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Abstract

In this study the observed non-linearity in the spatial pattern and time evolution of El Niño Southern Oscillation (ENSO) events is analyzed. It is shown that ENSO skewness is not only a characteristic of the amplitude of events (El Niños being stronger than La Niñas) but also of the spatial pattern and time evolution. It is demonstrated that these non-linearities can be related to the non-linear response of the zonal winds to sea surface temperature (SST) anomalies. It is shown in observations as well as in coupled model simulations that significant differences in the spatial pattern between positive (El Niño) versus negative (La Niña) and strong versus weak events exist, which is mostly describing the difference between central and east Pacific events. Central Pacific events tend to be weak El Niño or strong La Niña events. In turn east Pacific events tend to be strong El Niño or weak La Niña events. A rotation of the two leading empirical orthogonal function modes illustrates that for both El Niño and La Niña extreme events are more likely than expected from a normal distribution. The Bjerknes feedbacks and time evolution of strong ENSO events in observations as well as in coupled model simulations also show strong asymmetries, with strong El Niños being forced more strongly by zonal wind than by thermocline depth anomalies and are followed by La Niña events. In turn strong La Niña events are preceded by El Niño events and are more strongly forced by thermocline depth anomalies than by wind anomalies. Further, the zonal wind response to sea surface temperature anomalies during strong El Niño events is stronger and shifted to the east relative to strong La Niña events, supporting the eastward shifted El Niño pattern and the asymmetric time evolution. Based on the simplified hybrid coupled RECHOZ model of ENSO it can be shown that the non-linear zonal wind response to SST anomalies causes the asymmetric forcings of ENSO events. This also implies that strong El Niños are mostly wind driven and less predictable and strong La Niñas are mostly thermocline depth driven and better predictable, which is demonstrated by a set of 100 perfect model forecast ensembles.
Analysis of the Non-linearity in the !"
Pattern and Time Evolution of El Niño #"
Southern Oscillation !$"
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1. Introduction!;U"
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2. Data!and!Models!UR"
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3. Observed!El!Niño!pattern!non=linearity!!##"
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8. Summary!and!Discussion!H##"
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!R$R"
Acknowledgements!R$U"
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6&3>933&,-3" *-6" >,))'-(3G" o'" *83," 8&]'" (," (?*-]" (\," *-,-5),93" +'@'+''3" @,+"R;!"
(?'&+" >,))'-(3G" 1?&3" 3(965" \*3" 39VV,+('6" 25" (?'" FL7" 7'-(+'" ,@" P`>'88'->'" &-"R;#"
78&)*('" =53(')" =>&'->'" N7P!!SSS!S#RQ0" (?'" FL7" V+,b'>(" uv4'5,-6" (?'" 8&-'*+"R;$"
65-*)&>3",@"(?'"P8"[&-,"=,9(?'+-"K3>&88*(&,-uN%a!#S!S!;;#Q"*-6"(?'"%'9(3>?'"R;;"
:,+3>?9-."O')'&-3>?*@("N%K!S$Rr<Z!QG"R;<"
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"!R;H"
References!R;R"
"#$! %&! '&$! (&! )&! *+,$! -&! %&! ./0$! 1&! 1&! -2#$! +#3! '&! %&! .+#0$! 45567! 89! :2#;! <+! :2#+!R;U"
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2*3'6",-"(?,3'"@,+'>*3(3"'-3')28'3"(?*("?*C'"*-"[Y[K$"==1"*-,)*85"q"!GSrnZ!GS"!S<#"
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"!S<<"
Figure!14j"7,)V,3&('"I,'C),'88'+"6&*.+*)3",@"(?'"'g9*(,+&*8"a*>&@&>"*3"&-":&.G!!"!S<E"
29("@,+"(?'"LP7IKcZaF7K"3&)98*(&,-G"!S<H"
Figure 1
a) EOF-1 (44%) b) EOF-2 (10%)
c) PDF PC-1 d) PDF PC-2
42 0 2 4
0
50
100
150
PC1
probability density
γ1 = 0.2
γ2 = 0.1
Figure 1: (a) EOF-1 and (b) EOF-2 patterns of the HADISST linear detrended monthly
mean SST anomalies from 1950 to 2010 are shown in the upper panels. The associated PC
probability density distribution function of (c) PC-1 and (d) PC-2 with the distribution
shape parameters skewness (γ1)andkurtosis(γ2)areshowninthelowerpanels.
Figure 2
a) strong El Ni˜no b) strong La Ni˜na c) dierence (a)-(b) (0.8)
d) weak El Ni˜no e) weak La Ni˜na f) dierence (e)-(d) (0.8)
g) dierence (a)-(d) (0.8) h) dierence (e)-(b) (0.7) i) EOF-2
Figure 2: Composites of HADISST linear detrended monthly mean SST anomalies from
1950 to 2010 based on NINO3.4 SST anomalies. (a) for all data with NINO3.4 SST
anomalies >1oK, (b) with NINO3.4 SST anomalies <1oK, (d) with NINO3.4 SST
anomalies >0.5oKand <1oKand (e) with NINO3.4 SST anomalies <0.5oKand
>1oK. All Composites are normalized by their mean NINO3.4 SST anomalies. Thus (b)
and (e) are shown with the reversed sign of the mean SSTs, as the mean NINO3.4 SST
anomalies of these composites is negative. (c) is the dierence between (a) (b), (f) the
dierence between (e) (d), (g) the dierence between (a) (d) and (h) is dierence between
(e) (b). Panel (i) shows the EOF-2 pattern of Fig.1b for comparison. The headings in
(c), (f), (g) and (h) show the dierence pattern correlation values with the EOF-2 pattern.
