Differences between 1999 and 2010 across the Falkland Plateau: fronts and water masses

Ocean Sci., 13, 577–587, 2017 h ps://doi.o g/10.5194/os-13-577-2017 © Au ho (s) 2017. This wo k is dis ibu ed unde he C ea i e Commons A ibu ion 3.0 License. Di e ences be ween 1999 and 2010 ac oss he Falkland Pla eau: on s and wa e masses M. Dolo es Pé ez-He nández1,5, Alonso He nández-Gue a1, Isis Comas-Rod íguez1, Ve ónica M. Bení ez-Ba ios1, Eugenio F aile-Nuez2, Josep L. Peleg í3, and Albe o C. Na ei a Ga aba o4 1Ins i u o de Oceanog a ía y Cambio Global (IOCAG), Uni e sidad de Las Palmas de G an Cana ia (ULPGC), Las Palmas, Spain 2Cen o Oceanog á ico de Cana ias, Ins i u o Español de Oceanog a ía, San a C uz de Tene i e, Spain 3Ins i u de Ciències del Ma , Consejo Supe io de In es igaciones Cien í icas, Ba celona, Spain 4Uni e si y o Sou hamp on, Na ional Oceanog aphy Cen e, Sou hamp on, UK 5Depa men o Physical Oceanog aphy, Woods Hole Oceanog aphic Ins i u ion, Woods Hole, USA Co espondence o: M. Dolo es Pé ez-He nández ([email p o ec ed]) Recei ed: 17 No embe 2016 – Discussion s a ed: 13 Decembe 2016 Re ised: 6 June 2017 – Accep ed: 7 June 2017 – Published: 7 July 2017 Abs ac . Decadal di e ences in he Falkland Pla eau a e s udied om he wo ull-dep h hyd og aphic da a collec ed du ing he ALBATROSS (Ap il 1999) and MOC-Aus al (Feb ua y 2010) c uises. Di e ences in he uppe 100 dba a e due o changes in he seasonal he mocline, as he AL- BATROSS c uise ook place in he aus al all and he MOC- Aus al c uise in summe . The in e media e wa e masses seem o be e y sensi i e o he wind condi ions exis ing in hei o ma ion a ea, showing cooling and eshening o he decade as a consequence o a highe An a c ic In e media e Wa e (AAIW) con ibu ion and o a dec ease in he Sub- an a c ic Mode Wa e (SAMW) s a um. The deepe laye s do no exhibi any signi ican change in he wa e mass p op- e ies. The Suban a c ic F on (SAF) in 1999 is obse ed a 52.2–54.8◦W wi h a ela i e mass anspo o 32.6 S . In con as , he SAF ge s wide in 2010, s e ching om 51.1 o 57.2◦W ( he Falkland Islands), and weakening o 17.9 S . Changes in he SAF can be linked wi h he wes e ly winds and mainly a ec he no hwa d low o Suban a c ic Su ace Wa e (SASW), SAMW and AAIW/An a c ic Su ace Wa- e (AASW). The Pola F on (PF) ca ies 24.9 S in 1999 (49.8–44.4◦W), while in 2010 (49.9–49.2◦W) i na ows and s eng hens o 37.3 S . 1 In oduc ion The An a c ic Ci cumpola Cu en (ACC) lows eas wa ds a ound he An a c ic con inen , anspo ing oughly be ween 100 and 173 S (1 S =106m3s−1≈109kg s−1; he ea e S will be he uni used) (O si e al., 1995; Cunningham e al., 2003; Donohue e al., 2016). Along i s pa h, i con- nec s he A lan ic, Paci ic and Indian basins, exchanging hea and eshwa e among o he p ope ies. Al hough con e - gence o ne lux es ima es has been achie ed on basin scales (Ganachaud and Wunsch, 2003), he ACC low in o he A - lan ic Ocean is c i ical o es ablish he magni ude and pa h- ways o he Sou he n Ocean con ibu ion o he deep global ocean en ila ion. Pe e son and Whi wo h III (1989) sugges ed ha he Sub- an a c ic F on (SAF) and he Pola F on (PF), whe e he majo eloci y bands o he ACC occu , u n no hwes - wa d ac oss he Falkland Pla eau o he wes o he Mau- ice Ewing Bank, along he Pa agonian con inen al slope. This was suppo ed by Pe e son (1992), who es ima ed he la ge con ibu ion o he ACC o he Falkland Cu en (60– 70 S ), e ealing he impo ance o he o e low o sou he n wa e s o he Sou h A lan ic bounda y ci cula ion. Pe e son and Whi wo h III (1989) loca ed he SAF nea 53◦W, as co obo a ed by A han e al. (2002), a a loca ion whe e he ocean dep h is 2000 m. Se e al s udies ha e la e examined he pa h o he PF a ound he Mau ice Ewing Bank (T a han e al., 2000) and i s b anching a ound 49–50◦W (A han e Published by Cope nicus Publica ions on behal o he Eu opean Geosciences Union. 