al., 2004; G egg e al., 2005; Beh en eld e al., 2006; Polo -
ina e al., 2008). Howe e , he link be ween declines in su ace chlo ophyll and
inc eased s a i ica ion has mos o en been in e ed om co ela ions be ween
chlo ophyll and sea su ace empe a u es, lea ing he unde lying physical mech-
anisms unexplo ed. Fu he mo e, a deba e exis s abou he ecen ly obse ed
92
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
a iabili y being a clima e-change induced end (Polo ina e al., 2008; I win and
Oli e , 2009) o pa o a mul idecadal oscilla ion (Ma inez e al., 2009). A ecen
s udy o sa elli e ocean colo da a and ou pu om h ee biogeochemical mod-
els sugges ed dis inguishing be ween clima e-change d i en ends and na u al
a iabili y will equi e abou 40 yea s o con inuous sa elli e chlo ophyll mea-
su emen s (Henson e al., 2010).
Su ace chlo ophyll in he eas e n sub opical No h A lan ic has a ma ked
seasonal cycle; ye , in e annual a iabili y can be as la ge o la ge han he sea-
sonal changes (Fig. 4.1). In his pape , we use 49 yea s o ou pu om an ocean
gene al ci cula ion model, coupled o a s a e o he a biogeochemis y model, o
explo e he physical mechanisms ha d i e in e annual chlo ophyll a iabili y
in his egion. The use o models allows in-dep h s udy o se e al hypo hesized
physical d i e s o chlo ophyll a iabili y such as changes in s a i ica ion and
he la ge scale wind ield. The model’s skill a simula ing chlo ophyll spa ial and
empo al a iabili y in he s udy egion is i s es ed by compa ing i s ou pu
o nine yea s o Sea- iewing Wide Field-o -View Senso (SeaWiFS) da a (sec ion
4.3) and sa elli e measu emen s o sea su ace heigh (sec ion 4.4). We examine
links be ween chlo ophyll and sea su ace heigh o explo e he p emise ha sea
su ace heigh gi es an insigh o possible mechanisms explaining chlo ophyll
a iabili y. Finally, we asses he d i e s o chlo ophyll a iabili y in he model
ou pu du ing he las hal o he wen ie h cen u y. Speci ically, we es a ious
hypo hesis in sec ion 4.5 h ough he assessmen o each e m in a nu ien bud-
ge . The conclusions a e p esen ed in sec ion 5, including a discussion abou he
ole o sub opical gy e bounda y shi s.
93
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
4.2 Sa elli e da a, model ou pu and me hods
We use Le el 3 SeaWiFS mon hly chlo ophyll downloaded om oceancolo .
gs c.nasa.go a 9 km esolu ion o he pe iod No embe 1997 o Decembe
2007. Absolu e dynamic opog aphy (SSH), p oduced by Ssal o/Duacs and dis-
ibu ed by A iso, wi h suppo om Cnes, is downloaded om h p://www.
a iso.oceanobs.com/duacs/. I co esponds o me ged da a om Topex/ Posei-
don and ERS sa elli es. We a e aged he o iginal weekly da a on o mon hly, hen
smoo hed he 1/3◦spa ial esolu ion wi h a 3x3 (1◦x 1◦) boxca il e o educe
mesoscale ea u es.
The gene al ocean ci cula ion model used is Ve sion 4 o he Geophysical Fluid
Dynamics Labo a o y’s Modula Ocean Model (MOM4) (G i ies e al., 2008),
o ced wi h he Common Ocean-Ice Re e ence Expe imen (CORE) da a se (G i -
ies e al., 2009; La ge and Yeage , 2009). We use e sion 2 o he CORE eanalysis
e o , which includes six-hou ly in e annual a ying me eo ological ields o
he pe iod 1958 - 2006 (10 m ai empe a u e, humidi y, ai densi y, zonal wind,
me idional wind, and sea le el p essu e). Daily a ying sho wa e and long-
wa e adia i e luxes a e a ailable om 1983 and mon hly a ying p ecipi a ion
since 1979. P io o hose yea s, da a a e illed wi h he clima ological annual cy-
cle. Con inen al uno is a ailable as a clima ological annual mean. The ocean
model has i y le els in he e ical di ec ion, a longi udinal esolu ion o 1◦and
a la i udinal esolu ion a ying be ween 1◦in he ex a opics and 1/3◦on he
equa o . This simula ion uses he Boussinesq app oxima ion and he e o e s e ic
e ec s do no in luence he sea le el heigh . S e ic e ec s include he expansion o
con ac ion o he wa e column due o empe a u e o salini y changes. MOM4
94
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
has he abili y o espond o a mosphe ic loading. Howe e , o a ious easons,
mos coupled models do no ac ually apply he sea le el p essu e (SLP) on he
ocean. One eason is ha in e pola ing SLP o he ocean can be augh wi h e -
o , especially nex o land-sea bounda ies whe e land has high ele a ion. The e-
o e, he model does no conside a ia ions in SLP, so ha all changes in SSH
espond o he ocean in e nal dynamics. The simula ion was ini ialized om hy-
d og aphic mean p ope ies aken om he Wo ld Ocean A las 2001 (Conk igh
e al., 2002). I was spun-up o 348 yea s wi h o cing om a clima ological yea
calcula ed using he mean CORE da a du ing he yea s 1958-1977 be o e he inal
loop wi h he 49 yea s o CORE in e annual a iabili y was in eg a ed.
The biogeochemical componen is gi en by he model T ace s o Ocean Phy-
oplank on wi h Allome ic Zooplank on (TOPAZ), which simula es p ognos i-
cally all majo nu ien elemen s (N, P, Si and Fe). The ecosys em is based in
h ee classes o phy oplank on. The small class domina es when g ow h a es a e
low; his size class esis s sinking. La ge phy oplank on ep esen dia oms and
o he phy oplank on ha bloom and sink quickly. Finally, diazo ophs ix ni o-
gen di ec ly. Phy oplank on g ow h a es a e modeled as a unc ion o a iable
chlo ophyll o ca bon a ios and colimi ed by nu ien s and ligh . A mo e de ailed
desc ip ion can be ound in Appendix C.