Units are in K/K(in NINO3.4 region) for (a)-(h) and non-dimensional for (i).
Figure 3
a) PCs time series
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
4
2
0
2
4
time
PC values
PC1
PC2
b) composite strong El Ni˜nos c) composite strong La Ni˜nas
20 10 0 10 20
2
1
0
1
2
3
time relative to Dec. [mon]
PCs values
PC1
PC2
20 10 0 10 20
2
1
0
1
2
time relative to Dec. [mon]
PCs values
Figure 3: (a) time series of PC-1 (blue) and PC-2 (red). The shaded areas mark the ±1
intervals, which roughly matches the composites selection criteria for Fig. 2. The mean
lag/lead time evolution of the PC-1 and PC-2 time series for (b) composite mean time
evolution for the 6 strongest El Ni˜no events (1966, 1973, 1983, 1988, 1992 and 1998) and
(c) the same for the 6 strongest La Ni˜na events (1955, 1971, 1974, 1989, 1999 and 2008) are
shown. The shaded areas around the PCs mark the 90% confidence interval. The x-axis
values refer to the month relative to December of each ENSO event.
Figure 4
a) PC-1 vs. PC-2 b) rotated PC
LaNina vs. PC
ElNino
321 0 1 2 3 4
4
3
2
1
0
1
2
3
4
PC1
PC2
PC2(PC1)
mean PC2(PC1 intervals)
quadratic fit
321 0 1 2 3
3
2
1
0
1
2
3
4
5
PCLa Nina
PCEl Nino
c) El Ni˜no Pattern d) (-) La Ni˜na Pattern
e) PDF PC
ElNino f) PDF PC
LaNina
42 0 2 4
0
50
100
150
PCEl Nino
probability density
γ1 = 1.3
γ2 = 2.7
42 0 2 4
0
50
100
150
PCLa Nina
probability density
γ1 = 0.6
γ2 = 0.2
Figure 4:(a) scatter plot of tropical Pacific SST PC-1 and PC-2datapairs(blackdots)for
each linear detrended monthly mean SST anomaly from 1950 to 2010 corresponding to the
EOFs of Fig.1. In addition the mean PC-2 values for 0.2 intervals of PC-1 are shown (blue
open circles). The non-linear model of PC-2 as function of PC-1 (eq.[1]) is marked by a red
line. (b) is a counter clockwise 45o rotation of (a), defining the new rotated axes PC
ElNino
and PC
LaNina. (c) shows the pattern corresponding to the rotated PC
ElNino and (d) the
pattern corresponding to the rotated PC
LaNina.(e)and(f)areasinFig.1candd,butfor
(e) PC
ElNino and (f) PC
LaNina.
Figure 5
a) strong El Ni˜no b) strong La Ni˜na c) dierence (a)-(b) (0.3)
d) weak El Ni˜no e) weak La Ni˜na f) dierence (e)-(c) (0.0)
g) dierence (a)-(d) (0.2) h) dierence (e)-(b) (-0.5) i) EOF-2
Figure 5:Composites of HADISST SST anomalies as Fig.2, but for the data reconstructed
with the 20 leading EOF-modes and using PC 2residual (see eq.[2]) instead of PC-2, thus
excluding the non-linear interaction between the EOF-1 and EOF-2 modes.
Figure 6
Strong Events
a) El Ni˜no b) (-) La Ni˜na c) dierence
longitude
time relative to dec. [mon]
150 200 250
20
15
10
5
0
5
10
15
20
longitude
150 200 250
20
15
10
5
0
5
10
15
20
longitude
150 200 250
20
15
10
5
0
5
10
15
20
1
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
1
Wea k Event s
d) El Ni˜no e) (-) La Ni˜na f) dierence
longitude
time relative to dec. [mon]
150 200 250
20
15
10
5
0
5
10
15
20
longitude
150 200 250
20
15
10
5
0
5
10
15
20
longitude
150 200 250
20
15
10
5
0
5
10
15
20
1.5
1
0.5
0
0.5
1
1.5
Figure 6: Composite Hoevmoeller diagrams of the equatorial (averaged between 5oSto
5oN)PacicSSTanomaliesfor(a)strong(PC
ElNino >1.0 in December) and (d) weak ((0.5
<PC-1 <1.0 in December) El Ni˜no events and (b) strong ((PC
LaNina <-1.0 in December)
and (d) weak -1.0 <PC-1 <-0.5 in December) La Ni˜na events are shown. All Composites
are normalized by their mean NINO3.4 SST anomalies in the Decembers that passed the
selection criteria. Thus (b) and (e) are shown with the reversed sign of the mean SST
anomalies, as the mean December NINO3.4 SST anomalies of these composites is negative.
(c) is the dierence between (a) (b) and (f) the dierence between (d) (e). The y-axis
values refer to the month relative to the Decembers that passed the selection criteria for
each ENSO composite. Units are in K/K (in NINO3.4 region).
Figure 7
SLP
a) El Ni˜no b) (-) La Ni˜na c) dierence
longitude
time relative to dec. [mon]
180 240
20
15
10
5
0
5
10
15
20
longitude
180 240
20
15
10
5
0
5
10
15
20
longitude
[hPa/K]
180 240
20
15
10
5
0
5
10
15
20
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
Zonal Wind
d) El Ni˜no e) (-) La Ni˜na f) dierence
longitude
time relative to dec. [mon]
180 240
20
15
10
5
0
5
10
15
20
longitude
180 240
20
15
10
5
0
5
10
15
20
longitude
[m/s/K]
180 240
20
15
10
5
0
5
10
15
20
1
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
1
Thermocline Depth
g) El Ni˜no h) (-) La Ni˜na i) dierence
longitude
150 200 250
20
15
10
5
0
5
10
15
20
longitude
150 200 250
20
15
10
5
0
5
10
15
20
longitude
[m/(m/s)]
150 200 250
20
15
10
5
0
5
10
15
20
40
30
20
10
0
10
20
30
40
Figure 7: As Fig.6 a-c, but for (a-c) SLP and (d-f) zonal wind stress from NCEP reanalysis
and for (g-i) thermocline depth from the MPI-OM reanalysis.