578 M. D. Pé ez-He nández e al.: Di e ences be ween 1999 and 2010 ac oss he Falkland Pla eau 33.5 34 34.5 35 0 1 2 3 4 5 6 7 8 Salini y θ(°C) SASW SAMW AAIW + AASW UCDW LCDW 72° W 64° W 56° W 48° W 40° W 56° S 54° S 52° S 50° S 48° S 46° S 57911 13 15 17 19 21 23 25 27 29 31 170 168 166 164 162 160 158 156 152 150 148 146 144 142 MOC Aus al ALBATROSS Falkland Pla eau MOC Aus al ALBATROSS (b) (a) 26.90 27.20 27.60 28.00 Figu e 1. (a) Hyd og aphic s a ions ac oss he Falkland Pla eau ca ied ou du ing he ALBATROSS (1999, ed s a ion numbe s) and MOC- Aus al (2010, black s a ion numbe s) c uises. (b) θ−Sdiag am o bo h c uises. Red solid lines ep esen he γn alues (26.90, 27.20, 27.60 and 28.00 kg m−3) de ining he di e en wa e masses in he egion. al., 2002), wi h a possible meande ing o he on acco ding o Na ei a Ga aba o e al. (2002). The i s hyd og aphic c uise along he Falkland Pla eau was ca ied ou in 1999. The ALBATROSS (An a c ic La ge- scale Box Analysis and he Role o he Sco ia Sea) c uise explo ed he ACC h ough he D ake Passage and he Sco- ia Sea (Fig. 1). The da a o his c uise ha e been used o es ima e ela i e anspo , wa e masses, luxes and mixing ac oss he pla eau (Na ei a Ga aba o e al., 2003) and o p o- ide a de ailed explana ion o he deep wa e s in he Sco ia Sea (Na ei a Ga aba o e al., 2002). La e on, his sec ion was compa ed wi h hyd og aphic c uises ca ied ou no h and sou h o he Falkland Pla eau o achie e a be e knowl- edge o his a ea (A han e al., 2002; Smi h e al., 2010). Howe e , i is no un il 2010 ha his sec ion is epea ed. In his s udy, he wa e masses, ela i e geos ophic eloc- i ies and anspo s ac oss an almos zonal hyd og aphic sec- ion ca ied ou in 2010 along he Falkland Pla eau a e e al- ua ed. These da a, oge he wi h he ALBATROSS c uise, a e he only high- esolu ion hyd og aphic da a a ailable in he egion. Thus, esul s om he 2010 c uise a e compa ed wi h hose ob ained om he 1999 c uise in he same a ea (Na ei a Ga aba o e al., 2003), wi h he objec i e o assess- ing possible ela i e anspo and wa e mass di e ences be- ween he wo su eys. Fo changes in he ela i e anspo , he posi ion o he on s and he season in which each c uise ook place will be conside ed. Changes in wa e masses a e decomposed in o changes in he θ/S isoba ic su aces and esul s om he Bindo and McDougall (1994) model. 2 Da a and me hods The MOC-Aus al c uise was ca ied ou in Feb ua y 2010 onboa d he BIO Hespé ides. As shown in Fig. 1a, 27 ull- dep h conduc i i y, empe a u e and dep h (CTD) s a ions we e occupied ac oss he Falkland Pla eau, acking along he cas s p e iously conduc ed in be ween 41 and 57◦W a he nominal la i ude o 51◦S du ing he ALBATROSS c uise in Ap il 1999 (Na ei a Ga aba o e al., 2003). Wi h a spa- ial sepa a ion o 30 o 50 km, empe a u e and salini y p o- iles we e ob ained using a SeaBi d 911+CTD wi h dual conduc i i y and empe a u e senso s. The CTD was sen o SeaBi d o calib a ion be o e he c uise. The empe a u e senso has an accu acy o 0.001 ◦C. The conduc i i y senso s we e calib a ed on boa d wi h bo le sample salini ies. To ha end, wa e samples we e analyzed on a Guildline AUTOSAL 8400B salinome e wi h accu acy be e han 0.002 o single samples (salini y is exp essed in he P ac ical Salini y Scale, UNESCO, 1981). Rela i e geos ophic eloci ies and mass anspo s a e es- ima ed o bo h he ALBATROS and MOC-Aus al c uises using he sea bo om as he le el o no mo ion. The wa e col- umn is di ided in o 18 neu al densi y laye s ollowing he wo k o Na ei a Ga aba o e al. (2003) (see Table 1). Wed- dell Sea Deep Wa e (WSDW) is no ound along he pla eau; hus, i s densi y laye s a e no conside ed he e. Bindo and McDougall (1994) desc ibe a model o e al- ua e he empe a u e and salini y a ia ions in he wa e col- umn. This model ela es he empe a u e and salini y in bo h p essu e and densi y changes h ough he ollowing equa ion: dψ d z =dψ d γn −dp d γn dψ dp, which shows ha o a gi en p ope y (ψ, empe a u e o salini y), he a ia ions along isoba ic le els dψ d zcan be desc ibed as he sum o changes along isoneu al su aces dψ d γnand changes due o e ical displacemen s o he densi y su aces, e e ed o as hea ing dp d γn dψ dp. This allows he compa