We use he dis ance om he coas o he 0.2 mg/m3isoline o chlo ophyll
(DCHL) o cha ac e ize he size o he high chlo ophyll egion be ween he olig-
o ophic sub opical gy e and he A ican coas . La ge dis ances indica e a la ge
a ea wi h chlo ophyll highe han 0.2 mg/m3. S udies on chlo ophyll a iabil-
i y in coas al upwelling sys ems gene ally ha e de ined he a eas o high p o-
duc i i y as hose ha ing >1 mg/m3(e.g. Nixon and Thomas, 2001; Ca , 2002;
95
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
2006), and using ae osol op ical dep h as a p oxy o dus ( om SeaWiFS da a),
we ound ha 1998 was he dus ies yea du ing he SeaWiFS 1997 - 2007 pe iod
(no shown). In he model, we and d y dus deposi ion luxes a e p esc ibed
om he mon hly clima ology o Ginoux e al. (2001) and hus no in e annual
a iabili y exis s. This is an ad an age o he pu pose o his s udy, as i allows
us o ocus on chlo ophyll a iabili y caused by ocean dynamics.
E en excluding he e y high posi i e anomaly men ioned abo e, he wid h
o he high chlo ophyll egion a ies in e annually by abou 500 km o sa elli e
and 600 km in he model ou pu , compa able o he seasonal cycle o 780 km.
Quali a i ely, he anomalies show simila pa e ns; he high chl egion is la ges
om 1999 o 2004 and dec eases om 2005 o 2008. The anomalies o he high
chlo ophyll egion a e aged om 10 o 24◦N in he model and SeaWiFS a e sig-
ni ican ly co ela ed beyond he 1% le el (Fig. 4.3). The sh inking o he high
chlo ophyll egion in he la e yea s has been in e p e ed as a possible global
wa ming ela ed end and co ela ed wi h a end in sea su ace empe a u e
(Polo ina e al., 2008). In he nex sec ion we use he model o place he a iabili y
o e he SeaWiFS e a in he con ex o se e al decades and asses he mechanisms
d i ing his empo al a iabili y.
4.4 Sea su ace heigh and chlo ophyll
Sa elli e measu emen s p o ide global SSH da a a ailable du ing he SeaWiFS
decade. We explo e he ela ion be ween SSH and chlo ophyll, as p ocesses ha
may in luence su ace chlo ophyll a e likely e lec ed in SSH. We assume as a i s
app oach ha SSH a iabili y e lec s su ace nu ien a ailabili y in he egion,
102

Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
98 99 00 01 02 03 04 05 06 07
10
12
14
16
18
20
22
24
−1500
−1000
−500
0
500
1000
1500
600
400
200
0
-200
-400
-600
98 99 00 01 02 03 04 05 06 07
10
12
14
16
18
20
22
24
−1500
−1000
−500
0
500
1000
1500
98 99 00 01 02 03 04 05 06 07
10
12
14
16
18
20
22
24
−1500
−1000
−500
0
500
1000
1500
D (km)
CHL D (km)
SSH
Figu e 4.4: Ho mölle plo s o mon hly anomalies o sa elli e-de i ed dis ance
om he coas o (le ) he 0.2 mg/m3isoline o chlo ophyll and ( igh ) he 37 cm
SSH isoline, o e he SeaWiFS da a pe iod (No embe 1997 o Decembe 2007).
because o he in e se ela ionship be ween SSH and he mocline dep h (S am-
me , 1997; Maye e al., 2001), and he common coincidence o he he mocline
dep h and nu icline dep h (Wilson and Coles, 2005; Signo ini e al., 1999). The
empo al co ela ion o chlo ophyll and SSH anomalies in bo h he model and
da a subs an ia e his assump ion (Fig. 4.2, bo om): when SSH is dep essed,
chlo ophyll is high. Likewise, dis ances o he chlo ophyll and SSH isolines a e
posi i ely co ela ed (Fig. 4.4 and 4.5). A la ge dis ance o he SSH isoline e-
lec s lowe SSH in he eas , which is expec ed o be linked o a shallowe he -
mocline and a shallowe nu icline. Thus, when he dis ance o he SSH isoline is
la ge we expec an inc ease in chlo ophyll, and he e o e a la ge dis ance o he
0.2 mg/m3chlo ophyll isoline. Using SSH as a p oxy o he mocline dep h es-
sen ially assumes ha , o la ge scales, he su ace ocean esponds o i s in e nal
ba oclinici y. This is equi alen o saying ha , o he conside ed egion, wa-
e mass is in balance. I he balance is di e gen (con e gen ) he whole egion
103
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 00 02 04 06
10
12
14
16
18
20
22
24
−1200
−600
0
600
1200
58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 00 02 04 06
10
12
14
16
18
20
22
24
−1500
−1000
−500
0
500
1000
1500
D (km)
CHL
D (km)
SSH
Figu e 4.5: Ho mölle plo s o mon hly anomalies o model-de i ed dis ance
om he coas o ( op) he 0.2 mg/m3isoline o chlo ophyll and (bo om) he -18
cm SSH isoline, o e he simula ed pe iod (1958 o 2006).
104
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
would ha e a mean dec ease (inc ease) in SSH.
In line wi h hese expec a ions, in e annual a iabili y in he wid h o he
high chlo ophyll egion and he low SSH egion show a ema kable ag eemen in
he sa elli e obse a ions o he pe iod No embe 1998 o Decembe 2007 (Fig.
4.4). Bo h indices show nega i e anomalies (SSH and chlo ophyll isolines close
o coas ) in he i s yea o he ime se ies and posi i e anomalies (enla gemen
o he high chlo ophyll and low SSH egion) in he ollowing yea s, especially
o SSH. Nega i e anomalies a e la ge in no he n la i udes and p og ess sou h-
wa ds in he second hal o he ime se ies. The shi in he bounda y be ween
high and low chlo ophyll wa e s is mi o ed by a simila shi in he bounda y
be ween low and high SSH. The sub le shi in his bounda y o e he SeaWiFS
e a is ela ed o he expansion o he oligo ophic gy e (Polo ina e al., 2008).
Placing he a iabili y in he con ex o i y yea s o eanalysis- o ced model
simula ion, he ecen end appea s o be embedded in a mul idecadal oscilla ion
(Fig. 4.5). As wi h sa elli e obse a ions, he e is a s ong co ela ion be ween
modeled chlo ophyll and SSH dis ances ( =0.73, p<<0.01). Sou h o 20◦N, he
high chlo ophyll egion is highes om 1959 o 1976, coinciden wi h posi i e
anomalies o he dis ance o he -18 cm isoline o SSH. The chlo ophyll and SSH
ime se ies a e hen ollowed by 23 yea s o consis en nega i e anomalies. F om
1999 o 2004, chlo ophyll and SSH show less ag eemen (Fig. 4.5). In hese yea s,
he wid h o he low SSH egion is 1000 km abo e he mean be ween 13 and
18◦N, while he high chlo ophyll egion emains es ic ed o he coas . Low
SSH is linked o he shoaling o he nu icline, bu ha ela ionship is weakes
in he no he n pa o ou domain. In 1999-2004 he anomalously la ge egion
o dep essed SSH (high SSH dis ance) lies no hwa d o i s posi ion in he 1959-
105
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
1964 pe iod (Fig. 4.5). Because he co ela ion be ween nu icline dep h and SSH
is weake in he no he n pa o he domain, he anomalously high SSH dis ances
in he la e pe iod do no co espond wi h an equally s ong esponse in nu ien
supply o he eupho ic zone. Thus, he o sho e ex ension o he high chlo ophyll
egion esponds mo e weakly o he enla gemen o he no he n po ion o he
low SSH egion han an enla gemen o he sou he n egion.