Figure 8
CMIP3 models selection criteria
0.4 0.2 0 0.2 0.4 0.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1
1: BCCR
2
2: CCCMA T63
3
3: CCCMA T47
4
4: CNRM
5
5: CSIRO MK3.0
6
6: CSIRO MK3.5
7
7: GFDL 2.0
8
8: GFDL 2.1
9
9: GISS CM2.0
10 10: GISS CM2.1
11
11: GISS AOM
12
12: IAP
13
13: INGV
14
14: INM
15
15: IPSL
16
16: MIROC hires
17
17: MIROC medres
18
18: MIUB
19
19: MPI
20
20: MRI
21
21: NCAR CCSM3
22
22: NCAR PCM
23
23: UK HadCM3
24
24: UK HadGEM1
normalized eq. eastwest difference [K/KNINO3]
normalized laglead difference [K/KNINO3]
Figure 8: CMIP3 models selection criteria eq and time for two dierent observational
data sets (circle for HADISST and triangle for ERSST) and for the24CMIP3models. The
shaded regions are considered to be dierent from observations.
Figure 9
a) PC-1 vs. PC-2 b) rotated PC
LaNina vs. PC
ElNino
321 0 1 2 3 4
4
3
2
1
0
1
2
3
4
PC1
PC2
PC2(PC1)
quadratic fit
321 0 1 2 3 4
4
3
2
1
0
1
2
3
4
PCLa Nina
PCEl Nino
PC2(PC1)
quadratic fit
c) Pattern El Ni˜no d) (-) Pattern La Ni˜na
e) PDF PC
ElNino f) PDF PC
LaNina
42 0 2 4
0
5
10
15
PCEl Nino
probability density
γ1 = 0.7
γ2 = 2.3
42 0 2 4
0
5
10
15
PCLa Nina
probability density
γ1 = 0.7
γ2 = 1.1
Figure 9: Scatter plot of tropical Pacific SST PC -1 and PC-2 data pairs as in Fig.4, but
for the combined data set of the 4 CMIP3 models that passed the selection criteria eq and
time.
Figure 10
SST
a) El Ni˜no b) (-) La Ni˜na c) dierence
longitude
time relative to dec. [mon]
150 200 250
20
15
10
5
0
5
10
15
20
longitude
150 200 250
20
15
10
5
0
5
10
15
20
longitude
150 200 250
20
15
10
5
0
5
10
15
20
1
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
1
Zonal Wind
d) El Ni˜no e) (-) La Ni˜na f) dierence
longitude
time relative to dec. [mon]
180 240
20
15
10
5
0
5
10
15
20
longitude
180 240
20
15
10
5
0
5
10
15
20
longitude
[m/s/K]
180 240
20
15
10
5
0
5
10
15
20
1.5
1
0.5
0
0.5
1
1.5
Thermocline Depth
g) El Ni˜no h) (-) La Ni˜na i) dierence
longitude
time relative to dec. [mon]
180 240
20
15
10
5
0
5
10
15
20
longitude
180 240
20
15
10
5
0
5
10
15
20
longitude
[m/(m/s)]
180 240
20
15
10
5
0
5
10
15
20
15
10
5
0
5
10
15
Figure 10: Composite Hoevmoeller diagrams of the equatorial Pacific as in Fig.6 a-c, but
for combined data set of the 4 CMIP3 models for (a-c) SST and (d-f) zonal wind stress
and for (g-i) thermocline depth. The thermocline depth composites are normalized by the
mean zonal wind stress anomalies in the central Pacific (160oEto 120oW)andnotbythe
NINO3.4 SST anomalies.
Figure 11
RECHOZ SST
a) El Ni˜no b) (-) La Ni˜na c) dierence
longitude
time relative to dec. [mon]
150 200 250
20
15
10
5
0
5
10
15
20
longitude
150 200 250
20
15
10
5
0
5
10
15
20
longitude
150 200 250
20
15
10
5
0
5
10
15
20
1
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
1
Zonal Wind
d) El Ni˜no e) (-) La Ni˜na f) dierence
longitude
time relative to dec. [mon]
150 200 250
20
15
10
5
0
5
10
15
20
longitude
150 200 250
20
15
10
5
0
5
10
15
20
longitude
[m/s/K]
150 200 250
20
15
10
5
0
5
10
15
20
4
3
2
1
0
1
2
3
4
Figure 11: Composite Hoevmoeller diagrams of the equatorial Pacific as in Fig.6 a-c but for
the RECHOZ model (a-c) SST anomalies and (d-f) zonal wind anomalies. As a selection
criteria the NINO3.4 SST is used instead of the PC
ElNino and PC
LaNina as no such
corresponding non-linear PCs exist in the RECHOZ model.