ison be ween he isoba ic changes and he sum o he wo decomposed componen s, which ep esen he a ia ions o he wa e masses (wa ming and eshening) Ocean Sci., 13, 577–587, 2017 www.ocean-sci.ne /13/577/2017/ M. D. Pé ez-He nández e al.: Di e ences be ween 1999 and 2010 ac oss he Falkland Pla eau 579 Table 1. SAF, PF and ne geos ophic mass anspo (S ) pe c uise and wa e mass. The las ow shows he ne anspo , while he las column indica es he anspo di e ence be ween c uises. ALBATROSS (1999) MOC-Aus al (2010) Di e ence (2010–1999) SAF PF Ne SAF PF Ne SAF PF Ne SASW γn< 26.90 3.2 2.0 4.5 1.9 1.6 2.3 −1.3 −0.4 −2.2 SAMW 27.00 < γn< 27.20 5.1 1.0 4.9 1.4 1.8 2.2 −2.9 0.8 −2.7 AAIW/AASW 27.30 < γn< 27.60 15.5 8.3 16.8 10.9 9.7 11.8 −4.6 1.4 −5.0 UCDW 27.70 < γn< 28.00 8.6 12.0 12.4 3.8 20.1 11.7 −4.8 8.1 −0.7 LCDW 28.05 < γn0.1 1.6 0.3 0.0 4.1 1.7 −0.1 2.5 1.4 Ne 32.6 24.9 38.9 17.9 37.3 29.7 −14.7 12.4 −9.2 33.6 33.8 34 34.2 34.4 34.6 34.8 Salini y 0 1 2 3 4 5 6 7 8 9 Po . empe a u e (° C) (a) ALBATROSS 26.90 27.20 27.60 28.00 33.6 33.8 34 34.2 34.4 34.6 34.8 Salini y 0 1 2 3 4 5 6 7 8 9 (b) MOC Log coun s 26.90 27.20 27.60 28.00 0 0.5 1 1.5 2 Figu e 2. A olume ic po en ial empe a u e–salini y diag am o he (a) ALBATROSS and (b) MOC-Aus al c uises. Red solid lines ep esen he γn alues (26.90, 27.20, 27.60 and 28.00 kg m−3) de ining he di e en wa e masses in he egion. Do size and colo indica e he loga i hm o coun s. and he hea ing. To apply his me hodology, empe a u e and salini y a e in e pola ed on o a g id wi h a p essu e in e al o 20 dba ( om 10 o 3500 dba ) and he ollowing neu al den- si y (kg m−3) alues: om 26 o 27.6 each 0.02, om 27.7 o 28 each 0.01 and om 28.005 o 28.5 each 0.005. This ec o is selec ed o p ope ly ep esen he di e en s uc u es ound in he wa e column. In addi ion, sea su ace heigh (SSH) was downloaded om AVISO (h p://www.a iso.oceanobs.com/, Diba bou e e al., 2015). Da a be ween 10 and 20 Feb ua y 2010 a e used o he analysis o he MOC-Aus al c uise esul s. Da a o Feb ua y and Ap il om 1993 o 2016 a e used o es ima e he a e age seasonal geos ophic anspo in each mon h. 3 Resul s 3.1 Wa e masses Wa e masses in he s udy egion a e labeled ollowing Na ei a Ga aba o e al. (2003). The isoneu als 26.90, 27.20, 27.60 and 28.00 kg m−3( ed solid lines in Fig. 1b) di ide he wa e column in o Suban a c ic Su ace Wa e (SASW), Sub- an a c ic Mode Wa e (SAMW), An a c ic In e media e Wa- e (AAIW) mixed wi h An a c ic Su ace Wa e (AASW), Uppe Ci cumpola Deep Wa e (UCDW) and Lowe Ci - cumpola Deep Wa e (LCDW), espec i ely. Figu e 2 shows ha , in bo h c uises, he Ci cumpola Deep Wa e (CDW) is he mos homogenous wa e mass. The UCDW in he ALBATROSS c uise p esen s a wide empe a u e ange and i is less homogeneous han in he MOC-Aus al c uise. Figu e 2 also shows ha he s a a o AAIW +AASW and SAMW a e qui e di e en be ween c uises. The AAIW +AASW s a um o he MOC-Aus al c uise p esen s a minimum ha consis s o empe a u es be- low 1.2 ◦C and salini ies a ound 34. This minimum indica es ha he con ibu ion o AAIW is highe in 2010 han in 1999. In con as , in he same s a um, he ALBATROSS c uise shows a hicke laye o AASW. The SASW in 1999 eaches highe salini ies and empe a u es han in he MOC-Aus al c uise ( his can be be e obse ed in Fig. 1b, g ey do s). I is also wo h men ioning he exis ing di e ence be ween he SAWM s a a o bo h c uises, as he one o he ALBATROSS www.ocean-sci.ne /13/577/2017/ Ocean Sci., 13, 577–587, 2017 580 M. D. Pé ez-He nández e al.: Di e ences be ween 1999 and 2010 ac oss he Falkland Pla eau 0 500 1000 1500 2000 2500 3000 170 167 164 160 157 155 151 149 147 145 142 SAF PF 26.9 27.2 27.6 28 56 54 52 50 48 46 44 42 −60 −40 −20 0 20 40 60 Longi ude (o W) Dep h (m) SASW SAMW AAIW UCDW LCDW (a) 5 7 9 11 13 15 17 19 22 24 26 28 30 SAF PF 26.9 27.2 27.6 28 56 54 52 50 48 46 44 42 SASW SAMW AAIW UCDW LCDW (b) Figu e 3. Geos ophic eloci y (cm s−1, posi i e no hwa d) ela i e o he bo om o (a) he ALBATROSS c uise and (b) he MOC-Aus al c uise. Black dashed lines ma k 0 cm s−1 eloci