RESEARCH ARTICLE| EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES

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Contribution of anthropogenic aerosols to persistent La Niña-­likeconditions in the early 21st century
Yen-­Ting Hwanga,1, Shang-­Ping Xieb

, Po-­Ju Chena,c, Hung-­Yi Tsenga, and Clara Deserd,1Downloaded from https://www.pnas.org by Natl Ctr Atmosphere Research Library Serials on January 22, 2024 from IP address 128.117.70.146.

Contributed by Clara Deser; received September 8, 2023; accepted December 8, 2023; reviewed by Kay McMonigal and Michael J. McPhaden

The discrepancy between the observed lack of surface warming in the eastern equatorial
Pacific and climate model projections of an El Niño-­like warming pattern confronts
the climate research community. While anthropogenic aerosols have been suggested
as a cause, the prolonged cooling trend over the equatorial Pacific appears in conflictwith Northern Hemisphere aerosol emission reduction since the 1980s. Here, using
CESM, we show that the superposition of fast and slow responses to aerosol emission
change—an increase followed by a decrease—can sustain the La Niña-­like condition fora longer time than expected. The rapid adjustment of Hadley Cell to aerosol reduction
triggers joint feedback between low clouds, wind, evaporation, and sea surface temperature in the Southeast Pacific, leading to a wedge-­shaped cooling that extends to thecentral equatorial Pacific. Meanwhile, the northern subtropical cell gradually intensifies,
resulting in equatorial subsurface cooling that lasts for decades.

SignificanceThe tropical Pacific plays a crucialrole in global energy andmoisture distribution, andaccurate prediction of its changes
is vital for society’s adaptation to
growing climate change. The
cause of surface cooling trends
observed over the past 40 y in
the tropical eastern Pacific eludes
a satisfactory explanation. Our
study shows that anthropogenic
aerosol effects on the equatorial
Pacific Ocean may have reacheda peak and that the responsefeatures a La Nina-­like cooling
pattern. Prolonged aerosol
influence is clarified through dual
timescale responses involving
air–sea flux and ocean dynamics.

climate change | atmosphere–ocean interaction | climate teleconnection |sea surface temperature pattern

The sea surface temperature (SST) anomaly pattern in the tropical Pacific has significant
influence on global weather and climate. Over the past four decades, the equatorial cold
tongue has cooled while the rest of the tropics have warmed (1), modulating the pace of
global surface warming (2, 3), amplifying tropical cyclone genesis over the Northwest
Pacific (4), and heightening the likelihood of drought occurrence in the western UnitedStates (5). The aforementioned trends might cease or even reverse if the cold tongue warms
up and Walker Cell weakens, as anticipated by climate models. How the tropical Pacificmay change under anthropogenic forcing has been a subject of active debate for decades
and is still ongoing. The complex interplay among global dynamics, air–sea interactions,and radiative processes has given rise to multiple unresolved conundrums, including theinterpretation of the observed trend, the discrepancy between models and observations(1, 6), and diverging theories (6, 7).
One plausible interpretation for the observed eastern tropical Pacific cooling trend,
particularly during the early 2000s, is that it is predominantly driven by internal variability,
either through the negative phase of Pacific Decadal Variability (PDV) (2, 8–13) or the
decadal variation of independent ENSO events (14–16). Despite accounting for a range
of internal variability, it remains uncommon to find historical simulations with fully
coupled climate models that accurately replicate the strengthening of the zonal SST gradient observed over the past 50 y (1, 6). Furthermore, the cooling trend observed in thesubtropical South Pacific and equatorial eastern Pacific has persisted despite the transition
toward a positive PDV phase after 2013, prompting the need for an alternative explanation,
namely that a portion of the observed trend may be driven by external forcing that current
models fail to simulate adequately.
The exploration of idealized experiments, which enable the decomposition of the complex response to increasing carbon dioxide (CO2) into a fast and a slow component, has
yielded significant insights (17–19). The fast component closely aligns with the observed
trend (20) and the “ocean thermostat” hypothesis (21). This stands in stark contrast to
the slow component, which exhibits a reduced zonal gradient consistent with long-­term
model projections and the thermodynamic constraints imposed by radiative cooling (22)and evaporative damping (23, 24). The framework of multiple timescale responses offers
a promising avenue for resolving discrepancies between models and observations, as well
as reconciling the various competing mechanisms at play in this intricate system (19, 25).
It is hypothesized that the strength of the ocean thermostat may be underestimated bycurrent models, and the enhanced warming over the equatorial cold tongue may eventually
manifest (19, 25).
Evidence from single forcing experiments indicates that anthropogenic aerosols exert
a comparable influence to greenhouse gases in shaping the SST pattern during the
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Author affiliations: aDepartment of Atmospheric Sciences,
National Taiwan University, Taipei 10617, Taiwan; bScripps
Institute of Oceanography, University of California San
Diego, La Jolla, CA 92093; cDepartment of Atmospheric,
Oceanic, and Earth Sciences, George Mason University,
Fairfax, VA 22030; and dClimate and Global Dynamics
Division, National Center for Atmospheric Research,
Boulder, CO 80305