Figure 12
RECHOZ model
a) SST evolution b) Thermocline depth
20 10 0 10 20
1
0.5
0
0.5
1
lag lead [mon]
NINO3 SST [K/K]
elnino
lanina
20 10 0 10 20
8
6
4
2
0
2
4
6
8
lag lead [mon]
thermocline depth [m/K]
REOSC-MC model
c) SST evolution d) Thermocline depth
20 10 0 10 20
1
0.5
0
0.5
1
lag lead [mon]
NINO3 SST [K/K]
20 10 0 10 20
10
5
0
5
10
lag lead [mon]
thermocline depth [m/K]
Figure 12: Lag-lead composites of strong El Ni˜no and La Ni˜na events defined as in Fig. 11
for (a) NINO3 SST anomalies and (b) equatorial mean thermoclinedepthanomaliesofthe
RECHOZ simulation. (c) and (d) are as in (a) and (b), but for the linear (dotted lines) and
non-linear (solid lines) REOSC-MC model.
Figure 13
RECHOZ SST correlation skill
0 2 4 6 8 10 12
0
0.2
0.4
0.6
0.8
1
lead time [mon]
anomaly correlation skill
all
strong El Ninos
strong La Ninas
Figure 13: Anomaly correlation skills for RECHOZ perfect modelforecastensembles. The
strong El Ni˜no/La Ni˜na (red/blue) anomaly correlation skills are based on those forecasts
ensembles that have an NINO3 SST anomaly >1.0/<1.0at12monthlead-timeinthe
RECHOZ control simulation. The blue shaded area marks the 90% confidence interval for
the La Ni˜na forecasts.
Figure 14
RECHOZ-PACO SST
a) El Ni˜no b) (-) La Ni˜na c) dierence
longitude
time relative to dec. [mon]
150 200 250
20
15
10
5
0
5
10
15
20
longitude
150 200 250
20
15
10
5
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Figure 14: Composite Hoevmoeller diagrams of the equatorial Pacific as in Fig.11 but for
the RECHOZ-PACO simulation.
... La aplicación de las FOE a conjuntos de datos oceanográficos surgió casi una década después, con el estudio de Trenberth [5], que relacionó las oscilaciones atmosféricas en el hemisferio sur con las observaciones de TSM. Diversos autores [6], [7], [8], [9], [10], [11], [12], entre otros, han llevado a cabo estudios aplicados de las FOE en campos de TSM. ...
... De acuerdo con [11] y [12] , los primeros FOEs de la ATSM en el Pacífico Tropical no representan fenómenos distintos (es decir ENOS y EMI), sino que reflejan el desarrollo no lineal del ENOS. Los patrones C y E representan eventos cálidos extremos en la parte este y eventos cálidos moderados o fríos en la parte central del Pacífico Ecuatorial, los cuales corresponden a diferentes evoluciones de los regímenes. ...
... Se llevó a cabo un análisis de las series de tiempo Esto también puede observarse siguiendo los procedimientos descritos en [12]. Se grafican las dos series de tiempo, PC1 y PC2 (ver Figura 28). ...
Article
Full-text available
In this research, Empirical Orthogonal Function (EOF) analysis is applied to reduce the number of variables in a sea surface temperature (SST) dataset to a second dataset containing a much smaller number of variables. The condition is that these new variables retain the maximum possible fraction of information from the original dataset. This second dataset is derived by finding the eigenvalues and eigenvectors of the covariance matrix. The objective is to identify the most significant spatial and temporal patterns (principal components) of SST variability in the Equatorial Pacific Ocean (Latitude: 30°N - 30°S, Longitude: 140°E - 70°O) and subsequently associate these patterns (or modes) with phenomena such as the El Niño-Southern Oscillation (ENSO). Finally, to validate the estimated patterns, they are compared with those obtained by national and international institutions.
... This leads to overconfident forecasts and this framework for understanding El Niño and its forecasts is consistent with the results presented here. Comparison with studies that use a similar theoretical framework provides an explanation for why this a problem with El Niño forecasts and not La Niña forecasts 8,51 . The inclusion of state-dependent noise forcing in a recharge oscillator model decreases El Niño predictability without impacting La Niña predictability and could explain the difference in (150E-180, 5S-5N). ...
... Generally, the total NOMs of CP ENSO are more than those of EP ENSO (Figs. 6a, b). In particular, NOMs of CP La Niña are greater than those of CP El Niño (Fig. 6b), consistent with previous studies that the La Niña usually tends to be a CP type Hu et al., 2018;Okumura, 2019;Yu et al., 2023;Dommenget et al., 2013). However, there is large spread in the total NOMs among different ENSO indices. ...
Article
https://authors.elsevier.com/a/1knb42weQ%7ElMu (A personalized URL providing 50 days' free access to this article before May 07, 2025) El Niño-Southern Oscillation (ENSO) exhibits a wide range of spatial patterns, causing distinct global impacts. Over the past decades, various ENSO indices have been proposed to capture its diversity. Although the classification systems for ENSO flavors differ by definition methodology, it has generally been categorized into two distinct eastern Pacific (EP) and central Pacific (CP) types. However, how different indices characterize these ENSO flavors remains unclear. Here, we reveal the significant differences among these ENSO indices in terms of locally explained variance, type, frequency, and the phase. Specifically, some ENSO indices capture more signals in the equatorial EP or CP, but some others explain relatively insufficient local explanatory variance in the tropical EP and CP, respectively. Moreover, different definition methodologies can result in varying ENSO frequencies identified, even under the same identification criteria and classification process. The most notable discrepancies are observed among CP El Niño and EP La Niña flavors. Furthermore, we discuss the macro-regulation of the Pacific Decadal Oscillation phase on the frequency of each ENSO flavor with different definition. Our findings indicate that each index processes unique characteristics, yet none can comprehensively describe all aspects of ENSO complexity. This highlights the need to consider multiple indices for a comprehensive understanding of ENSO.