ies. Thick black lines s and o he ep esen a i e isoneu als (26.90, 27.20, 27.60 and 28.00 kg m−3) de ining he wa e masses in he egion. S a ion numbe ing and he on (SAF and PF) loca ion a e displayed on he op axis. c uise has a wide ange o salini ies han he one o MOC- Aus al (Fig. 2). The uppe laye s a e less compa able as he c uises ook place in di e en seasons, which implies di - e en p ecipi a ion/e apo a ion and winds ha will di ec ly a ec he SASW s a um. 3.2 F on s In Fig. 3, he p ominen slope o he γnsu aces oge he wi h he in ensi ied ela i e eloci ies poin ou he p es- ence o he SAF and PF nea hei his o ically epo ed lo- ca ions (O si e al., 1995). In 1999, a no hwa d- lowing je accompanies he SAF, ex ending he on ’s in luence om he su ace down o app oxima ely 1500 m be ween 52.2 and 54.8◦W (Fig. 3a, s a ions 160 o 165). In con as , he SAF is displaced o he wes in 2010, ex ending om he Falkland sho e (57.2◦W) o 51.1◦W (s a ions 5 o 17), and he ho - izon al densi y g adien and associa ed ela i e geos ophic eloci ies a e weake (Fig. 3b). Rega ding he PF, i s quasi- ba o opic p esence and e ec on he wa e column a e mos no iceable in 2010 (Fig. 3b, s a ions 20 and 21), when i in- ensi ies, displaying he s onges low o he no h a ound 49.5◦W. Figu e 3a shows how his on is weake in 1999, when i ex ends app oxima ely be ween 44.4 and 49.8◦W and no in ense je s a e igge ed by i s p esence (s a ions 146 o 156). I can be obse ed how hese on s a e e ealed by he sloping isoneu als, su e ing signi ican changes be- ween he wo oceanog aphic c uises. The e o e, i is impo - an o de e mine which a ia ions in po en ial empe a u e and salini y a e due o wa e mass changes and which a e caused by he displacemen o he γnsu aces. 3.3 The θ/S isoba ic changes Figu e 4 e eals ha in he decade, he wa e s shallowe han 50 dba ( oughly he SASW s a um) exhibi a signi ican in- c ease in empe a u e and salini y, being 0.5 ◦C and 0.12, espec i ely (Fig. 4a–d). This su ace inc ease is p obably caused by he ac ha he a ea was sampled in e y di e en seasons: while he MOC-Aus al c uise ook place in he aus- al summe , he ALBATROSS c uise was ca ied ou du ing he aus al all. In he wa e s immedia ely benea h ( om 50 o 500 dba ), he in e media e s a a o SAMW and AAIW +AASW p esen a dec ease in empe a u e o 0.8 and 0.4 ◦C, espec- i ely (Fig. 4a and b). In con as , while salini y o he AAIW +AASW s a um also dec eases by 0.01, he salin- i y o he SAMW inc eases by 0.02 (Fig. 4c and d). In hese in e media e s a a, a oughly he loca ion o he on s (be- ween s a ions 5–17 and 20–21), a ema kable dec ease in empe a u e can be seen (Fig. 4a). In be ween s a ions 9–12, whe e he SAF s ands, he UCDW exhibi s a ema kable in- c ease in salini y. This inc ease ge s compensa ed in he a e - age wi h he dec ease obse ed h oughou he s a um. The same is obse ed in he a ea o he PF, whe e an inc ease in salini y is egis e ed a he UCDW and AAIW/AASW s a a (Fig. 4c). The UCDW and LCDW do no show any signi ican changes in empe a u e. The UCDW inc eases by 0.01 in salini y, while he LDCW does no show any signi ican di - e ence in salini y (Fig. 4c and d). Ocean Sci., 13, 577–587, 2017 www.ocean-sci.ne /13/577/2017/ M. D. Pé ez-He nández e al.: Di e ences be ween 1999 and 2010 ac oss he Falkland Pla eau 581 Figu e 4. Ve ical sec ions o di e ences in (a) po en ial empe a u e and (c) salini y in isoba ic le els, o he 1999 and 2010 c uise. The lines supe imposed o e he e ical sec ions (g ey lines o he 1999 sec ion and black lines o he 2010 sec ion) s and o he isoneu als de ining he di e en wa e masses in he egion (26.90, 27.20, 27.60 and 28.00 kg m−3). MOC-Aus al s a ion numbe ing and he on (SAF (g ey 1999, black 2010) and PF) loca ions a e displayed on he op axis. Side panels show he zonally a e aged di e ences in empe a u e (b) and salini y (d), espec i ely (solid black lines), oge he wi h hei 95 % con idence in e al based on a S uden ’s - es (dashed g ey lines). www.ocean-sci.ne /13/577/2017/ Ocean Sci., 13, 577–587, 2017 