Author contributions: Y.-­T.H. designed research; P.-­J.C.
and H.-­Y.T. performed research; C.D. contributed new
reagents/analytic tools; Y.-­T.H., P.-­J.C., and H.-­Y.T. analyzed
data; S.-­P.X. provided important insights for interpreting
the results and forming the article’s storyline; C.D.
provided important insights for interpreting the results;
and Y.-­T.H., S.-­P.X., and C.D. wrote the paper.
Reviewers: K.M., University of Alaska Fairbanks; and
M.J.M., National Oceanic and Atmospheric Administration
Pacific Marine Environmental Laboratory.
The authors declare no competing interest.
Copyright © 2024 the Author(s). Published by PNAS.
This open access article is distributed under Creative
Commons
Attribution-­NonCommercial-­NoDerivatives
License 4.0 (CC BY-­NC-­ND).
1

To whom correspondence may be addressed. Email:ythwang@ntu.edu.tw or cdeser@ucar.edu.
This article contains supporting information online athttps://www.pnas.org/lookup/suppl/doi:10.1073/pnas.​
2315124121/-­/DCSupplemental.
Published January 22, 2024.

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observational period (26–32). However, our current understanding of the Earth system’s responses to anthropogenic aerosols on
different timescales remains limited (33), hindering our ability toaccurately attribute past climate change and assess uncertainties
in climate change projections. For instance, a notable reduction
in aerosol concentrations has already been observed in North
America and Europe since 1980, as well as in East Asia since 2000.
Naively predicting that the decrease in aerosols would lead to a
weakening of the observed La Niña-­like SST pattern trend appears
to contradict both the available observational records and theoutcomes of single forcing simulations.
In the following, we explore the possible role of non-­monotonic
evolution of anthropogenic sulfate aerosols in slowing down theequatorial Pacific warming. Our findings reveal that the equatorial
Pacific response to aerosol radiative forcing exhibits distinct fast andslow components. We then put forth a hypothesis that these components interact and result in an equatorial cooling that is stronger
in the early 21st century than in the 1980s. The superposition of
the fast and slow response to the Northern Hemisphere sulfate aerosol emission trajectory, which consists of an increase followed bya decrease, may sustain the La Niña-­like SST pattern for an extended
duration that is longer than previous estimations.
The Experimental Design
We differentiate the fast and slow components of the climate system’s response to anthropogenic aerosols by conducting a 15-­
ensemble-­member step function experiment using the Com­munity
Earth System Model version 1 (CESM1) at 2° resolution for theatmosphere and land components and 1° resolution for the oceanand sea ice components. The last-­millennium ensemble (34) andthe CMIP5 archive also utilized the same version and resolution,
identified as “CESM1 (CAM5.1, FV2).” Note that the CESM1
model at 2° resolution is particularly skillful in simulating someof the key processes involved in the response, including low cloud-­
SST feedback (35) and the Pacific mean state (36). The experimentconsists of suddenly introducing and then maintaining a constantpattern of sulfate aerosol emissions representative of 1980s levels
taken from CMIP5 historical forcing data (SI Appendix, Fig. S1A)
for the first 30 y of the simulation, followed by an abrupt returnto pre-­industrial (year 1850) levels for an additional 30 y. Ensemble
members are generated by initializing the experiments by the conditions on January 1st of the last 15 consecutive years in the control
simulation. The evolving forced response anomaly is calculated bysubtracting the climatology of the 90-­y pre-­industrial control simulation from the average of the 15 ensemble members. Detailed
definitions for the fast and slow components of aerosol emission
and aerosol removal can be found in the Methods. The fast andslow components of the SST response are very similar to thoseobtained with a similar set of experiments using the 2020s distribution of sulfate aerosol emissions (SI Appendix, Fig. S1 D and F).
The similarity implies that the proposed mechanisms shaping thefast and slow components of SST response are operating across both
historical simulations and future projections, despite the different
patterns of aerosol emissions.
Multiple Timescale Responses in the Equatorial
Pacific Region
The equatorial Pacific, located far from the Northern Hemisphereaerosol emission source regions, shows a clear indication of a multiple timescale response. The imposed step-­function in aerosol
emissions results in a complex evolution of oceanic temperatures,as shown in Fig. 1. Initially (within the first 3 y), SSTs in the
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Fig. 1. Temporal evolutions of anomalous oceanic temperature in the idealized
aerosol experiments relative to the climatology in the control simulation. (A) The
3-­y running mean of meridionally averaged SST anomalies in the equatorial
Pacific (5oS~5oN). (B) The 3-­y running mean of the equatorial Pacific temperature
response which is zonally and meridionally averaged (5oS~5oN; Pacific basin).
Dotted region indicates statistically significant values of temperature anomalies
at the 95% confidence level based on two-­sided t test.