... During La Niña, almost all the processes are reversed, which drives an amplified CO 2 outgassing in the tropical Pacific Ocean (Feely et al., 2002(Feely et al., , 2006Ishii et al., 2009). However, these reversed processes are not completely symmetrical in terms of amplitude, distribution, and evolution between La Niña and El Niño events (Dommenget et al., 2013;Larkin & Harrison, 2002;Timmermann et al., 2018). This asymmetry probably arises from asymmetrical atmospheric response to sea surface temperature (SST) anomaly (Kang & Kug, 2002), oceanic nonlinear dynamical heating (An & Jin, 2004;Song et al., 2022), and nonlinearities in the wind-SST feedback (Karamperidou et al., 2017). ...
Article
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Plain Language Summary The El Niño‐Southern Oscillation (ENSO) is a dominant factor in the interannual variation of global air‐sea CO2 flux. In the ocean, ENSO manifests itself as a transition between El Niño and La Niña. Because of the asymmetry between the two, it may exert a non‐zero forcing, inducing imbalanced carbon responses to ENSO. In this study, we have studied the asymmetry of air‐sea CO2 flux caused by La Niña and El Niño events in the tropical Pacific Ocean. During La Niña, ocean CO2 emissions to the atmosphere increase abnormally (0.1–0.2 PgC/yr), which lasts about 3 years with a broader poleward extension (15°S–15°N). During El Niño, ocean CO2 emissions to the atmosphere decrease abnormally (0.2–0.4 PgC/yr), which is more intense and lasts about 1 year with a narrower poleward extension (10°S–10°N). What's more, the ocean carbon anomalies caused by La Niña tend to be more westward than those caused by El Niño. Our study shows that this is due to the combination of asymmetries in wind, rainfall, circulation anomalies, and biological processes. This study will help people understand the ocean carbon sink deeply and improve the accuracy of carbon budget estimation.
... The traditional way to identify ENSO events is based on the Niño indices, areamean SST anomaly in the equatorial Pacific (Trenberth 1997;Trenberth & Stepaniak 2001). Recently, a method based on the reconstruction of Niño indices has created relatively pure EP and CP patterns, where the new indices are given by the E-index and C-index, also called the E-C space (Dommenget et al. 2012;Santoso et al. 2017;Takahashi et al. 2011). Such a space has a better performance in distinguishing changes in EP and CP ENSO in a warming climate (Cai et al. 2018;Geng et al. 2022). ...
Article
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The onset of the South China Sea (SCS) Summer Monsoon (SCSSM) is usually in May, strongly influencing the beginning of the wet season over the surrounding regions, such as South China and the Philippine Islands. The onset date is significantly affected by the El Niño-Southern Oscillation (ENSO), which manifests as two major types: the Eastern-Pacific (EP) and Central-Pacific (CP) patterns. However, the impacts of these patterns on the SCSSM onset await quantification. Here, using a novel statistical metric, binary combined linear regression, we find a dominant role of the CP pattern rather than the EP pattern, as a moderate CP El Niño/La Niña tends to cause SCSSM onset delay/advance by around 4.7/5.1 days in its decaying year. The anomalous SST structure of CP El Niño leads to persistent anomalous anti-cyclonic winds over the western North Pacific from April to June, suppressing the local convection and impeding the SCSSM winds. In comparison, the EP El Niño impact decays quickly in May and hardly affects the SCSSM onset. This, hence, weakens the relationship between the SCSSM onset and ENSO. These results help improve the SCSSM onset prediction by involving diverse ENSO impacts.
... The first EOF resembles the typical pattern of El Niño and the second is a dipole that represents shifts in longitude. The PCs are distributed around a quadratic curve that is now familiar from work on different types of El Niño (Monahan and Dai 2004;Takahashi et al. 2011;Dommenget, Bayr, and Frauen 2013). ...
Article
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The response of February to April (FMA) Northeast Brazil precipitation to tropical sea surface temperature anomaly (SSTA) patterns is analysed by making ensemble seasonal forecasts with a simple GCM. The physical and dynamical link between the SSTAs and regional precipitation is examined for a set of tropical Pacific and Atlantic SSTAs, both separately and in combination. Five canonical SSTA patterns are considered: El Niño (strong eastern Pacific and moderate central Pacific); La Niña and the tropical North and South Atlantic. The model's anomaly responses to persisted SSTAs correspond quite well to observed precipitation regressions against SSTA timeseries: dry for El Niño and warm North Atlantic, wet for La Niña and warm South Atlantic. Experiments with reversed SSTAs show that model nonlinearity has a drying effect over northeastern South America. Combined tropical Atlantic SSTA patterns result in wet conditions over northern Northeast Brazil for a warm Atlantic or a negative dipole, and dry conditions for the contrary, so the South Atlantic has a greater impact than the North Atlantic. Combining SSTA patterns in the Pacific and Atlantic gives results that are similar to the sum of individual effects, with the stronger Pacific SSTAs exerting more influence despite the model's sensitivity to Atlantic conditions. Dry dynamical experiments with fixed basic states and heating perturbations confirm that interactive moist thermodynamics is crucial for accurately representing continental rainfall responses, even over short timescales. These results are generally promising for applications in operational seasonal forecasting, and this is reported in the companion paper, part 2.
... As a stronger tendency in the transition from one phase to another corresponds to a shorter duration of the event, while a weaker tendency in the transition harmonizes with a longer duration (An and Kim 2017), duration asymmetry is often associated with transition asymmetry (TA) (Choi et al. 2013). A regular consensus is that asymmetric atmospheric nonlinearity without invoking external forcing is key to asymmetric transition and evolution (An and Kim 2017;Ohba and Ueda 2009;Dommenget et al. 2013). This nonlinearity includes the feedback process from the wind stress and the surface heat fluxes (Bayr et al. 2021;Fang and Chen 2023;Fang et al. 2015;Chen et al. 2018;Chen et al. 2019). ...