582 M. D. Pé ez-He nández e al.: Di e ences be ween 1999 and 2010 ac oss he Falkland Pla eau -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 ∆θ2010–1999 (°C) -3000 -2500 -2000 -1500 -1000 -500 0 P essu e (dba ) (a) θz θn -Nθz Sum -0.14 -0.12 -0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 ∆S2010–1999 -3000 -2500 -2000 -1500 -1000 -500 0 P essu e (dba ) (b) Sz Sn -NSz Sum 33.8 34 34.2 34.4 34.6 34.8 Salini y 0 1 2 3 4 5 6 7 Po en ial empe a u e (° C) SASW SAMW AAIW/AASW UCDW LCDW 26.90 27.20 27.60 28.00 (c) Moc Aus al Alba oss Figu e 5. Isoba ic changes om 1999 o 2010 (θz, black line) de- composed in o changes along neu al su aces (θn,Sn, blue line) and changes due o he e ical displacemen o isoneu als (−Nθz, −NSz, ed line) o (a) po en ial empe a u e and (b) salini y. The g ey line shows he sum o bo h componen s. The lowe panel (c) shows he a e age p o ile o θ/S o each c uise oge he wi h he densi ies ha di ide he wa e column in o he di e en wa e masses (26.90, 27.20, 27.60 and 28.00 kg m−3, ed lines) and he link in be ween poin s o equal p essu e (dashed blue lines). 3.4 Resul s o applying he Bindo and McDougal (1994) analysis The empe a u e and salini y isoba ic changes, hei decom- posi ion and he sum o he wo componen s a e plo ed in Fig. 5a and b, espec i ely. Excep o ce ain dep h anges, he sum o he componen s (g ey line) compa es easonably well wi h he isoba ic change (black line, θzand Sz), indica - ing ha he decomposi ion has been success ully pe o med. The ew disc epancies obse ed will be analyzed a he end o he sec ion. The su ace and in e media e empe a u e and salini y a ia ions a e a ec ed by bo h mechanisms: changes along neu al su aces (θnand Sn, blue lines) and changes due o e ical displacemen o he isoneu als (−Nθzand −NSz, ed lines) (Fig. 5a and b). In he SASW s a um (p es- su e < 100 dba ) an inc ease o 0.7◦C in empe a u e and 0.1 in salini y pe decade is obse ed (Fig. 5a and b, espec- i ely). This inc ease can also be disce ned in Fig. 5c. These inc eases come oge he wi h a empe a u e-d i en e ical displacemen o he isoneu als (Fig. 5a, ed line). As he c uises ook place in di e en seasons, he mos plausible ex- plana ion o his shoaling is he di e en dep hs o he sea- sonal he mocline, being shallowe in summe (2010) han in all (1999). In con as o he uppe laye , he SAMW and AAIW/AASW s a a p esen a decadal dec ease in em- pe a u e (−0.6 ◦C) and salini y (−0.07) be ween 100 and 500 dba (Fig. 5a and b). These changes can also be obse ed in he a e age θ/S-diag am o he AAIW/AASW (Fig. 5c). The SAMW and AAIW/ASW s a a occupy he same dep h ange, bu he AAIW/ASW wa e mass spans o e a highe a ea (Fig. 3). Hence, he decomposi ion shown in Fig. 5a and b mainly shows he beha io o he AAIW/AASW s a um and, he e o e, i does no ma ch wi h he inc ease in salini y obse ed in Fig. 5c o SAMW. In Fig. 5c he lines linking poin s o equal p essu e o he SAMW and AAIW/AASW s a a a e no pa allel o he isopycnals, indica ing, as well, displacemen o he isoneu als’ su aces. This displacemen is a deepening o he isoneu als, mainly d i en by he salin- i y. A he le el o he UCDW no changes a e obse ed (Fig. 5a–c). In con as , he LCDW s a um shows a deepen- ing o he isoneu als d i en by bo h empe a u e and salin- i y, al hough no changes along neu al su aces a e obse ed (Fig. 5a–c). As seen in Fig. 5, he sum o he componen s compa es easonably well o he isoba ic changes. Howe e , a ca e- ul inspec ion e eals some disc epancies, which ake place be ween 52 and 57◦W and a ound 49.5◦W. These a e he app oxima e loca ions o he SAF and PF. These g adien s cause he e ical displacemen o mo e han 200 dba o some isoneu als, in alida ing a hese speci ic loca ions he linea expansion used o de i e he p oposed decomposi ion model, as was also ound in he Gul S eam by A bic and Owens (2001). Thus, in Fig. 6 a sensi i i y analysis is ca ied ou by using he model o Bindo and McDougall (1994) wi hou he s a ions in ol ed in he on s, aking in o ac- coun only he s a ions 18 (157) and 28 o 31 (145 