Two physical p ocesses go e n he ime e olu ion o sea su ace heigh in
MOM4. One is con e gence o di e gence o ho izon al cu en s, caused p ima -
ily by changes in he wind ield. The second one is mass luxes h ough he su -
ace such as p ecipi a ion, e apo a ion, i e uno and ice mel . Again, no s e ic
e ec s a e included as he model conse es olume a he han mass. The e o e,
he modeled changes in SSH espond p edominan ly o a iabili y in he con-
e gence o di e gence o he ho izon al eloci ies, as he p ecipi a ion minus
e apo a ion and i e un-o e m has a low impac in he s udy a ea.
4.5 Mechanisms o chlo ophyll a iabili y
The size o he high chlo ophyll egion, as de ined by he dis ance be ween he
coas and he 0.2 mg/m3chlo ophyll isoline, is ela ed o he chlo ophyll con-
cen a ion in he s udy egion enclosed by he box in Fig. 4.2. In bo h model
and da a, he highe he chlo ophyll concen a ion in he egion, he g ea e he
dis ance o he isoline ( =0.97 o model ou pu , =0.73 o sa elli e da a, bo h
p<<0.001). In he model, chlo ophyll concen a ions depend bo h on phy o-
plank on concen a ions and on he chlo ophyll o Ca bon a io (Chl:C) in hei
cells. Chl:C is a unc ion o i adiance, i on (Fe) and g ow h a e in he model.
106
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
In he s udy egion, he seasonal a iabili y o Chl:C is mos ly go e ned by Fe
inpu and g ow h (maximum alues in win e , minimum in summe ). Chl:C has
also signi ican in e annual a iabili y which is posi i ely co ela ed o chlo o-
phyll concen a ions ( =0.90). Thus, an inc ease in chlo ophyll may indica e an
inc ease in phy oplank on biomass and/o a highe Chl:C. TOPAZ uses he Gei-
de e al. (1997) pho oacclima ion model which adjus s Chl:C o ambien condi-
ions. When g owing condi ions a e op imal, bo h g ow h a es (mo e biomass)
and Chl:C (mo e chlo ophyll) inc ease. Consequen ly, biomass and Chl:C a e
igh ly ela ed. He ea e we will s udy changes in ca bon biomass which do no
depend on Chl:C o exclude any a iabili y caused by changes in cellula Chl:C.
G ow h a es and biomass a e modeled as a unc ion o i adiance, nu ien
a ailabili y and empe a u e. In ou s udy egion, i adiance and empe a u e
a e no dominan con ols on biomass. P io o 1983, he model i adiance is p e-
sc ibed as a clima ological annual cycle. F om 1983 onwa ds, he model uses a i-
abili y o incoming adia ion, bu we ind no co ela ion o biomass. An inc ease
in empe a u e would inc ease g ow h a es. In ou s udy egion, he a e age
empe a u e in he op 80 m is nega i ely co ela ed o biomass; his co espon-
dence is likely due o he ela ion be ween empe a u e and upwelling. The e o e,
in es iga ing he di e en pa hs o nu ien supply in o his egion and hei el-
a i e impo ance will help elucida ing mechanisms ha cause biomass changes
and a iabili y in he size o he high chlo ophyll egion.
107

Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
4.5.1 Nu ien supply
In o de o add ess in e annual a iabili y in nu ien supply, we conside each
e m in he PO4budge (Eq. 4.3) o he s udy egion ou lined in Fig. 4.2, abo e
he a e age dep h o he 1 Wa ligh laye (80 m). S udying hese e ms p o ides
a mechanis ic iew o he physical con ols on phy oplank on biomass. Because
a numbe o p e ious s udies ha e linked s a i ica ion a iabili y wi h chlo o-
phyll and biomass a iabili y (McClain e al., 2004; G egg e al., 2005; Beh en eld
e al., 2006; Polo ina e al., 2008), we also compa e ou biomass ime se ies wi h
a common measu e o s a i ica ion, he densi y di e ence be ween 200 m and
he su ace. Howe e , his measu e o s a i ica ion gi es no aluable in o ma-
ion in ou egion, as densi y a ia ions a 200 m a e small, and he me ic hus
e lec s only densi y a iabili y a he su ace, p edominan ly d i en by empe -
a u e changes. Highe empe a u es a he su ace a e indeed co ela ed wi h
lowe biomass, bu his gi es li le indica ion o he physical mechanisms espon-
sible o he dec ease. The densi y di e ence be ween he su ace and he base
o he eupho ic zone (80 m) may be a mo e sui able measu e o he s a i ica ion
impac ing he sunli laye . The densi y di e ence be ween he su ace and 80
m is ac ually posi i ely co ela ed wi h biomass ( =0.18, p<<0.001) and su ace
chlo ophyll concen a ions ( =0.13, p<0.01), opposi e o s a i ica ion exe ing a
leading con ol on he nu ien supply. Ins ead, his sligh posi i e co ela ion
sugges s a dis inc mechanism con olling phy oplank on a iabili y ha is no
supp essed unde inc easing s a i ica ion, which we nex sea ch o among all
he nu ien supply e ms in he egion.
The PO4budge equa ion (Eq. 4.3) includes la e al ad ec ion, e ical ad ec-
108
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
-5
0
5
-1000
0
1000
-5
0
5
-5
0
5
58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 00 02 04 06
-5
0
5
D (km)
∂ zwPO x 10
4
(mol m )
-2 s-1
∂ y PO x 10
4
(mol m )
-2 s-1
mixing x 10
(mol m )
-2 s-1
a
b
c
d
10 10 10
CHL
biomass
Biomass
(mg C m )
-3
Figu e 4.6: Time se ies o mon hly anomalies a e aged in he s udy egion (la i-
udes 10 o 24◦N, om 40◦W o he coas , op 80 m dep h). (a) Dis ance o he 0.2
mg/m3chlo ophyll isoline, and phy oplank on biomass as he black line, (b) e -
ical PO4con e gence, (c) me idional PO4con e gence and (d) PO4inpu due o
mixing. Anomalies o zonal PO4con e gence a e no shown as hei magni ude
is no no able as compa ed o me idional and e ical con e gence anomalies.
