Internal Multidecadal and Interdecadal Climate Oscillations: Absence of Evidence Is No Evidence of Absence [original]

Internal Multidecadal and
Interdecadal Climate Oscillations:
Absence of Evidence Is No Evidence
of Absence
Gisela Müller-Plath *
Department of Psychology and Ergonomics, Technische Universität Berlin, Berlin, Germany
Keywords: Paci ﬁ c Decadal Oscillation, Atlantic Mul tidecadal Oscillation, sulfate aerosols, climate osci llations,
multi-taper method of singular value decomposition spectral analysis (MTM-SVD), CMIP5 model simulation
INTRODUCTION
The present paper contributes a critical commentary on the recent ﬁ nding by Mann, M. E., Steinman,
B. A. and Miller, S. K (2020). Absence of internal multidecadal and interdecadal oscillations in
climate model simulations. Nat. Commun. 11, 1 – 9.
Climate oscillations are recurring large-scale ﬂ uctuations in the surface temperatures of the
oceans in connection with the atmosphere. This commentary focuses on the Paci ﬁ c Decadal
Oscillation (PDO, interdecadal timescale) and the Atlantic Multidecadal Oscillation (AMO,
multidecadal timescale), which have been regarded as intrinsic climate drivers on the adjacent
continents in numerous studies based on observations and paleoclimate reconstructions ( Henley,
2017 ; O ’ Reilly et al., 2017 ). In a recent paper, Michael E. Mann and colleagues ( Mann et al., 2020 ,
hereafter M20) fail to ﬁ nd a PDO signal in global measured and modeled temperatures that is
statistically different from noise. They further propose that the signi ﬁ cant AMO-like signal is mainly
due to anthropogenic aerosols in the 20th century, and to statistical artifacts before. Therefore they
doubt the intrinsic nature of the two oscillations. The present paper shows that M20 ’ s results are
largely artifacts themselves with issues ranging from using inadequate data and referencing improper
literature on anthropogenic aerosols with regards to the AMO to inappropriately interpreting the
results with regards to the PDO.
After brie ﬂ y sketching the rationale and method of M20, I will elaborate on these three points.
M20 (p. 3) argue that any truly oscillatory AMO or PDO signals should generate a spatially
coherent and large-scale variability pattern in the climate system with a narrowband signature in the
frequency domain. They search for such signals in global (observed and modeled) temperature grids
of different time lengths with the multi-taper method of singular value decomposition (MTM-SVD),
which was developed and widely applied by Mann and Park ( Mann and Park, 1994 ; Mann et al.,
1995 ; Mann and Park, 1999 ). Signi ﬁ cance tests of the test statistic LFV (local fractional variance) are
carried out with Monte Carlo simulations generated according to the null hypothesis of colored (red)
noise. The method can generally be applied to reconstruct the time course and the spatial pattern of
any potential oscillatory climate signal.
INADEQUATE DATA
M20 examine three global sets of temperature data for oscillatory signals, all spanning a minimum
length of 158 years: Control simulations (control runs of the IPCC model ensemble CMIP5, using
pre-industrial conditions of the atmosphere without any external forcing so that “ any apparently
Edited by:
Jing-Jia Luo,
Bureau of Meteorology, Australia
Reviewed by:
Elsa Mohino,
Complutense University of Madrid,
Spain
*Correspondence :
Gisela Müller-Plath
gisela.mueller-plath@ tu-berlin.de
Specialty section:
This article w as submitted to
Atmospheric Science,
a section of the journal
Frontiers in Earth Science
Received: 05 May 2020
Accepted: 04 November 2020
Published: 26 November 2020
Citation:
Müller-Plath G (2020) Internal
Multidecadal and Interdecadal Climate
Oscillations: Absence of Evidence Is
No Evidence of Absence.
Front. Earth Sci. 8:559337.
doi: 10.3389/feart.2020.559337
Frontiers in Earth Scien ce | www.frontiersin.o rg November 2020 | Volume 8 | Article 559337 1
OPINION
published: 26 November 2020
doi: 10.3389/feart.2020.559337

oscillatory behavior must arise from internal variability ” ),
historical observations (annualized global monthly average
surface temperatures from the HadCRUT4 land and ocean
surface temperature dataset), and historical simulations (IPCC
model ensemble CMIP5, containing external anthropogenic and
natural forcing). They ﬁ nd robust signi ﬁ cant spectral peaks in the
multidecadal AMO range (period 40 – 70 years) in the historical
observations and the historical simulations, but not in the control
simulations. However, the latter data set is the only one that
covers a greater length of time, with almost half of the model runs
spanning 500 years or more.