o 142) o he 2010 (1999) su ey. Fo he su ace SASW wa e mass, he same beha io is ound wi h o wi hou on s: an in- c ease in empe a u e and salini y, hough sligh ly highe in he decomposi ion done wi hou he on s (0.9 ◦C and 0.15 s. 0.7 ◦C and 0.1), and a empe a u e-d i en shoaling o he isopycnals. Likewise, in he ange 100–500 dba , whe e he SAMW and AAIW/AASW s a a appea , he decomposi ion shows he same pa e n: a sligh ly smalle dec ease in em- pe a u e (−0.4 ◦C) and salini y (−0.04) again accompanied by a salini y-d i en deepening o he isopycnals. In con as , om 500 dba o he bo om wo di e ences appea be ween bo h decomposi ions. The i s one occu s be ween 500 and 2000 dba : in bo h decomposi ions a sligh Ocean Sci., 13, 577–587, 2017 www.ocean-sci.ne /13/577/2017/ M. D. Pé ez-He nández e al.: Di e ences be ween 1999 and 2010 ac oss he Falkland Pla eau 583 -2 -1 0 1 2 -3000 -2500 -2000 -1500 -1000 -500 0 P essu e (dba ) 8JUIGSPOUT B -2 -1 0 1 2 θ Di e ences(°C) -3000 -2500 -2000 -1500 -1000 -500 0 P essu e (dba ) 8JUIPVUGSPOUT D θz θn -Nθz Sum -0.1 0 0.1 -3000 -2500 -2000 -1500 -1000 -500 0 C -0.1 0 0.1 S Di e ences -3000 -2500 -2000 -1500 -1000 -500 0 E Sz Sn -NSz Sum Figu e 6. Compa ison be ween he isoba ic changes om 1999 o 2010 ca ied ou wi h he whole da ase o each yea (a,b, same as Fig. 5) and wi hou he s a ions whe e he on s a e loca ed (c, d). Isoba ic changes om 1999 o 2010 (θz, black line) decomposed in o changes along neu al su aces (θn,Sn, blue line) and changes due o he e ical displacemen o isoneu als (−Nθz,−NSz, ed line) o empe a u e (a, c) and salini y (b, d). The g ey line shows he sum o bo h componen s. inc ease in empe a u e and salini y is obse ed, bu in he one ca ied ou wi hou he on s i appea s wi h a salini y- d i en shoaling o he isopycnals. This dep h ange is mainly occupied by he UCDW s a um. The second signi ican change be ween bo h decomposi ions appea s a he bo - om o he p o ile, in he domain o he LCDW. As he s a- ions eas o MOC-Aus al s a ion 28 (ALBATROSS s a- ion 148) a e shallowe han 2400 dba , his decomposi ion mainly shows he changes a s a ion 18. This s a ion is lo- ca ed be ween bo h on s, and shows a empe a u e-d i en deepening o he isopycnals. The esul in his s a um can be neglec ed, as one s a ion canno be conside ed s a is ically signi ican o p o ide ep esen a i e esul s. 3.5 Rela i e geos ophic mass anspo changes Some signi ican di e ences a e obse ed in he ela i e mass anspo es ima es o 1999 and 2010 ac oss he hy- d og aphic line along he Falkland Pla eau (Table 1). The accumula ed anspo s show he impo an ole played by he SAF and PF in he ela i e mass anspo ac oss he sec ion du ing bo h ealiza ions (Fig. 7). Du ing he MOC- Aus al c uise he SAF-associa ed je is displaced wes wa d and weakens by 14.7 S as compa ed wi h he ALBATROSS obse a ions (Fig. 4 and Table 1). This a ec s he ela i e mass anspo o all s a a bu LCDW. In con as , he loca- ion o he PF emains unchanged be ween bo h c uises, bu i s eng hens up o 37.3 S du ing he MOC-Aus al c uise ( s. 56 54 52 50 48 46 44 42 −5 0 5 SASW 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 56 54 52 50 48 46 44 42 −5 0 5 10 SAMW 56 54 52 50 48 46 44 42 0 10 20 AAIW/AASW 56 54 52 50 48 46 44 42 −10 0 10 20 UCDW Mass anspo (S ) 56 54 52 50 48 46 44 42 −5 0 5 LCDW 56 54 52 50 48 46 44 42 0 20 40 Ne Longi ude (° W) MOC Aus al Alba oss SAF SAF PF Figu e 7. Wes - o-eas accumula ed ela i e geos ophic mass ans- po , compu ed ac oss he ALBATROSS and MOC-Aus al hyd o- g aphic sec ions. S a ion numbe ing and he on (SAF and PF) loca ions a e displayed on he op axis. No e he di e en e ical scales. he 24.9 S egis e ed in he ALBATROSS su ey), a ec ing he ela i e mass anspo s o all wa e s a a. In 2010, im- media ely eas o he PF, a 47.8◦W, a coun e cu en appea s ca ying −8.8 S o he sou h. Figu e 8 shows he a e age SSH o he 2010 MOC-Aus al c uise wi h he aim o un- de s anding he sou ce o his coun e - low. In his igu e, he PF lows o he no h a ound s a ion 20 and pa ially di e s sou hwa d a s