109
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
Figu e 4.7: Co ela ion be ween (le ) e ical PO4ad ec ion, wPO4, and e i-
cal PO4g adien , ∂zPO4, and ( igh ) e ical PO4ad ec ion, wPO4, and e ical
eloci y, w.
ion and mixing. Anomalies o di usi e mixing o PO4only play a ela i ely im-
po an ole in he mon hs o Feb ua y and Ma ch (Fig. 4.6d), when con ec ion
and e ical di usion injec PO4in o he no he n pa o his egion. On an in e -
annual imescale, anomalies o PO4di usi e mixing explain less han 4% o he
biomass a iabili y (o 6% o he changes in he dis ance o he 0.2 mg/m3chlo o-
phyll isoline). Con e sely, ad ec ion o PO4seems o be a key ac o (co ela ion
o 0.86 wi h biomass). Amongs e ical, me idional and zonal PO4con e gence,
we ind ha in e annual a iabili y o biomass is highes co ela ed o e ical
con e gence ( =0.82), al hough co ela ion o me idional con e gence is also high
( =0.73). In e annual a iabili y o he e ical PO4 luxes a e caused by changes
in e ical eloci ies a he han changes in he e ical PO4g adien , calcula ed
as he PO4di e ence be ween 75 m and 85 m (Fig. 4.7). Mos o he me id-
110
Chap e 4. Physical d i e s o in e annual chlo ophyll a iabili y
-1
-0.5
0
0.5
1
wPO
4uPO
4 PO
4
58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 00 02 04 06
-6
-4
-2
0
2
4
6
T anspo (S )
V
coas Vcu l
(mol m )
-2 s-1
ad ec ion *109
∂ z∂ x∂ y
b
a
Figu e 4.8: Time se ies o mon hly means a e aged in he s udy egion (la i udes
10 o 24◦N, om 40◦W o he coas , op 80 m dep h). (a) Ad ec i e e ms in
he PO4budge ( e ical ∂zwPO4, zonal ∂xuPO4, me idional ∂y PO4), and (b)
wa e olume upwelled due o o sho e cu l-d i en upwelling (Vcu l) and coas al
Ekman anspo (Vcoas ).
ional anspo o PO4in o he s udy egion akes place h ough he sou he n
bo de (95% on a yea ly a e age). This is consis en wi h sou he n wa e s ha ing
highe nu ien concen a ions. Ne e heless, he magni ude o bo h he mon hly
means and he anomalies o e ical con e gence a e much la ge han hose o
he me idional componen (Fig. 4.6b and c, Fig. 4.8a), sugges ing ha upwa d
ad ec i e luxes a e he mos impo an supplie o nu ien s in o his egion and
he dominan con ol on phy oplank on biomass a iabili y and he size o he
high chlo ophyll egion.
111

Chap e 5
Conclusions and ou look
5.1 Conclusions
How e ec i e is he Cape Ve de on as a ba ie be ween he wo cen al
wa e masses?
Cen al wa e s masses om no he n and sou he n o igin ha e di e en phy-
sico-chemical p ope ies. NACW is wa me , mo e saline, and has less nu ien s
and mo e dissol ed oxygen, as compa ed o SACW. The Cape Ve de on ep-
esen s he mee ing o hese wo wa e masses in he eas e n sub opical No h
A lan ic. The on has been desc ibed as a ba ie be ween he wo wa e masses
whe e epipycnal and less equen ly diapycnal mixing occu s (Tomczak, 1981;
Zenk e al., 1991). He e we p opose ha double di usi e mixing enhances ho -
izon al hea ans e esul ing in he smoo hing o empe a u e g adien s ac oss
he on , as compa ed o o he p ope ies such as sal , nu ien s o dissol ed
oxygen.
119
Chap e 5. Conclusions and ou look
How does he coas al upwelling on in e ac wi h he Cape Ve de on al
sys em?
In addi ion o he Cape Ve de on ha sepa a es cen al wa e s o no he n
and sou he n o igin, a second on exis s in he coas al egion be ween newly
upwelled wa e s, and he o sho e wa e s. We p opose ha he in e lea ing e-
quen ly ound in he Cape Ve de on may o igina e as mesoscale ea u es in he
coas al upwelling on , which hen con e ge o Cape Blanc and a e anspo ed
wes wa ds.
Do wa e mass and nu ien exchanges be ween he coas al upwelling egion
and he open ocean display seasonal a iabili y?
O sho e anspo along he NW A ican coas has i s maximum nea Cape
Blanc, a abou 21◦N. Con e gence o he sou hwa d Cana y Upwelling Cu en
and he no hwa d Mau i ania Cu en du ing summe and all is expec ed o
p oduce in ense o sho e expo . Du ing win e and sp ing he along-slope low
con e gence is expec ed o be subs an ially educed. The da a collec ed du ing
sp ing 1973 and all 1975 display la ge o sho e anspo s o Cape Blanc bu ,
con a y o expec ed, he maximum anspo is ound du ing sp ing in he a ea
sou h o Cape Blanc.
Does he posi ion o he Cape Ve de on a y in ime?
T adi ionally, he Cape Ve de on posi ion has been de ined by he loca ion
o he 36 isohaline a 150 m dep h. Using his de ini ion, he on is loca ed a
21◦N du ing sp ing 1973, a 22.5◦N du ing all 1975, and a 23.3◦N du ing all
2008. A clea e pic u e o he on is gi en by he wa e mass pe cen ages. Using
120
Chap e 5. Conclusions and ou look
he 50% isoline o wa e mass con en as he di ide be ween wa e s o no he n
and sou he o igin, he on is seen o shi no hwa ds wi h dep h in all 2008:
i s loca ion is 24◦N in he op 250 m, and 25◦N below. This il may be caused by
he sou hwa d NACW anspo associa ed wi h he Cana y Upwelling Cu en .
Wha is he o igin o he subsu ace salini y minimum o en ound in he
Sou h A lan ic Cen al Wa e domain?
The ela i e salini y minimum and oxygen maximum ound a abou 300 m
wi hin he SACW domain, and clea ly isible in θ- S diag ams, made us won-
de i his was he imp in o less dilu ed SACW o opical o igin. The dis inc-
ion o an addi ional cen al wa e s ype o sou he n o igin in he OMP analysis
has allowed o in e a p opaga ion pa h o he less saline, oxygen ich a ie y
(SACW*). This wa e mass could be ad ec ed no hwa ds by he Polewa d Un-
de cu en , while he adi ional SACW would ha e al eady expe ienced some
mixing wi h NACW.
How is he along-slope la i udinal dis ibu ion o cen al and in e media e
wa e masses o NW A ica?