Article
El Niño is frequently followed by La Niña, while La Niña tends to sustain into the next year or even longer, exhibiting a notable phase transition asymmetry (TA) of El Niño-Southern Oscillation (ENSO). Here this study explores the potential influences of tropical Indian Ocean (TIO) decadal variability on TA, based on a comparative analysis of the relationship between the TIO sea surface temperature anomalies (SSTA) and ENSO during different periods. Generally, the TIO SSTA strengthened TA before the 1980s, corresponding to a highly positive relationship between the whole TIO SSTA and ENSO. However, the weakening effect was exhibited after the 1980s when the correlation diminished. After the late 1990s, ENSO was only positively correlated with western TIO, with the westerly exhibit of the SSTA center leading to smaller impacts on TA. Besides, TIO SSTA tends to weaken TA by promoting the transition efficiency of La Niña, while bringing little effect on that of El Niño. Physically, compared to the mid-1970s, TIO SSTA triggered westerly wind anomalies during the autumn and winter of the La Niña development phase in the central equatorial Pacific in the late 1990s, which sped up the decay of La Niña. It then regenerated westerly anomalies in the following winter, facilitating the development of El Niño. This study quantifies the impact of the TIO SSTA on TA in seasonal signals and investigates the decadal variability of such influence, aiming to further understand phase transition asymmetry and offer valuable insights for the prediction of multi-year La Niña.
... Then, while the LIM's predictable dynamics would remain linear, this additional "correlated additive-multiplicative" (CAM) noise term would drive non-Gaussian variability including some key aspects of the observed asymmetry in the intensity and duration of ENSO events [Martinez-Villalobos et al., 2019]. However, ENSO asymmetry has also been attributed to slower nonlinear processes in the tropical Pacific [Dommenget et al., 2013, Takahashi et al., 2011, 2019, Hayashi et al., 2020, which might not be simply represented by CAM noise. Given that the LIM already shows substantial predictive skill, then, we could systematically investigate its forecast error residual for predictable nonlinear dynamics, and add the resulting correction of the LIM. ...
Preprint
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While deep-learning models have demonstrated skillful El Ni\~no Southern Oscillation (ENSO) forecasts up to one year in advance, they are predominantly trained on climate model simulations that provide thousands of years of training data at the expense of introducing climate model biases. Simpler Linear Inverse Models (LIMs) trained on the much shorter observational record also make skillful ENSO predictions but do not capture predictable nonlinear processes. This motivates a hybrid approach, combining the LIMs modest data needs with a deep-learning non-Markovian correction of the LIM. For O(100 yr) datasets, our resulting Hybrid model is more skillful than the LIM while also exceeding the skill of a full deep-learning model. Additionally, while the most predictable ENSO events are still identified in advance by the LIM, they are better predicted by the Hybrid model, especially in the western tropical Pacific for leads beyond about 9 months, by capturing the subsequent asymmetric (warm versus cold phases) evolution of ENSO.
Article
Aim We aim to review present uncertainties in projecting fine‐scale future precipitation in an area of high model disagreement, which is also data poor, topographically complex, and experiences climate‐driven threats to endemic biodiversity. Location Hawaiian Islands. Time Period We primarily focused on downscaling studies from the past decade and studies comparing the most recent iterations of the Coupled Model Intercomparison Project. Major Taxa Studied Hawaiian honeycreepers. Methods We explored sources of uncertainties in two major categories: (1) downscaling general circulation models (GCMs) to islands and (2) systematic biases in the representation of the tropical Pacific climate. We framed this discussion in the context of management planning for endangered Hawaiian forest birds. We also explored a brief case study exploring the impact of differing precipitation projections on Hawaiian forest bird ranges. This involves the use of maximum entropy software to model suitable habitat for Kiwikiu ( Pseudonestor xanthophrys ) using baseline climate data and projecting that model to two different dynamically downscaled precipitation projections for Hawaii. Results The selection of downscaling methodology can affect as much as the sign of change for precipitation in areas of complex topography, especially forest bird habitat at higher elevations. We identified dynamical downscaling as the most used method for island climate predictions globally. Of statistical downscaling methods, machine learning proved to be the most common in recent island studies. The major sources of persistent uncertainty of GCM simulations in the tropical Pacific are the double Inter‐Tropical Convergence Zone bias, the cold tongue bias, and westward‐extended El Niño‐Southern Oscillation sea surface temperature anomalies. These biases complicate the prediction of winter precipitation and future drought prevalence in Hawaii. The differences in precipitation projections from our case study show a large impact on range estimations of suitable habitat for Kiwikiu, especially on the leeward side of Maui. Main Conclusions Despite its limitations, dynamical downscaling may be better suited than statistical downscaling for simulating precipitation in Hawaii. Of statistical downscaling methods, perfect prognosis and machine learning show the most promise in accurate spatial representation of precipitation. Selected GCMs have recently achieved improved representations of the mean state tropical Pacific climate and more realistic El Niño –Southern Oscillation nonlinear feedbacks. To benefit from these improvements, future research could be dedicated to finding which models within the Coupled Model Intercomparison Project have the lowest precipitation bias over the northern central tropical Pacific. Future drought predictions in Hawaii will impact the planning of conservation actions such as predator control, conservation introductions, and novel disease management techniques.