a ion 23. This meande ing o he PF has al- eady been epo ed in p e ious s udies (Na ei a Ga aba o e al., 2002). The ela i e ne mass anspo du ing he MOC-Aus al c uise is 9.2 S weake han in he ALBATROSS c uise as an ou come o a mo e in ense SAF in 1999. SASW, AAIW/AASW and UCDW p esen lowe alues in 2010 han in 1999, he su ace and in e media e s a a being he ones wi h he highes decadal anspo di e ences (Fig. 7 and Table 1). This is p esumably due o a weake SAF in 2010 (17.9 S ) han in 1999 (32.6 S ) (Table 1). The SAF does no ha e a con ibu ion om he LDCW s a um due o he shal- www.ocean-sci.ne /13/577/2017/ Ocean Sci., 13, 577–587, 2017 584 M. D. Pé ez-He nández e al.: Di e ences be ween 1999 and 2010 ac oss he Falkland Pla eau 46° S 48° S 50° S 52 ° S 54° S 56° S 72 ° W 60° W 48° W Longi ude 36° W 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 Figu e 8. AVISO SSH o he MOC-Aus al c uise. Isolines ha e a sepa a ion o 5 cm. low ba hyme y. Thus, he only no hwa d con ibu ion o his s a um is done by he PF, which is s onge in 2010 han in 1999 om he SAMW s a um o he bo om. The o al mass anspo s o he PF a e 37.3 S in 2010 s. 24.9 S in 1999. Figu e 9 exhibi s he e ical s uc u e o he calcula ed mass anspo in he di e en laye s, which de ine each wa e mass. The geos ophic mass anspo s in he AL- BATROSS (1999) and MOC-Aus al (2010) hyd og aphic c uises beha e likewise ac oss he wa e column. The mass anspo s om he su ace o he UCDW s a um a e a - ec ed by a no iceable no hwa d ne mass anspo dec ease o 10.6 S om 1999 o 2010. In con as , he LCDW ex- hibi s an inc ease o 1.4 S in he no hwa d low in 1999 and 2010. To pu all he es ima ed mass anspo s in o con ex , he mon hly 1993–2016 a e aged geos ophic eloci ies om AVISO a e in e pola ed o he s a ion pai s o bo h c uises and in eg a ed by using he s a ions’ dis ance and he a e age dep h o he SASW s a um (50 m). This is shown in Fig. 10, whe e he 1993–2016 a e age o all Feb ua ies (Ap ils) is con as ed wi h he es ima ed ela i e anspo o he MOC- Aus al (ALBATROSS) uppe s a um. I is seen ha he e is no signi ican clima ological di e ence be ween he es i- ma ions o bo h mon hs. Hence, he posi ions and anspo s (exp essed as mean ±s anda d de ia ion) o he SAF and PF in he AVISO-de i ed anspo s a e 2.5 ±0.5 S a he lon- gi udinal ange 52.97–56.96◦W and 1.1 ±0.7 S a 47.49– 51.34◦W, espec i ely. The SAF AVISO es ima ed anspo is app oxima ely he a e age be ween he SASW anspo s o ALBATROSS (3.2 S ) and MOC-Aus al (1.9 S ). The PF obse ed in he AVISO da a co e s a wide ange o longi- udes han he ones o he hyd og aphic su eys. I s anspo is sligh ly smalle han o he ALBATROSS c uise (2.0 S ) and non-signi ican ly di e en om MOC-Aus al (1.6 S ) a he SASW s a um (Table 1). 4 Discussion and conclusions The decadal di e ences in he Falkland Pla eau a e s udied om ull-dep h hyd og aphic da a collec ed du ing he AL- -5 -4 -3 -2 -1 0 1 2 3 4 5 Mass anspo (S ) Su ace 26.90 27.20 27.60 28.00 28.26 Bo om γn (kg m-3) SASW SAMW AAIW/AASW UCDW LCDW WSDW MOC Aus al Alba oss Figu e 9. Rela i e geos ophic mass anspo pe laye ac oss he ALBATROSS and MOC-Aus al sec ions. -58 -56 -54 -52 -50 -48 -46 -44 -42 -40 Longi ude (° W) 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Accumula ed anspo (S ) Ap il AVISO ALBATROSS Feb ua y AVISO MOC Aus al Figu e 10. Wes - o-eas accumula ed ela i e geos ophic mass anspo om AVISO a e aged om 1993 o 2016, oge he wi h hei s anda d de ia ions: Ap il (g ey solid line) and Feb ua y (black solid line). Fo his calcula ion, he dep h o 50 m has been consid- e ed o compu e he mass anspo ha co esponds o he SASW s a um. Dashed lines a e he wes - o-eas accumula ed ela i e geos ophic mass anspo s shown in Fig. 7 o he SASW o AL- BATROSS (g ey) and MOC-Aus al (black) ca ied ou in Ap il and Feb ua y, espec i ely. BATROSS (Ap il 1999) and MOC-Aus al (Feb ua y 2010) c uises. Wa e mass changes a e explo ed in e ms o changes along neu al su aces and changes due o e ical displace- men s o γnsu aces, applying he model p oposed by Bindo and McDougall (1994). Va iabili y in he SAF and PF loca ion and mass anspo is in e ed om ela i e geos ophic eloci ies es ima ed by using he sea bo om as he le el o no mo ion. Ocean Sci., 13, 577–587, 2017 www.ocean-sci.ne /13/577/2017/ M. D. Pé ez-He nández e al.: Di e ences be ween 1999 and 2010 ac oss he Falkland Pla eau 585 ( a) Win e a e age ----- 32° 5 40° 5 48° 5 56° 5 64°5 140° W 120° w 100° w 80° W 60° W 40° W 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 32° 5 40° 5 48° 5 56° 5 ( b) Win e anomaly 1998 . . . � • • ••• : • i • • .