The Cape Ve de on al sys em ep esen s a apid ansi ion be ween cen al
wa e s o sou he n and no he n o igin. The less dilu ed SACW a ie y does no
ex end no h om his on bu some o he locally dilu ed is ound no h o he
Cape Ve de on along he slope. A in e media e le els he ansi ion be ween
An a c ic and Medi e anean wa e s is much smoo he han in he cen al le els.
Does he s a e-o - he-a nume ical coupled MOM4/TOPAZ model p ope ly
ep oduce he obse ed dynamics in he eas e n sub opical No h A lan ic?
121
Chap e 5. Conclusions and ou look
The biogeochemis y model TOPAZ, coupled o he global ocean ci cula ion
model MOM4, has p o en o ep oduce he main dynamical ea u es and he
obse ed seasonal and in e annual chlo ophyll a iabili y in he s udy egion.
Model ou pu om a his o ical un o ced wi h 49 yea s o in e annually a ying
da a is hus analyzed o in es iga e chlo ophyll a iabili y and i s d i ing mech-
anisms in he eas e n sub opical No h A lan ic.
Wha is he dominan nu ien anspo amongs e ical ad ec ion, ho i-
zon al ad ec ion and di usi e mixing?
An assessmen o he nu ien budge in he eupho ic laye ( op 80 m) o
he s udy egion shows ha e ical nu ien ad ec ion is he main d i e o in-
e annual chlo ophyll a iabili y, al hough no hwa d anspo a he sou he n
bounda y is also conside able. Di usi e mixing only explains 9% o he chlo o-
phyll in e annual a iabili y.
Is he a iabili y in he e ical nu ien anspo o igina ed by changes in
he eloci y ield o in he nu ien g adien ?
In e annual a iabili y in he e ical nu ien anspo a 80 m is domina ed
by changes in eloci y, a he han changes in he nu ien ese oi .
Which p ocess plays a mo e decisi e ole in he upwa d nu ien anspo ,
coas al upwelling o o sho e upwelling?
Two main p ocesses b ing nu ien s o he eupho ic laye in he eas e n sub-
opical No h A lan ic. One is he coas al upwelling; he second, Ekman pump-
ing d i en by wind s ess cu l. We ha e shown ha o sho e e ical eloci ies e-
122
Chap e 5. Conclusions and ou look
la ed o wind s ess cu l domina e o e coas al eloci ies induced by along-sho e
winds in se ing he a ea o chlo ophyll concen a ion abo e 0.2 mg m-3.
Do shi s in he sou heas sub opical gy e bounda y de e mine he a ea o
high chlo ophyll?
The in ensi y o Ekman pumping in he eas e n sub opical No h A lan ic is
he leading con ol on se ing he in e annual ex ension o he high chlo ophyll
egion unde s udy. Wind-d i en shi s in he bounda y be ween he upwelling
( opical gy e) and downwelling (sub opical gy e) domains a e also a key ac o
ha de e mine he ex ension o he high chlo ophyll egion.
5.2 Fu u e esea ch
∗The his o ical da a analyzed in chap e 2 was digi alized om p in ed col-
lec ions. This da a will be made a ailable in a public da abase o he scien-
i ic communi y use.
∗We would like o use an idealized model o ep esen he ho izon al empe -
a u e smoo hing ac oss he Cape Ve de on . Fo his pu pose, we could use
an exponen ial equa ion (modi ied om Rod íguez-San ana, 1997) o model
he unequal p ope y dis ibu ions ac oss he on , and s udy i s e ec on
p ope y-p ope y diag ams. In pa icula , his p ocedu e should allow us
o es ima e he ela i e impo ance o isopycnal and diapycnal mixing lead-
ing o he wa e p ope ies in he on al egion.
123

Chap e 5. Conclusions and ou look
∗Op imum Mul ipa ame e analysis (OMP) has been used o in e mixing be-
ween wa e masses a he medium-scale o mo ion. The analysis assumes
iden ical exchange coe icien s o all p ope ies (e.g. empe a u e, salin-
i y, phospha e, dissol ed oxygen). Howe e , ou esul s sugges ha em-
pe a u e di uses as e han all o he pa ame e s in he Cape Ve de on al
egion. Fu u e esea ch should add ess he de elopmen o an OMP o mu-
la ion ha includes he e ec o di e en ial exchange coe icien s.
∗The OMP echnique es ablishes he con ibu ion o he di e en wa e ypes
om he dis ances be ween he da a poin s in p ope y-p ope y spaces,
implici ly assuming he e a e no di e ences be ween along-isopycnal o di-
apycnal mixing p ocesses. These dis ances could be decomposed in along-
isopycnal and diapycnal con ibu ions, wi h ela i e weigh s in acco dance
o he size o he co esponding di usi e coe icien s.
∗The nu ien con en in he su ace laye a ies depending on he sou ce
wa e masses upwelling a he coas . A high- esolu ion physical-biological
model would be use ul o in es iga e he impac o he a iabili y in he
Cape Ve de on posi ion on p ima y p oduc ion.
∗In a mo e ecen analysis, Peña Izquie do e al. (2011) ha e linked he wa e
mass analysis p esen ed in chap e 3 o he low ield o e he con inen al
slope. This app oach allows o iden i y a connec ion be ween he cen al
wa e masses, wi h SACW being he esul o NACW and SACW* mixing in
he Cape Ve de on al egion. Fu he mo e, he no hwa d p opaga ion o
SACW* is linked o he no hwa d b anch o he cyclonic ci cula ion a ound
he Guinea Dome. Due o he seasonally a iable na u e o he dome, i is
124
Chap e 5. Conclusions and ou look
expec ed ha anspo o SACW* oscilla es also du ing he yea . Fu u e
di ec ions should explo e he seasonal a iabili y o SACW* anspo and
i s connec ion wi h he Guinea Dome ci cula ion.
∗In chap e 4, model ou pu has been analyzed o unde s and chlo ophyll
a iabili y in he eas e n sub opical No h A lan ic. The same coupled
ocean ci cula ion-biogeochemical model, o a highe esolu ion model i
a ailable, could be used o in es iga e he Guinea Dome seasonal nu ien
eplenishmen as sugges ed by Peleg í e al. (2006).
∗The modeled chlo ophyll in e annual a iabili y in he eas e n sub opical
No h A lan ic could be examined in e ms o la ge-scale a iabili y in cli-
ma e modes. I would be impo an o u he explo e he link be ween
chlo ophyll a iabili y and a ious clima ic indices such as he No h A -
lan ic Oscilla ion o he A lan ic Me idional Mode.