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The Pacific Decadal Oscillation (PDO) and the North Pacific Meridional Mode (NPMM) are prominent climate modes in the North Pacific. Recent research has highlighted the influence of NPMM on PDO variability. However, how the relationship between these two modes will evolve from historical to future climates remains uncertain. This study investigates the evolving relationship between the NPMM and PDO under a warming climate. The shift in the NPMM-PDO relationship is attributed to changes in the NPMM and the El Niño-Southern Oscillation (ENSO) transition cycle. Results from the Community Earth System Model Version 2 large ensemble suggest that external forcing has played a key role in triggering these shifts. Additionally, our findings indicate that the influence of NPMM on PDO weakens in future climates due to altered tropical teleconnection patterns and enhanced NPMM intensity, which can further modify the PDO pattern, offering new insights into North Pacific climate variability projections.
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El Nino events, characterized by anomalous warming in the eastern equatorial Pacific Ocean, have global climatic teleconnections and are the most dominant feature of cyclic climate variability on subdecadal timescales. Understanding changes in the frequency or characteristics of El Nino events in a changing climate is therefore of broad scientific and socioeconomic interest. Recent studies(1-5) show that the canonical El Nino has become less frequent and that a different kind of El Nino has become more common during the late twentieth century, in which warm sea surface temperatures (SSTs) in the central Pacific are flanked on the east and west by cooler SSTs. This type of El Nino, termed the central Pacific El Nino (CP-El Nino; also termed the dateline El Nino(2), El Nino Modoki(3) or warm pool El Nino(5)), differs from the canonical eastern Pacific El Nino (EP-El Nino) in both the location of maximum SST anomalies and tropical-midlatitude teleconnections. Here we show changes in the ratio of CP-El Nino to EP-El Nino under projected global warming scenarios from the Coupled Model Intercomparison Project phase 3 multi-model data set(6). Using calculations based on historical El Nino indices, we find that projections of anthropogenic climate change are associated with an increased frequency of the CP-El Nino compared to the EP-El Nino. When restricted to the six climate models with the best representation of the twentieth-century ratio of CP-El Nino to EP-El Nino, the occurrence ratio of CP-El Nino/EP-El Nino is projected to increase as much as five times under global warming. The change is related to a flattening of the thermocline in the equatorial Pacific.
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This study examines preindustrial simulations from Coupled Model Intercomparison Project, phase 3 (CMIP3), models to show that a tendency exists for El Niñ o sea surface temperature anomalies to be located farther eastward than La Niñ a anomalies during strong El Niñ o–Southern Oscillation (ENSO) events but farther westward than La Niñ a anomalies during weak ENSO events. Such reversed spatial asymmetries are shown to force a slow change in the tropical Pacific Ocean mean state that in return modulates ENSO amplitude. CMIP3 models that produce strong reversed asymmetries experience cyclic modulations of ENSO intensity, in which strong and weak events occur during opposite phases of a decadal variability mode associated with the residual effects of the reversed asymmetries. It is concluded that the reversed spatial asymmetries enable an ENSO–tropical Pacific mean state interaction mechanism that gives rise to a decadal modulation of ENSO intensity and that at least three CMIP3 models realistically simulate this interaction mechanism.
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Warm and cold phases of El Nino-Southern Oscillation (ENSO) exhibit a significant asymmetry in their transition/duration such that El Nino tends to shift rapidly to La Nina after the mature phase, whereas La Nina tends to persist for up to 2 yr. The possible role of sea surface temperature (SST) anomalies in the Indian Ocean (IO) in this ENSO asymmetry is investigated using a coupled general circulation model (CGCM). Decoupled-IO experiments are conducted to assess asymmetric IO feedbacks to the ongoing ENSO evolution in the Pacific. Identical-twin forecast experiments show that a coupling of the IO extends the skillful prediction of the ENSO warm phase by about one year, which was about 8 months in the absence of the IO coupling, in which a significant drop of the prediction skill around the boreal spring (known as the spring prediction barrier) is found. The effect of IO coupling on the predictability of the Pacific SST is significantly weaker in the decay phase of La Nina. Warm IO SST anomalies associated with El Nino enhance surface easterlies over the equatorial western Pacific and hence facilitate the El Nino decay. However, this mechanism cannot be applied to cold IO SST anomalies during La Nina. The result of these CGCM experiments estimates that approximately one-half of the ENSO asymmetry arises from the phase-dependent nature of the Indo-Pacific interbasin coupling.
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A simple mechanism is offered that accounts for a change in the long-term (decadal scale) mean of ocean temperatures as the El Nino-Southern Oscillation (ENSO) amplitude changes. It is intended as an illustration of a kinematic effect of oscillating a nonlinear temperature profile with finite-amplitude excursions that will cause the Eulerian time mean temperature to rise ( fall) where the curvature of the temperature is positive ( negative) as the amplitude of the oscillations increases. This mechanism is found to be able to mimic observed changes in the mean sea surface temperatures in the Pacific between the 1920s, 1960s, and 1990s due to the changing ENSO amplitude. The effects alter both the calculated mean surface temperatures and the time mean temperatures at depth. It also results in a skewness of the temperature distribution that shares many properties with the observed SST. In this model, the time-local gradients of temperature never change if referenced to a single isotherm (i.e., the Lagrangian description is one of DT/Dt = 0). This implies that changes in the amplitude of ENSO will have no influence on the stability of the underlying system, and that the simple Eulerian decadal mean temperature structure has no predictive value. This is in direct contrast to recent work that ascribes a change in ENSO statistics as due to a change in the background state.