The absence of robust multidecadal AMO oscillations in the
control simulations stands in sharp contrast to numerous studies
ﬁ nding the opposite in paleoclimatic data ( Kerr, 2000 ; Gray et al.,
2004 ; Chylek et al., 2011 ; Lanci and Hirt, 2015 ). Even with the
same MTM-SVD method, the main author of M20 himself
formerly identi ﬁ ed robust and signi ﬁ cant AMO frequencies in
four independent sets of global proxy temperature data ( Mann
and Park, 1994 ). The most obvious explanation for the
discrepancy is that control runs of the CMIP5 models have
little to do with reality ( Power et al., 2017 ). Since it is unclear
to which extent the modeled “ internal variability ” re ﬂ ects real
conditions, the control simulations cannot be trusted unless their
results were validated with paleoclimate data.
On the other hand, the historical observations and the
hi st or ic al si mu la ti on s a re in su f ﬁ ci en t in leng t h in orde r t o
de te ct AM O freq ue nc i es wi th su f ﬁ ci e nt st ati st ic a l po wer . Th ey
co ve r bare ly tw o cy cl es o f the put at iv e os cil la ti o n, w hich ma y span
80 yea rs and mo re ( Sc hle sing er and Ram anku tty , 1994 ) and may
co nsis t of a bro ade r band of lo w-fr eque ncy si gnals ( O ’ Re ill y et al. ,
20 17 ; Su tto n et al., 20 18 ) th an ca n be id ent i ﬁ ed wi th the shor t tim e
se ri es. Fu rth ermo re, M20 le ave s some c onfu sion abo ut the a ctua l
le ngt h of th ei r co nt ro l and h isto rica l sim ulat ion s. Al thoug h they
st ate i n the t ext tha t a min imu m leng th of 158 year s is requi red a nd
fu l ﬁ lled by N  44 con tr ol ru ns and N  118 hist or ica l run s of the
CM IP 5 mode ls, Ta ble 1 in thei r S up plem ent rev eals th at only N 
42 (n ot 44 ) from th e altog et he r N  47 cont rol sim ula tio ns and
on ly N  8 (no t 118) fr om the alt oge ther N  16 4 his to ri ca l
si mu la tion s satis ﬁ ed th e requi reme nt of a minim um leng th of
15 8 ye ars. S o e ven tho ugh the la tter da ta set show ed a rob ust
si gni ﬁ can t AMO fr eque ncy aro un d a ∼ 45 yea r pe riod , ot her
mu lt ide cada l freq uenc ies mi ght ha ve bee n miss ed.
IMPROPER LITERATURE REFERENCE
Having found signi ﬁ cant AMO frequencies only in the historical
observations and simulations for the industrial time, M20 suggest
that this multidecadal ﬂ uctuation is due to anthropogenic
aerosols rather than to an intrinsic climate oscillation. They
argue 1) that no multidecadal ﬂ uctuation is present under
preindustrial conditions, 2) that in industrial times its phase is
synchronized across three independent global time series, which
would be unlikely if it were an intrinsic oscillation, whereas 3) the
ﬂ uctuation with positive (warm) peaks near 1940 and 2000 and a
negative (cool) peak near 1980 coincides with the response of the
climate system to anthropogenic sulfate aerosol emissions. The
ﬁ rst part of the argument is questioned above. The second refers
to the observation that three global time series, namely two
speci ﬁ c models and the historical observations, are roughly in
phase (Figure 3 of M20). I agree with the authors that this would
be unlikely if they were (stochastically) independent, but
“ internal ” / “ intrinsic ” is not necessarily the same as
“ independent ” : The same phase points to a structural
relationship, but it says nothing about whether its cause is
external or internal to the climate system. For the third part of
the argument, M20 refer to a recent paper by Kasoar et al. (2018) .
However, this referenced paper supports only the spatial
correspondence of sulfate aerosol effects and the putative
AMO signal, both emphasizing the North Atlantic region.