central and eastern equatorial Pacific SST undergo warming before
transitioning to cooling thereafter (Fig. 1A). It is notable that the
maximum cooling takes place in the 3 y immediately following
the abrupt cessation of aerosol forcing at year 30 (Fig. 1A).
Although the surface cooling gradually decreases after year 35,
subsurface cooling persists for decades after the radiative forcing
has been removed (Fig. 1B). Additionally, the subsurface coolingreaches maximum strength during years 33 to 45, after the aerosol
emissions return to preindustrial levels. Both the surface and subsurface signals indicate a persistent cooling long after the aerosol
emissions are abruptly terminated.
The multiple timescale responses identified in the idealized
experiments can also be seen in a more realistic setting. Fig. 2
compares the Niño3.4 index and subsurface ocean temperaturein the equatorial Pacific for both the idealized step function experiment and the standard CESM experiments with realistic emission
scenarios (historical and RCP8.5) (29) (Methods). The average
global aerosol emission reaches its highest point around 1970 to
1990, causing the Niño3.4 region’s SST and subsurface temperature to experience cooling that show little sign of weakening evenafter year 1990. The equatorial subsurface cooling reaches maximum strength several decades after the aerosol emissions peak.
According to the CESM single forcing experiment, we are about
to experience the peak equatorial subsurface cooling (around year
2030 to 2050) induced by aerosols.
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Fig. 2. Temporal evolutions of imposed sulfate aerosol emissions and equatorial Pacific climate response. Top row: the global mean sulfate emission imposedin the idealized experiment (A) and CESM LE All minus XAERindus (B). The gray lines denote the global mean anomalous SW radiation in clear sky. Middle andBottom rows: the evolutions of averaged SST in the Niño3.4 region (5oS~5oN; 170oW~120oW) and averaged potential temperature in the subsurface equatorial
Pacific (200 to 400 m; 5oS~5oN; Pacific basin) in the idealized experiment (Left) and CESM-­LE (Right). All indices except the global mean sulfate emission aresmoothed by 3-­y running mean in the idealized experiment and by a 5-­y running mean in CESM-­LE. The thick black curves and the thin gray bars indicate theensemble mean and the 1 SD among ensemble members, respectively.