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This study examines the slow modulation of El Nino-Southern Oscillation (ENSO) intensity and its un- derlying mechanism. A 10-15-yr ENSO intensity modulation cycle is identified from historical and paleo- climate data by calculating the envelope function of boreal winter Nino-3.4 and Nino-3 sea surface temperature (SST) indices. Composite analyses reveal interesting spatial asymmetries between El Nino and La Nina events within the modulation cycle. In the enhanced intensity periods of the cycle, El Nino is located in the eastern tropical Pacific and La Nina in the central tropical Pacific. The asymmetry is reversed in the weakened intensity periods: El Nino centers in the central Pacific and La Nina in the eastern Pacific. El Nino and La Nina centered in the eastern Pacific are accompanied with basin-scale surface wind and thermocline anomalies, whereas those centered in the central Pacific are accompanied with local wind and thermocline anomalies. The El Nino-La Nina asymmetries provide a possible mechanism for ENSO to exert a nonzero residual effect that could lead to slow changes in the Pacific mean state. The mean state changes are char- acterized by an SST dipole pattern between the eastern and central tropical Pacific, which appears as one leading EOF mode of tropical Pacific decadal variability. The Pacific Walker circulation migrates zonally in association with this decadal mode and also changes the mean surface wind and thermocline patterns along the equator. Although the causality has not been established, it is speculated that the mean state changes in turn favor the alternative spatial patterns of El Nino and La Nina that manifest as the reversed ENSO asymmetries. Using these findings, an ENSO-Pacific climate interaction mechanism is hypothesized to ex- plain the decadal ENSO intensity modulation cycle.
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Open access to an unprecedented, comprehensive coordinated set of global coupled climate model experiments for twentieth and twenty-first century climate and other experiments is changing the way researchers and students analyze and learn about climate. The history of climate change modeling was first characterized in the 1980s by a number of distinct groups developing, running, and analyzing model output from their own models with little opportunity for anyone outside of those groups to have access to the model data. This was partly a consequence of relatively primitive computer networking and data transfer capabilities, along with the daunting task of collecting and storing such large amounts
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Nonlinear principal component analysis (NLPCA) is a generalization of traditional principal component analysis (PCA) that allows for the detection and characterization of low-dimensional nonlinear structure in multivariate datasets. The authors consider the application of NLPCA to two datasets: tropical Pacific sea surface temperature (SST) and tropical Indo-Pacific sea level pressure (SLP). It is found that for the SST data, the low-dimensional NLPCA approximations characterize the data better than do PCA approximations of the same dimensionality. In particular, the one-dimensional NLPCA approximation characterizes the asymmetry between spatial patterns characteristic of average El Niño and La Niña events, which the ID PCA approximation cannot. The differences between NLPCA and PCA results are more modest for the SLP data, indicating that the lower-dimensional structures of this dataset are nearly linear.
Article
This study examines preindustrial simulations from Coupled Model Intercomparison Project, phase 3 (CMIP3), models to show that a tendency exists for El Nin˜o sea surface temperature anomalies to be located farther eastward than La Nin˜ a anomalies during strong El Nin˜ o–Southern Oscillation (ENSO) events but farther westward than La Nin˜ a anomalies during weak ENSO events. Such reversed spatial asymmetries are shown to force a slow change in the tropical Pacific Ocean mean state that in return modulates ENSO amplitude. CMIP3 models that produce strong reversed asymmetries experience cyclic modulations of ENSO intensity, in which strong and weak events occur during opposite phases of a decadal variability mode associated with the residual effects of the reversed asymmetries. It is concluded that the reversed spatial asymmetries enable an ENSO–tropical Pacific mean state interaction mechanism that gives rise to a decadal modulation of ENSO intensity and that at least three CMIP3 models realistically simulate this interaction mechanism.
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El Niño and La Niña are not a simple mirror image, but exhibit significant differences in their spatial structure and seasonal evolution. In particular, sea surface temperature (SST) anomalies over the equatorial Pacific cold tongue are larger in magnitude during El Niño compared to La Niña, resulting in positive skewness of interannual SST variations. The associated atmospheric deep convection anomalies are displaced eastward during El Niño compared to La Niña because of the nonlinear atmospheric response to SST. In addition to these well-known features, an analysis of observational data for the past century shows that there is a robust asymmetry in the duration of El Niño and La Niña. Most El Niños and La Niñas develop in late boreal spring/summer, when the climatological cold tongue is intensifying, and they peak near the end of the calendar year. After the mature phase, El Niños tend to decay rapidly by next summer, but many La Niñas persist through the following year and often reintensify in the subsequent winter. Throughout the analysis period, this asymmetric feature is evident for strong events in which Niño-3.4 SST anomalies exceed one standard deviation in December. Seasonally stratified composite analysis suggests that the eastward displacement of atmospheric deep convection anomalies during El Niño enables surface winds in the western equatorial Pacific to be more affected by remote forcing from the Indian Ocean, which acts to terminate the Pacific events.
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An observational-based analysis of coupled variability in the equatorial Atlantic and its seasonality is presented. Regression analysis shows that the three elements of the Bjerknes positive feedback exist in the Atlantic and are spatially similar to those of the Pacific. The cross-correlation functions of the elements of the Bjerknes feedback are also similar and consistent with an ocean-atmosphere coupled mode. However, the growth rate in the Atlantic is up to 50% weaker, and explained variance is significantly lower. The Bjerknes feedback in the Atlantic is strong in boreal spring and summer, and weak in other seasons, which explains why the largest sea surface temperature anomalies (SSTAs) occur in boreal summer. Its seasonality is determined by seasonal variations in both atmospheric sensitivity to SSTA and SSTA sensitivity to subsurface temperature anomalies.