∗We ha e seen ha he ex ension o he high-chlo ophyll a ea in he Cape
Blanc egion is ela ed o la ge-scale p ocesses, a ec ing he whole eas e n
sub opical gy e. We ha e iewed changes in his ex ension as a ising om
a ia ions o he dynamics in bo h sides o he Cape Ve de on al sys em,
speci ically in he along-slope con e gence, which is ela ed o he in ensi y
o coas al upwelling, and he o sho e su ace di e gence, o posi i e Ek-
man pumping. Bu hese wo egions a e no isola ed, hey a e connec ed
wi h ups eam wa e sou ces, i.e. he Azo es Cu en o he no he n e-
gion and he No h Equa o ial Coun e Cu en o he sou he n egion. I
emains o be s udied wha a e he la ge-scale mechanisms ha may con-
125
Chap e 5. Conclusions and ou look
ol he in ensi y o hese ups eam wa e sou ces and wha is hei e ec
o NW A ica.
126
Chap e 6
Resumen de la esis en español
A lo amien o en el ma gen o ien al del A lán ico
No e sub opical
6.1 In oducción y obje i os de la esis
6.1.1 In oducción
El ma gen o ien al del A lán ico No e sub opical es una egión de ele ado in-
e és po dos azones p incipales. P ime a, una acción signi ica i a de es e á ea
oceánica mues a una al a p oducción p ima ia que sos iene unos icos ecu sos
pesque os. Las pesque ías cons i uyen una uen e impo an e de alimen os pa a
el se humano y un sec o impo an e de la economía. La segunda azón es su
conexión con el clima global. En la egión o ien al del A lán ico No e sub opi-
cal, aguas subsu pe iciales en an en con ac o con la a mós e a e in e cambian
p opiedades como calo y dióxido de ca bono. En e ec o, las masas de agua e-
127
Chap e 6. Resumen en español
Figu e 6.3: Dig ama empe a u a-salinidad con los ipos de masas de agua cen-
ales, de las capas in e medias y p o undas en el Océano A lán ico, omado de
S e d up e al. (1942).
Masas de agua
La capa supe icial del A lán ico No es e sub opical (ap oximadamen e los p i-
me os 100 m) es á ca ac e izada po al as salinidades, una al a concen ación de
oxígeno disuel o y bajas concen aciones de nu ien es (F aga and Man íquez,
1974). Una ca ac e ís ica pa icula del gi o sub opical del A lán ico No e es el
máximo de salinidad subsupe icial (p.ej. Figu a 2.4 y 2.5). Aguas supe iciales
densas con salinidad al a, p oducidas po el exceso de e apo ación en e a p e-
cipi ación, subducen debido a la con e gencia po anspo e de Ekman y a la
con ección in e nal, y son anspo adas a a és de la ci culación a g an escala
del gi o sub opical (De an , 1936; Baue and Siedle , 1988).
Las masas de agua de la e moclina pe manen e son las masas de agua cen-
ales (S e d up e al., 1942). Es as aguas ienen su o igen en las egiones sub-
134

Chap e 6. Resumen en español
opicales de ambos hemis e ios, donde las aguas supe iciales con e gen y el
bombeo de Ekman es nega i o, y se ex ienden hacia las egiones ecua o iales. En
un diag ama Tempe a u a-Salinidad (TS) des acan po su elación ap oximada-
men e lineal (Figu a 6.3). Las aguas cen ales ocupan el ango de p o undidad
de ap oximadamen e 100 a 700 m. El Agua Cen al No a lán ica (NACW) iene
su o igen en la zona de con e gencia supe icial del A lán ico No e y alcanzan
la i udes meno es a a és de la ci culación e moclina (Sa mien o e al., 1982;
Kawase and Sa mien o, 1985). Únicamen e las masas de agua o madas en la
zona de subducción a inales de in ie no y comienzos de p ima e a pueden es-
capa de las capas supe iciales y se inyec adas en la e moclina pe manen e;
du an e el es o del año, las aguas subducidas quedan en la capa de mezcla ya
que las elocidades de escape gene adas po el bombeo de Ekman nega i o son
meno es que el a ance es acional de la capa de mezcla. La ex ensión e ical del
NACW en la i udes más bajas end á de e minada po el a lo amien o in e nal
de la isopicna más densa en la zona de subducción, ap oximadamen σθ=27.3
(Kawase and Sa mien o, 1985; Reid, 1994).
Eme y and Meincke (1986) subdi idió el NACW en dos ipos, O ien al (ENACW)
y Occiden al (WNACW). La di isión e lejaba dis in as egiones de o mación, al
su del F en e Subá c ico pa a WNACW y al su del F en e de Islandia-Fa oe
pa a ENACW. En cambio, Tomczak and God ey (1994), a gumen an que los
cambios de empe a u a y salinidad obse ados a lo la go de la cuenca ocánica
son el esul ado de la a iabilidad medioambien al den o de la egión de o ma-
ción. Po o o lado, Eme y and Meincke (1986) iden i ican sólo un ipo de Agua
Cen al Suda lán ica (SACW), con o mación ce ca de la zona de con e gencia
sub opical de B asil-Mal inas . Go don e al. (1992) y Sp in all and Tomczak
135
Chap e 6. Resumen en español
(1993) han mos ado que el agua cen al o mada en la con e gencia sub opical
del Océano Índico ep esen a una con ibución impo an e de la e moclina del
Océano A lán ico, in oducida en la cuenca A lán ica a a és de la Co ien e de
Agulhas.
El Agua Cen al Suda lán ica (SACW) iaja a a és de la e moclina del A lán-
ico su hacia el sis ema ecua o ial de co ien es y la egión opical del A lán-
ico No e. La zona on al de Cabo Ve de (CVFZ) cons i uye la on e a en e el
NACW y SACW y co esponde al lími e su de la eci culación e moclina del
A lán ico No e (S amma and Siedle , 1988; Zenk e al., 1991; A han e al., 1994).
La CVFZ se ex iende hacia el su oes e desde los 20◦N en la cos a de Á ica hacia
las Islas Cabo Ve de, y en onces adquie e una o ien ación más zonal a medida
que se a di undiendo p og esi amen e hacia el lado occiden al de la cuenca.
Las dos masas de agua ocupan el mismo ango de densidad, siendo el NACW
más salada y cálida que el SACW. Como esul ado, el en e es á compensado en
densidad lo cual p opicia la gene ación de mul i ud de in usiones y len es (Zenk
e al., 1991; Pé ez-Rod íguez e al., 2001; Pas o e al., 2008).
En la egión de es udio, la capa in e media es á ocupada po Agua In e media
An á ica (AAIW) y Agua Medi e ánea (MW), ap oximadamen e en e los 700 y
1500 m de p o undidad. El AAIW iene su o igen en el F en e Suban á c ico y
es anspo ada a a és del gi o sub opical del su a lán ico hacia los ópicos
(Suga and Talley, 1995). El AAIW puede se ácilmen e iden i icada po su baja
salinidad, con un mínimo de salinidad cen ado sob e los 800 m p o undidad.