In the following, we utilize the idealized experiment to investigate the distinct characteristics and underlying mechanisms
behind the fast and slow elements of the climatic response toanthropogenic aerosol forcing. The multiple timescale elements
explain why the peak aerosol influence on the tropical Pacificoccurs after emission reduction.
The Fast Component
The fast component reveals a remote influence of aerosols that has
the opposite sign from that expected from the imposed forcing, aphenomenon that has yet to be recognized in the existing

literature. In the case of aerosol emissions, this manifests as awedge-­shaped SST warming pattern reaching its maximum intensity in the central equatorial Pacific, extending southeastward, andfeaturing a sharp front north of the equator (Fig. 3A). The reduced
equatorial zonal SST gradient and easterlies are associated with aflatten thermocline and a typical seesaw temperature anomaly inthe equatorial band (Fig. 3C). Here, we disentangle how theanomalous radiative forcing, primarily concentrated in the northern extratropics, gives rise to a remote SST response in the equatorial Pacific with an opposite sign from that expected from theforcing. In the Summary and Outlook, we will discuss how thesame pattern with reversed sign explains the enhanced coolingFig. 3. The fast and slow responses in the idealized aerosol experiments. (A and B) Spatial patterns of SST (shaded) and surface wind stress (vector) anomalies.
(C and D) Vertical structure of potential temperature anomalies along the equator (5oS~5oN mean) as a function of depth (m). Dotted region indicates statisticallysignificant values of temperature anomalies at the 95% confidence level based on two-­sided t test.

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that takes place after the sudden removal of aerosol emissions in
our idealized experiment and contributes to the La Niña-­like trend
in more realistic simulations (and by extension the observed
trends).
We propose that the formation of the cross-­equatorial Hadley
Cell, along with its associated air–sea interactions, drives theobserved teleconnection pattern in the tropical Pacific. The energetic framework (37–39) offers a plausible interpretation for thedevelopment of this anomalous cross-­equatorial cell. In response
to aerosol emissions, an anomalous cross-­equatorial cell emerges,
transporting energy toward the north (Figs. 4 A and B and 5A).
The accompanying pattern of surface winds plays a pivotal role inestablishing the SST pattern in the tropical Pacific, as described
below.
First, the meridional winds in the equatorial eastern Pacific holda significant influence over thermocline depth and the strength ofthe cold tongue (40). A reduction in the trade winds across theequator suppresses mixing, limits upwelling, and results in a flatterthermocline (Fig. 3C). Despite very little radiative forcing in thetropics, the anomalous cross-­equatorial winds that are generated
remotely by aerosols in the northern extratropics can induce a
tropical SST response that resembles the dynamical thermostat
mechanism frequently discussed in the context of greenhousewarming or volcanic eruptions (19, 21, 25, 41, 42). We confirm
the key role of extratropical aerosols via performing additional
experiment that only considers tropical aerosol emissions, which
does not enable the dynamical thermostat mechanism and displaysa slight equatorial cooling instead for the fast response [seeSI Appendix, Fig. S2, contrary to the hypothesis presented in
Verma et al. (33) based on year 2000 emissions].
Second, the anomalous cross-­equatorial winds strengthen the
trade winds in the northern tropics and weaken them in the southern tropics. Weakened trades in the south inhibit coastal upwelling,
diminish stratus and stratocumulus clouds along the coast ofSouth America, and reduce the east-­west SST gradient in thesubtropics. Given that the climatological rainband is located northof the equator, the combined wind-­evaporation-­SST and low
cloud-­SST feedbacks can spread anomalous warming from theSoutheast Pacific to the equatorial central and eastern Pacific(43–46) (Fig. 3A). Meanwhile, the anomalous cooling caused bystrengthened trades and the joint wind-­evaporation-­low cloud-­SST
feedback is blocked by the rainband. It can only enter the equatorial region from the west. The resulting weakened zonal SST
gradient in the equatorial western-­central Pacific reduces the easterlies, thereby reducing zonal cold advection and meridional
Ekman energy divergence. This, in turn, further amplifies thewarming (47) (see Fig. 4D for the anomalous deep tropical subtropical cell (STC) driving the Ekman energy convergence and
SI Appendix, Fig. S3 for the energy budget analysis of the upper
0 to 50 m).
The Slow Component
The slow component of the climate response to aerosol forcing
exhibits an enhanced equatorial zonal SST gradient and an intensification of the Walker Cell, consistent with the literature (26,
28, 48) (Fig. 3B). The cooling of the slow component extends well
below the thermocline to at least 400 m depth. Intriguingly, thesesurface and subsurface cooling anomalies in the equatorial region
occur within decadal timescale and show little sign of recovery
after aerosol removal (Fig. 1), a phenomenon previously unreported in the literature. In this section, we delve into the ocean
dynamical mechanisms that drive the spread of aerosol-­induced
cooling to the equatorial region. The persistent nature of thisoceanic response emphasizes the potential to detect the coolinganomalies in the noisy observational records, which we comparewith the historical simulations in the last section.
We propose that the interhemispherically asymmetric surface
wind anomalies and the associated air–sea interactions in the fast
response drive the slow oceanic adjustments and set the stage forFig. 4. Climatological (gray contours) and anomalous (colored contours) meridional mass streamfunction in the atmosphere (A–C) and in the Indo-­Pacific ocean(D–F). Dotted region indicates statistically significant values of anomalies at the 95% confidence level based on two-­sided t test. The contour intervals for panels(A–C) are 2 × 1010 m3/s for climatology and 2 × 109 m3/s for anomalies. For (D–F), the contour intervals are 4 Sv for climatology and 0.8 for anomalies.

