Eastern Pacific Warming Research Articles

Biennial variability of boreal spring surface air temperature over India

In this study, we report significant biennial variability or oscillation (BO) in the boreal spring (March–May) Surface Air Temperature (SAT) over India and unravelled the causative mechanisms. The positive phase of the BO exhibit significant seasonal warming over India, whereas seasonal cooling is observed during the negative phase of BO. Heat wave days are more during the positive phase of BO compared to negative or neutral phase. The positive (negative) phase of BO is generally coherent with the central (eastern) Pacific warming (cooling) years. The anomalous low-level divergence associated with low-level anticyclonic circulation induces less cloudiness and intense surface solar radiation ovr India during the positive phase, favouring surface warming. The evolution of some positive and/or negative phases of BO without any large scale forcing from the equatorial Pacific suggested the possibility of alternate pathways. The strong anomalous upper-level (at 200 hPa) anticyclonic circulation provoked by mid-latitude Rossby waves is found contributing to the positive phase, thereby highlighting the role of dominant mid-latitude pathways in the biennial SAT variability in addition to El Niño forcing. The sinking motion associated with persistent high, and the associated adiabatic compression also supported surface heating during the positive phase of BO. On the other hand, the mid-latitude Rossby wave induced upper-level cyclonic circulation is found contributing to the negative phase. The sinking motion associated with persistent high, and the associated adiabatic compression also supported surface heating during the positive phase of BO. In contrast, negative soil temperature anomalies and high latent heat flux release to the atmosphere supported surface cooling during the negative phase.

Dominant contribution of atmospheric nonlinearities to ENSO asymmetry and extreme El Niño events

Extreme El Niño events have outsized impacts and strongly contribute to the El Niño Southern Oscillation (ENSO) warm/cold phase asymmetries. There is currently no consensus on the respective importance of oceanic and atmospheric nonlinearities for those asymmetries. Here, we use atmospheric and oceanic general circulation models that reproduce ENSO asymmetries well to quantify the atmospheric nonlinearities contribution. The linear and nonlinear components of the wind stress response to Sea Surface Temperature (SST) anomalies are isolated using ensemble atmospheric experiments, and used to force oceanic experiments. The wind stress-SST nonlinearity is dominated by the deep atmospheric convective response to SST. This wind-stress nonlinearity contributes to ~ 40% of the peak amplitude of extreme El Niño events and ~ 55% of the prolonged eastern Pacific warming they generate until the following summer. This large contribution arises because nonlinearities consistently drive equatorial westerly anomalies, while the larger linear component is made less efficient by easterly anomalies in the western Pacific during fall and winter. Overall, wind-stress nonlinearities fully account for the eastern Pacific positive ENSO skewness. Our findings underscore the pivotal role of atmospheric nonlinearities in shaping extreme El Niño events and, more generally, ENSO asymmetry.

Historical changes in wind-driven ocean circulation drive pattern of Pacific warming

The tropical Pacific warming pattern since the 1950s exhibits two warming centers in the western Pacific (WP) and eastern Pacific (EP), encompassing an equatorial central Pacific (CP) cooling and a hemispheric asymmetry in the subtropical EP. The underlying mechanisms of this warming pattern remain debated. Here, we conduct ocean heat decompositions of two coupled model large ensembles to unfold the role of wind-driven ocean circulation. When wind changes are suppressed, historical radiative forcing induces a subtropical northeastern Pacific warming, thus causing a hemispheric asymmetry that extends toward the tropical WP. The tropical EP warming is instead induced by the cross-equatorial winds associated with the hemispheric asymmetry, and its driving mechanism is southward warm Ekman advection due to the off-equatorial westerly wind anomalies around 5°N, not vertical thermocline adjustment. Climate models fail to capture the observed CP cooling, suggesting an urgent need to better simulate equatorial oceanic processes and thermal structures.

Drivers of Caribbean precipitation change due to global warming: analyses and emergent constraint of CMIP6 simulations

Using 31 CMIP6 models we have analyzed projected future Caribbean precipitation. The model mean projection results in a 0.5 mm/day (20%) drying under a SSP5-8.5 scenario for the end of this century over the Caribbean basin. The multi-model spread is large ranging from no drying to a 1 mm/day (40%) reduction in mean precipitation. Eastern and central Pacific warming, resembling an El Niño / positive phase of Pacific Decadal Oscillation (PDO), appears to be the main driver by shifting and weakening the Walker circulation and inducing subsidence over the Caribbean, especially during the wet season (May–November). This applies for the model mean as well as for the inter-model spread. During the dry season (December–April) the southward migration of the Intertropical Convergence Zone (ITCZ) and the advection of dry air from outside the Caribbean seem to be the dominant drivers of the projected drying. Another mechanism that contributes to the drying is the land-sea contrasts that induce divergence/convergence over the Caribbean. The incapability of CMIP6 models to simulate the current tropical Pacific warming and Walker circulation trends questions the reliability of precipitation projections in the Caribbean. Based on our understanding of the physical processes affecting Caribbean drying and on an emergent constraint analysis we state that the future drying in the Caribbean is likely to be weaker than the one projected by CMIP6 models.