Dos caminos anspo an AAIW hacia el ma gen o ien al A lán ico No e, uno
es a a és del sis ema de co in es de on e a oes e y la Co ien e de Azo es
(Kawase and Sa mien o, 1985; Tsuchiya e al., 1992). El segundo camino es a
136
Chap e 6. Resumen en español
a és del ma gen o ien al a lo la go de la cos a a icana (Machín and Peleg í,
2009), con una pene ación máxima du an e el o oño (Machín e al., 2010). El
MW se o ma en el Ma Medi e áneo y es in oducida en el A lán ico a a és
del Es echo de Gib al a ; desde donde se ex iende hacia el no e y el su , in-
luyendo sob e odo el A lán ico No e (Wo hing on, 1976). Su al a salinidad y
empe a u a ca ac e ís icas se pueden obse a en e ap oximadamen e los 600
y 1500 m p o undidad. Ambas aguas in e medias se encuen an sob e los 32◦N
(p.ej. Figu a 3.5), con el AAIW ocupando un ango de p o undidad lige amen e
más supe icial que el MW. Las p opiedades de las masas de agua in e medias
o iginales es án modi icadas po mezcla con aguas de las capas supe io es e in-
e io es, y sus p opiedades TS en la egión de es udio apa ecen como alo es
ex emos locales que se des ían sus ancialmen e de los alo es encon ados en
su egión de o mación (Figu a 6.3).
El Agua P o unda No a lán ica (NADW) se encuen a en la capa in e io , en-
e los 2000 y 4000 m ap oximadamen e. El NADW del A lán ico No o ien al es á
o mada p incipalmen e po Agua de Desbo damien o de Islandia-Escocia, mo-
di icada po Agua del Ma de Lab ado y Agua P o unda In e io (McCa ney,
1992; Dickson and B own, 1994; an Aken, 2000).
Va iabilidad in e anual y modos climá icos
La a iabilidad in e anual en el a lo amien o cos e o ha sido elacionada p inci-
palmen e con la Oscilla ion del A lán ico No e (NAO) y con El Niño - Oscilación
Su eña (ENSO). El índice NAO iene de inido po la anomalía en la di e encia
de la p esión a mos é ica a ni el del ma en e el sis ema de bajas p esiones de
137
Chap e 6. Resumen en español
Islandia y sis ema de al as p esiones de Azo es du an e el in ie no (de diciemb e
a ma zo). Un aumen o en el índice implica un aumen o en las al as p esiones
de Azo es y un aumen o del lujo hacia el es e sob e el no oes e a icano, dando
luga a un aumen o en el a lo amien o cos e o gene ado po los ien os alisios.
Analizando imágenes de sa éli e de empe a u a supe icial del ma (SST) en e
1982 y 2001, San os e al. (2005) encon a on un cambio decadal en la in ensidad
del a lo amien o pasando de un égimen de a lo amien o débil en la década de
los 80’s a un a lo amien o in enso en los 90’s, asociado a un cambio en el índice
NAO. Meine s (2007) y Meine s e al. (2010) ambién encon a on una co elación
posi i a en e el índice NAO y los ien os alisios al es udia impac os de la a ia-
bilidad climá ica en la dinámica de la me luza neg a en el no oes e de la cos a
a icana. Especí icamen e, el índice NAO podía explica el 53% de la a iabilidad
en la componen e me idional de la ensión del ien o en la cos a de Mau i ania y
Senegal en e 1960 y 2004.
Roy and Reason (2001) in es iga on conexiones en e el Índice Mul i a ia i o
ENSO y anomalías en la SST y en la ensión del ien o en la cos a del no oes e
a icano (en e 10 y 20◦N). Su abajo mos ó que pe iodos cálidos en el Pací-
ico du an e el o oño y p icipios de in ie no (si uación El Niño) daban luga a
un es ado elajado del a o amien o inducido po ien o en el lado o ien al de la
cuenca A lán ica y a e en os cálidos obse ados a lo la go de la cos a oes e de
Á ica a inales de in ie no y en p ima e a. Los e en os ENSO ambién in luyen
en el sis ema ecua o ial de co ien es del A lán ico y en la dinámica del Domo de
Guinea. Láza o e al. (2005) obse a on una in ensi icación de la NECC du an e
la p ima e a de 1997 y 1998, y del Domo de Guinea du an e el e ano de 1997,
asociada a un desplazamien o p ema u o hacia el no e de la ITCZ indicado po
138
Chap e 6. Resumen en español
En ield and Maye (1997), y coincidiendo con un acon ecimien o La Niña en el
Pací ico.
Va ios au o es ambién han iden i icado endencias en el a lo amien o cos e o,
aunque en ocasiones las endencias son con adic o ias. Bakun (1990) iden i icó
un aumen o signi ica i o en la ensión de ien o a o able al a lo amien o en un
pun o del no oes e a icano si uado en la la i ud 28◦N en e 1946 y 1981. La in en-
si icación de los ien os en es e si io coincidió con un aumen o en o os luga es
den o de los p incipales sis emas de a lo amien o cos e o del mundo. Es e au-
o hipo e izó que, en un escena io de calen amien o global, con el aumen o en
las concen aciones de gases de e ec o hin e nade o hab ía una in ensi icación
del a lo amien o cos e o debido a un aumen o en el g adien e de p esión en e
la egión cos e a y el ma . U ilizando da os de empe a u a de i ados a pa -
i de egis os sedimen a ios que se ex ienden 2500 años a ás, McG ego e al.
(2007) in i ie on un aumen o anómalo y sin p eceden es en el a lo amien o cos-
e o en Cabo Ghi du an e el siglo XX. En cambio, analizando da os de ien o
de QuickSca en e 2000 y 2007, Dema cq (2009) encon ó una endencia al de-
c ecimien o en la componen e me idional de la ensión del ien o en el no oes e
a icano. Aun así, es os esul ados pueden e leja únicamen e a iabilidad in e -
anual o decadal supe pues a a una endencia a escala de iempo mayo .