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STC (Figs. 4E and 5B). The northern STC intensifies within 2
to 3 y of imposing aerosol forcing, matching the time for baroclinic Rossby waves to cross the Pacific Basin (51, 52). In addition to the wind-­driven mechanism, the aforementioned
anomalous cooling introduced by the oceanic tunnel can reinforce and extend the intensified STC via altering density andvertical stability (53). The strengthened northern STC reaches
its peak at year 10 and persists for more than 10 y after switching
off aerosol emissions, leading to a continued export of energy
and a gradually intensified cooling in the equatorial region (54).
The cooler equatorial water is upwelled from the cold tongue,
kicking off a series of air–sea interactions and leading to a persistent La Niña-­like condition.

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Summary and Outlook

Fig. 5. Temporal evolutions of anomalous meridional circulations and theirindices in the idealized aerosol experiments. The timeseries are all smoothedby a 3-­y running mean. (A) displays the change in Hadley Cell (shaded) which isshown by the vertically averaged mass streamfunction (globally zonal mean;
300 to 700 hPa). The positive anomalies indicate anomalous northward flows
in the upper level and southward flows in the lower level, vice versa. The solidline denotes the strength of anomalous cross-­equatorial Hadley Cell using
the mass streamfunction averaged over the domain (10oS~10oN; 300 to 700hPa). (B) displays the change in the Meridional Overturning Circulation in thetropical Pacific (shaded) through the vertically averaged mass streamfunction(100 to 300 m). The solid line represents the index of the strength which is the
maximum of the Meridional Overturning Circulation in a domain (10oN~30oN; 0
to 400 m). Dotted region indicates statistically significant values of anomalous
mass streamfunction at the 95% confidence level based on two-­sided t test.