••. • ••• • .•• : • : •••• ,· • : • : • • ••• ,· • : • • •••.•• ; • • .••.•• .- • � •••• : • � • ·,. ' � ' ' ' ' ' ' ' � ' ,,,,,111 111111 1 64° 5 32° 5 40° 5 48° 5 56° 5 140° W I I ' I I I I I I ! � 120° w 100° w 80° W ( c) Win e anomaly 2009 . . . ·.· ·.· •: • � • ·.· •: • ·. • ·.· • _- •; • ·, • ·:· •: • ·, • ·, • • ,· •; •:. • ._. "-! •.:, • �- •.,• •.,:_ • ,.. •,• • (•-.,·"I" •I•·. . : . /. ·'· . ! �. � ..•. · -�. ,. .. ., .. ., . ,, .• ,. "". • ..... ·- I I I I � ' .... ' .;: ' • _..__,,,.I •• ; •• , •• I . '· . , . . i .. l . J ..•.• I.. l .. . . . ) . ·'· . ": . :--.. -. .,. i' ., 64° 5 .................................. . ' ' ... ..;: 140° W 120° w 100° w 80° W -0.05 0 � ' ' ' ' ' ' ·- -�· ',, ' ' ' 60° W 40° W 60° W 40° W 0.05 Figu e 11. Maps o NCEP-NCAR (a) win e (July–Sep embe ) mean wind s ess in Pa (a ows; colo indica es magni ude) o he ime pe iod 1998–2010. (b) Win e wind s ess anomaly (Pa) o he yea 1998. (c) Same as (b) bu o 2009. The SASW s a um p esen s a wide ange o salini ies and empe a u es in 1999 han in 2010, as shown in he θ/S di- ag am. In spi e o his, he θ/S isoba ic changes show an inc ease in su ace empe a u es and salini ies ma ching he Bindo and McDougall (1994) model’s esul o changes along neu al su aces. The model also exhibi s shoaling o he isopycnals. The mos plausible sou ce o hese di e - ences is he ac ha he hyd og aphic c uises ook place in di e en seasons (ALBATROSS in aus al all and MOC- Aus al in aus al summe ). Hence, he seasonal he mocline has p obably changed i s dep h due o he di e en seasonal hea ing and p ecipi a ion. The sca ce in si u obse a ions in he a ea do no allow any u he conclusions. SAMW expands o e a la ge dep h ange and p esen s a wide ange o salini ies in 1999 han in 2010 (Figs. 2 and 4). In con as , he θ/S diag am and isoba ic changes o he AAIW/AASW s a um show a dec ease in empe a- u e and salini y in 2010 when he AAIW/AASW occupies a highe dep h ange (Figs. 2 and 4). As bo h Bindo and Mc- Dougall (1994) model es ima ions (wi h and wi hou on al zones) ag ee in ha he changes in he in e media e s a a a e due o he displacemen o γnsu aces, some changes a e likely o ha e occu ed be ween 1999 and 2010 in he Falk- land Pla eau. The Bindo and McDougall (1994) model e- eals a deepening o he isoneu als a hese le els, whe e he AAIW/AASW s a um occupies a highe dep h ange han SAMW. An explana ion o changes in hose s a a can be ound in Na ei a Ga aba o e al. (2009). Figu e 11a shows he mean wind s ess o he win e s in he pe iod 1998– 2010. This igu e is analogous o Fig. 11a o Na ei a Ga a- ba o e al. (2009). In he clima ological mean a con inuous wind s ess magni ude sp eads wes om Sou h Ame ica (Fig. 11a). Figu e 11b and c exhibi s he p e ious win e anomalies o he ALBATROSS and MOC-Aus al c uises, espec i ely. These anomalies look e y di e en be ween hemsel es. Figu e 11b shows a la ge eas wa d (posi i e) wind s ess anomaly in he Sou h Paci ic. Na ei a Ga aba o e al. (2009) sugges ha his s uc u e causes a shi in he SAMW o ma ion a ea. This ma ches wi h he changes ob- se ed in Figs. 2 and 4, whe e he SAMW s a um a ea is educed. I also ag ees wi h he isoba ic changes epo ed, a dec ease in empe a u e o 0.8 ◦C and an inc ease in salini y o 0.02 om 1999 o 2010. Na ei a Ga aba o e al. (2009) also epo ed ha he 1998 wind s ess anomaly pa e n shown in Fig. 11b gene a es a shu down o he AAIW o ma ion. Due o his, a minimum www.ocean-sci.ne /13/577/2017/ Ocean Sci., 13, 577–587, 2017