P oducción p ima ia
La egión de a lo amien o cos e o del no oes e a icano es uno de los cua o Sis-
emas de A lo amien o de F on e a Es e (EBUS) en el océano global. Los EBUS
mues an una al a p oduc i idad e impo an es cap u as pesque as (Pauly and
139

Chap e 6. Resumen en español
0.03 0.1 0.3 1 3
CHL (mg m )
-3
40
30
20
10
40
30
20
10
-30 -20 -10 -30 -20 -10
VERANO
INVIERNO PRIMAVERA
OTOÑO
Cape Blanc Cape Blanc
Cape Blanc Cape Blanc
Figu e 6.4: Concen aciones de clo o ila p omedio (mg m-3) en in ie no (EFM),
p ima e a (AMJ), e ano (JAS) y o oño (OND). Los con o nos en neg o ma can
las isolíneas de 0.2 y 1 mg m-3 de clo o ila. Los da os co esponden al senso
SeaWiFS en e 1998 y 2007.
140
Chap e 6. Resumen en español
Ch is ensen, 1995). El A lán ico No des e sub opical iene la zona ac i a más
g ande de los cua o EBUS p incipales, á ea de inida como la egión donde las
concen aciones de clo o ila supe an el 1 mg m-3 (Ca , 2002). Es el segundo EBUS
más p oduc i o, con una p oducción p ima ia anual de 0.33 G de Ca bono po
año, después del EBUS de Benguela en el A lán ico Su (Ca , 2002).
La clo o ila p opo ciona una medida indi ec a de abundancia de i oplanc-
on y de ahí los nume osos es ue zos en desa olla un algo i mo pa a de i a
p oducción p ima ia a pa i de clo o ila medida po sa éli e (Beh en eld and
Falkowski, 1997; Ca , 2002; Ma a e al., 2003). La Figu a 6.4 mues a las concen-
aciones de clo o ial es acionales de i adas a pa i de 10 años de da os sa eli ales
del senso SeaWiFS. La egión en e 24◦N y el Es echo de Gib al a mues a una
a iabilidad es acional débil. Concen aciones de clo o ila po encima de 1 mg
m-3 es án con iadas a la pla a o ma con inen al, a pesa de la exis encia de a lo-
amien o du an e odo el año. La es echa ex ensión cos a a ue a de la clo o ila en
es a egión puede se causada po limi ación de nu ien es, ya que el o acional
nega i o de la ensión del ien o, ca ac e ís ico del gi o sub opical dep ime la
nu iclina (La huiliè e e al., 2008).
La egión en e 18 y 24◦N ambién mues a una es acionalidad débil, pe o en
es e caso las al as concen aciones de clo o ila se ex ienden conside ablemen e
cos a a ue a du an e odo el de año. La ca ac e ís ica más p edominan e de es a
banda la i udinal es el ilamen o gigan e de Cabo Blanco (Gab ic e al., 1993).
Du an e el e ano y o oño, la con e gencia de la CUC, que luye hacia el su ,
y la MC, que luye hacia el no e, esul a en un in enso anspo e hacia océano
abie o. En in ie no y p incipios de p ima e a, cuando el a lo amien o alcanza
más al su de Cabo Ve de, la con e gencia y el anspo e hacia océano abie o
141
Chap e 6. Resumen en español
se en educidos pe o es án aún p esen es (Peleg í e al., 2006). Es uc u as de
ipo ilamen o que ad ec an clo o ila cos a a ue a ambién puede se obse adas
en los p incipales cabos, como Cabo Ghi , donde la p oducción p ima ia alcanza
alo es de 5 g C m-2 año-1 (Ga cía-Muñoz e al., 2005; Peleg í e al., 2005).
En e los 18◦N y el Es echo de Gib al a elemen os mesoscala es, como ila-
men os y emolinos, y p ocesos ondula o ios con ibuyen a la a iabilidad diná-
mica y bioquímica de la egión, pe o no a a emos es os emas en es e es udio.
Ba on e al. (1998) y Ba on (1998) han p opo cionado un análisis ex enso de los
p ocesos mesoscala es y Hagen (2001) ha ealizado una e isión excelen e de la
p opagación de ondas.
La egión al su de 18◦N p esen a una g an ex ensión de la clo o ila cos a
a ue a du an e in ie no y p ima e a, cuando aguas icas en nu ien es llegan a
la supe icie g acias al a lo amien o cos e o. Du an e e ano, el o alecimien o
de la NECC y el desplazamien o cos a a ue a del á ea de bombeo de Ekman po-
si i o ele a las capas supe io es de la e moclina, y ya en o oño una al a p o-
ducción p ima ia puede se obse ada en el á ea del GD (localizado en 10◦N,
22◦O ap oximadamen e). Peleg í e al. (2006) han p opues o un mecanismo po
cual las capas subsu pe iciales del GD man ienen un ni el al o de concen ación
de nu ien es. El GD se desa olla du an e e ano y o oño y los nu ien es que
alcanzan la capa ó ica son u ilizados. Du an e el in ie no, los ien os alisios a-
o ables al a lo amien o se ex ienden al su de Cabo Blanco, y el GD se elaja. La
celda e ical asociada al a lo amien o cos e o necesi a un suminis o de aguas
subsu pe iciales desde el océano in e io , es e suminis o ees ablece los al os
ni eles de nu ien es de la egión del GD.
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Chap e 6. Resumen en español
6.1.2 Obje i os y esumen de la esis
Es a esis con iene es capí ulos cen ales en o ma o de a ículo cien í ico, p ece-
didos po una in oducción gene al y seguidos de unas conclusiones gene ales.
Pa a ealiza la esis, se ha examinado una g an a iedad de uen es de da os.
Da os his ó icos han sido eanalizados. También han sido examinamos da os
hid og á icos ecien emen e adqui idos y p opiedades medidas emo amen e po
senso es sa eli ales, como concen ación de clo o ila o al u a supe icial del ma .
Finalmen e, un modelo de ci culación ocánica gene al acoplado a un modelo bio-
geoquímico de úl ima gene ación ha pe mi ido una mejo comp ensión de los
mecanismos ísicos que gene an a iabilidad en p opiedades bioquímicas.
Capí ulo 2
Da os hid og á icos his ó icos de cua o campañas han sido combinados pa a
p oduci dos conjun os de da os, conc e amen e el conjun o de p ima e a de 1973
y el de o oño de 1975, que cub en la zona de ansición cos e a en e las la i udes
17 y 26◦N. Los da os es aban disponibles en colecciones imp esas y po ello se
u ie on que digi aliza an es de comenza el análisis. La hid og a ía de la zona
se desc ibe en cuan o a su empe a u a, salinidad, ni a os y oxígeno disuel o.
La mezcla en e aguas cen ales del no e y del su es explo ada, en a izando el
papel especial de la doble di usión en aumen a la di usión ho izon al de calo .
Finalmen e, se calculan in e cambios de agua y nu ien es en e la zona de ansi-
ción cos e a y el océano p o undo, di e enciando en e la con ibución geos ó ica
y la de Ekman .
Las p egun as que se in en a an con es a en es e capí ulo son:
143