the slow response. Since oceanic temperature and circulation
respond slowly, the resulting equatorial cooling does not become
significant until year 10 (Fig. 1A). Also, the removal of the aerosol
forcing does not result in an immediate recovery of oceanic circulation, as shown below.
The so-­called “oceanic tunnel” provides a linkage for the
Northern Hemisphere extratropical SST cooling of the fast
response to the tropical ocean (45, 49, 50). That is, the aerosol
cooling and the associated joint low cloud-­WES feedback lower
the temperature of waters subducted within the STC, causing
colder water to be advected downward and equatorward. Indeed,
we observe cooler water penetrating into the oceanic interior in
the eastern Pacific basin around 20~30oN and transporting equatorward to 15oN a few years after the imposition of aerosol forcing(SI Appendix, Fig. S4).
Targeting the equatorial Pacific oceanic energy budget, theloss of energy and the resultant cooling in the equatorial Pacific
Ocean are mainly caused by changes in the northern STC (refer
to SI Appendix, Fig. S5 for changes in meridional energy convergence dominating energy budgets of 0 to 200 m and 0 to 400
m and see SI Appendix, Fig. S6 for V ′ T    of the northern boundary outweighing other terms). The cooling in the northernmid-­latitudes leads to an increase in subtropical wind stress andwind-­stress curl, which then spins up the Pacific Gyre circulation
and, as a result of the Sverdrup balance, strengthens the northern
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We have examined the fast and slow components of tropical Pacific
SST responses to anthropogenic sulfate aerosol emissions. We
observe that the equatorial Pacific displays opposing signs of fastand slow components, and both components are insensitive toexact aerosol distributions outside the equatorial region. As summarized in Fig. 6, the unique patterns of both surface fast andsubsurface slow responses reinforce each other, resulting in a notably strong equatorial cooling that persists decades after the removalof anthropogenic aerosols. Following an abrupt removal of aerosols, the Hadley Cell responds quickly (Figs. 5A and 6A). The
intensified trades, the associated coastal Ekman upwelling, andthe chain of low cloud-­WES feedback processes, lead to a cooling
trend in the southeast Pacific extending toward the equatorial
central Pacific. The anomalous northern STC, which is the signature of the slow response of aerosol increase, persists for a few more
years after the aerosol removal (Figs. 5B and 6B). This, in turn,results in a continued cooling trend in the equatorial subsurface.
The surface air–sea interactions associated with the rapid Hadley
Cell adjustment and the prolonged anomalous energy transport
divergence in the ocean interior caused by the continuous northern STC strengthening both contribute constructively to the La
Niña-­like condition.
The characteristics of the two distinct components of response
can be seen in simulations conducted under more realistic conditions, as well as in the observed trend. The fast response, whichlasts only a few years in the idealized experiment, transforms
into a continuous linear trend in the simulations with realistic
aerosol emission trajectories, which exhibit increasing anddecreasing trends lasting for decades. Notably, the greatest spatial
correlation between the fast component identified in the idealized experiment and the linear trends observed in the CESM LE
single forcing experiment is found during the 1960s—a decademarked by a substantial increase in Northern Hemisphere aerosol
emissions (SI Appendix, Fig. S7). Conversely, La Niña-­like trends
emerge in the 1990s, coinciding with a more pronounced reduction in aerosol emissions in the CESM LE single forcing experiment. During 1981 to 2000, the period characterized by rapid
interhemispherically asymmetric changes in SST, the spatial
correlation between the fast component and the observed SST
trend reaches −0.63 (SI Appendix, Fig. S7). Furthermore, thepersistent cooling of the equatorial subsurface—a characteristicfeature of the slow response—can be seen not only in the single
forcing experiment but also in the standard historical simulationconsidering GHG warming and the reanalysis data (SI Appendix,
Fig. S8).
Our findings are based on a single model (CESM), and thetemporal and spatial characteristics of the two components identified here remain to be tested and compared with other models
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A

Fig. 6. A schematic illustrating the distinct characteristics of and the interplay between the fast component ofaerosol emission reduction and the slow component ofaerosol emission increase. Shadings represent anomalous oceanic potential temperature. Arrows in the twovertical planes are the anomalous cross-­equatorial
Hadley Cell and the anomalous oceanic STC. The thickarrows in the Top panel A illustrates intensified trades
and the thin blue arrows illustrate the associated air-­sea
fluxes. The thick arrows in the Bottom panel B illustrate
how the intensified STC induces cold water, which converges in west Pacific subtropics and upwells in the east.