With regard to the alleged temporal correspondence, the
reader is referred to Figure 3 of M20, which not only leaves
several questions open, but also ignores a substantial refutation in
the literature: M20 do not provide enough detail to clearly
understand what they are plotting in the time series of their
Figure 3. Is it just one realization or an average of the two and ﬁ ve
members that they have for the MPI-ESM-LR and HadGEM2-ES
models? If it is the latter, they are methodologically canceling out
any internal variability the model could show. If it is the former,
on which criteria did they select the member, and do the rest
behave similarly? Furthermore, in the MPI-ESM-LR model
(Figure 3C of M20) the signal amplitude and the signal-to-
noize ratio appear to be very low, which is re ﬂ ected in the
small percentage of explained variance. I doubt that this signal
exceeds the statistical signi ﬁ cance limit, which the authors do not
comment on. Regarding the HadGEM2-ES model, Booth et al.
showed already in 2012 that it closely reproduced the AMO-like
multidecadal North Atlantic sea surface temperature (NASST)
variability in the 20th century, and claimed that aerosols caused
this variability. However, Zhang et al. (2013) refuted this claim on
various methodological grounds, for example by comparing the
heat content anomaly of the upper ocean in the North Atlantic
with the HadGEM2-ES model with constant aerosols vs. all
drivers, or by showing differences between observed and
modeled spatial patterns of multi-decadal SST changes inside
and outside the North Atlantic, and observed and modeled
anomalies in salinity in the subpolar North Atlantic. Due to
the large and multivariate discrepancies in the mechanisms,
Zhang et al. concluded that the aerosol effects simulated by
HadGEM2-ES cannot be responsible for the multi-decadal
temperature variations observed in the North Atlantic in the
20th century. M20 make the same claim again now without
mentioning this debate in the literature.
INAPPROPRIATE INTERPRETATION
In the in terd eca dal ti me ran ge att ribu ted to th e PDO (16 – 20 yea rs
acc ord ing to th e aut ho rs), M20 fo und no ro bust si gni ﬁ can t spec tra l
pe aks in an y of th e thr ee dat a sets . Ag ain , thi s stan ds in sha rp
co ntra st to a la rge b ody of li te ra ture ( for a re view se e e. g., He nley ,
20 17 ), in clud ing pr evio us wor k by the ma in aut hor us ing the sa me
MT M- SVD meth od ( Ma nn and Park , 1994 ). M20 ex pl ain th e
di sc re pa ncy b y the t ime wi ndow b eing la rg er in thei r pres ent th an
Frontiers in Earth Scien ce | www.frontiersin.o rg November 2020 | Volume 8 | Article 559337 2
Müller-Plath Climate Oscillations Not Just Noise

in t heir fo rmer work, rend eri ng the pr esen t resu lt s mor e reli abl e.
Ho wev er, on cl ose r in spe cti on the re spec tive S uppl ement ary Fi gur e
S2 in M20 su gg ests an ot he r int erp reta tio n: The mo vi ng w indo w
sh ow s tha t the fr equ enci es on th e in terd ecada l sca le are no t
co nsta nt over ti me - in othe r words , narr owba nd PD O
os ci llati ons are ep isod ic (see F ig ure 1 ). This is fu lly con sist ent
wit h the wor k of Foll and et al. (2 002) , whi ch M2 0 t hems elv es
me nti on in th eir in tro duct ory di scus sion on w he the r the PDO ha s
a bro ad or na rr ow fr equ en cy ba nd. Howe ver, wi th refe renc e to
th eir o wn form er wor k ( Mann a nd Park , 1994 ; Man n and Pa rk,
19 99 ) the y deci de a prio ri tha t the PD O is con ﬁ ne d t o a
na rr owba nd 16 – 20 year peri od. Wh en t hey late r ﬁ nd th at this
na rr owba nd freq uenc y is not rob ust ov er a larg er tim e windo w,
th ey c oncl ude th at ther e is no in tri nsic PD O at all . How ever , a mo re
app ropr iate i nte rpre ta ti on wou ld have be en that th eir ass umpt ion
of a con tinuo us na rro wba nd PDO freq uenc y was pre matu re and
sh ou ld be re vise d.
CONCLUSION
Altogether, I conclude that the paper M20 is not advancing our
understanding of the nature of multi- and interdecadal
oscillations such as the AMO and PDO.
AUTHOR CONTRIBUTIONS
The author con ﬁ rms being the sole contributor of this work and
has approved it for publication.
ACKNOWLEDGMENTS
The author would like to thank a reviewer for valuable hints
concerning argumentation and literature.
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Con ﬂ ict of Interest: The author declare that the research was conducted in the
absence of any commercial or ﬁ nancial relationships that could be construed as a
potential con ﬂ ict of interest.
Copyright © 2020 Müller-Plath. This is an open-access article distributed under the
terms of the Creative Commons Attribution License (CC BY). The use, distribution
or reproduction in other forums is permitted, pro vided the original author(s) and the
copyright owner(s) are credited and that the original publication in this journal is
cited, in accordance with accepted academic practice. No use, distribution or
reproduction is permitted which does not comply with these terms.
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Müller-Plath Climate Oscillations Not Just Noise

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