Band different emission scenarios. Despite the caveat, the mechanistic understanding of the two components reported here provides valuable insights for unraveling the discrepancies between
global climate model simulations and observational records. For
instance, the well-­known double ITCZ biases, characterized by
the presence of the eastern Pacific rainband in the southern hemisphere for a longer duration compared to the real world, canhinder the connection between the southern subtropics and theequatorial region in the fast component. In some global climate
models, the spurious rainband obstructs the chain of air–sea interaction that typically brings the anomalous cooling from the South
Pacific to the equatorial region. Furthermore, the underestimationof the strength of the stratus cloud-­SST feedback, a bias common
to most global climate models [but not the two-­degree CESM1
that we use (35)], also impacts the relationship between SSTs inthe southeast and equatorial Pacific. Turning to the slow response,
biased low stratus cloud-­SST feedbacks in the northern subtropics
contribute to an inaccurate estimation of weakening in the STC
and the resulting equatorial cooling. Additionally, global climate
models tend to exhibit excessive stratification in the equatorial
Pacific, impeding the upwelling of STC-­induced cooling in theeastern equatorial Pacific. A process-­based understanding of model
biases is crucial for improved regional climate projections.
The implications of the proposed Hadley circulation and STC
responses to an interhemispheric thermal gradient extend beyond
the simulated effects of anthropogenic sulfate aerosols. The intensified warming trend observed in the mid-­to-­high latitudes of theNorthern Hemisphere and the muted warming in the Southern
Ocean, regardless of whether the causes are aerosol recovery, greenhouse gas-­induced warming, or ozone depletion, could potentiallytrigger similar adjustments in the Hadley Cell, air–sea interaction,and STC. Although our study primarily focuses on the prolonged
La Niña-­like cooling trend, the accelerated warming over theSouthern Ocean and the subtropical oceanic interior would

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engender a reverse mechanism and an El Niño-­like backgroundstate in the future. Additionally, the circulation adjustments acrossmultiple timescales and their impacts on the mean state of thetropical Pacific will also influence tropical climate variability, suchas the El Niño/Southern Oscillation.
Methods
Definitions of the Fast and Slow Components. We define the fast components

as the average of years 1 to 3 minus control and the average of years 31 to 35
minus years 11 to 20 (Fig. 3A and SI Appendix, Fig. S1C), based on the evolutionof the Niño 3.4 index (Fig. 2A). The slow components are defined as the averageof years 11 to 20 minus years 1 to 3 and the average of years 41 to 60 minus
years 31 to 35 (Fig. 3B and SI Appendix, Fig. S1E). Through subtracting the fastcomponent, the slow component partly excludes the cross-­equatorial Hadley Cell
response that develops quickly and highlights the enhanced equatorial responsethat is hemispherically symmetric. The overall patterns are not sensitive to the
choice of the definitions, although the absolute anomalies are.
CESM Single Forcing Large Ensemble. To evaluate the climate responses toaerosol forcing with realistic temporal and spatial variations, we take the difference between the 40-­member CESM large ensemble all forcing experiment (55)and the 20-­member “XAERindus” experiment from the CESM single forcing large
ensemble (29). The “XAERindus” uses the same forcing protocol as the all forcing
experiment, i.e., the CMIP5 historical and RCP8.5 forcing protocols, except thatthe industrial anthropogenic aerosols’ emission is held fixed at 1920 conditions.
The model used in the CESM all forcing and single forcing large ensemble is thesame as that of the idealized step function aerosol experiment used in this study
but at a higher resolution (1°).
Data, Materials, and Software Availability. All processed data to support
the analysis and figure codes publicly available at https://doi.org/10.6084/
m9.figshare.24581016 (56). Data from CESM1 Single Forcing Large Ensemble
Project is publicly available at: https://www.cesm.ucar.edu/working-­groups/
climate/simulations/cesm1-­single-­forcing-­le (29). Monthly outputs from theidealized aerosol experiments are available from Y.-­T.H. on request.

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ACKNOWLEDGMENTS. S.-­P.X. is sponsored by NSF AGS 2105654, C.D. by the NSF

thank Hsing-­Hung Chou for making the schematic (Fig. 6), Po-­Chun Chung andWan-­Yu Wu for their help at the initial stage of the project, Kun Wang for sharing
the codes for budget analysis, M.J.M. and K.M. for their constructive comments,
and John Chiang and Yu-­Heng Tseng for valuable discussions.

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Deposited 17 November 2023.

under Cooperative Agreement 1852977. Y.-­T.H., P.-­J.C., and H.-­Y.T. are supportedby Ministry of Science and Technology of Taiwan (MOST 110-­2628-­M-­002-­002,
MOST 111-­2628-­M-­002-­003-­, and NSTC 112-­2111-­M-­002-­016-­MY4). We

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