Explained predictions of strong eastern Pacific El Niño events using deep learning

Global and regional impacts of El Niño-Southern Oscillation (ENSO) are sensitive to the details of the pattern of anomalous ocean warming and cooling, such as the contrasts between the eastern and central Pacific. However, skillful prediction of such ENSO diversity remains a challenge even a few months in advance. Here, we present an experimental forecast with a deep learning model (IGP-UHM AI model v1.0) for the E (eastern Pacific) and C (central Pacific) ENSO diversity indices, specialized on the onset of strong eastern Pacific El Niño events by including a classification output. We find that higher ENSO nonlinearity is associated with better skill, with potential implications for ENSO predictability in a warming climate. When initialized in May 2023, our model predicts the persistence of El Niño conditions in the eastern Pacific into 2024, but with decreasing strength, similar to 2015–2016 but much weaker than 1997–1998. In contrast to the more typical El Niño development in 1997 and 2015, in addition to the ongoing eastern Pacific warming, an eXplainable Artificial Intelligence analysis for 2023 identifies weak warm surface, increased sea level and westerly wind anomalies in the western Pacific as precursors, countered by warm surface and southerly wind anomalies in the northern Atlantic.

Plausible Drying and Wetting Scenarios for Summer in Southeastern South America

Abstract Summer rainfall trends in southeastern South America (SE-SA) have received attention in recent decades because of their importance for climate impacts. More than one driving mechanism has been identified for the trends, some of which have opposing effects. It is still not clear how much each mechanism has contributed to the observed trends or how their combined influence will affect future changes. Here, we address the second question and study how the CMIP6 summer SE-SA rainfall response to greenhouse warming can be explained by mechanisms related to large-scale extratropical circulation responses in the Southern Hemisphere to remote drivers (RDs) of regional climate change. We find that the regional uncertainty is well represented by combining the influence of four RDs: tropical upper-tropospheric amplification of surface warming, the delay in the stratospheric polar vortex breakdown date, and two RDs characterizing recognized tropical Pacific SST warming patterns. Applying a storyline framework, we identify the combination of RD responses that lead to the most extreme drying and wetting scenarios. Although most scenarios involve wetting, SE-SA drying can result if high upper-tropospheric tropical warming and early stratospheric polar vortex breakdown conditions are combined with low central and eastern Pacific warming. We also show how the definition of the SE-SA regional box can impact the results since the spatial patterns characterizing the dynamical influences are complex and the rainfall changes can be averaged out if these are not considered when aggregating. This article’s perspective and the associated methodology are applicable to other regions of the globe. Significance Statement Summer rainfall in southeastern South America (SE-SA) affects an area where around 200 million people live. The observed trends suggest long-term wetting, and most climate models predict a wetting response to greenhouse warming. However, in this work, we find that there is a physically plausible combination of large-scale circulation changes that can promote drying, which means SE-SA drying is a possibility that cannot be ignored. We also show that the definition of the SE-SA regional box can impact regional rainfall analysis since the spatial patterns characterizing the dynamical influences are complex and the changes can be averaged out if these are not considered when aggregating. This perspective and the associated methodology are applicable to other regions of the globe.

The Impact of Eastern Pacific Warming on Future North Atlantic Tropical Cyclogenesis

AbstractTropical cyclogenesis in the Atlantic is influenced by environmental parameters including vertical wind shear, which is sensitive to forcing from the tropical Pacific. Reliable projections of the response of such parameters to radiative forcing are key to understanding the future of hurricanes and coastal risk. One of the least certain aspects of future climate is the warming of the eastern tropical Pacific Ocean. Using climate model experiments isolating the warming of the eastern Pacific and controlling for other factors including El Niño‐Southern Oscillation (ENSO), changes in Atlantic tropical cyclogenesis potential by the end of this century are ∼20% lower with enhanced eastern Pacific warming. The ENSO signal in Atlantic tropical cyclogenesis potential amplifies with global warming, and that amplification is larger with enhanced eastern Pacific warming. The largest changes and dependencies on eastern Pacific warming are found in the south‐central main development region, attributable to changes in zonal overturning.

Changes in the West Africa Monsoon Precipitation Extremes during ENSO developing Phases

ABSTRACT A 37-year record of rainfall gridded data covering West Africa and a Global Sea Surface Temperature (SST) dataset are used to investigate the remote influence of SST anomalies in the Equatorial Pacific on the interannual variability of West Africa’s extreme rainfall indices over the period 1981–2018. The top five (5) years with the strongest and weakest peak of Niño3.4 SST monthly anomalies are selected, and May-to-September (MJJAS) composite anomalies of the total and extreme rainfall indices are performed. Results reveal that Equatorial Pacific SST’s impacts on daily rainfall intensity are generally more robust than that on their frequency. The significant changes in the mean zonal atmospheric circulation associated with the SST lead to significant dynamic and thermodynamic changes that affect the West African monsoon system locally. During El Niño (La Niña) years, (i) a weakening (strengthening) of the Tropical Easterly Jet (TEJ), (ii) a strengthening (weakening), and southward (northward) shift position of the African Easterly Jet (AEJ), and (iii) a decrease (increase) of the monsoon flow are noted. These changes in the atmospheric circulation prevent (encourage) a supply of moisture, resulting in a reduction (increase) in extreme precipitation observed across West Africa. Equatorial eastern Pacific warming (cooling) is also shown to lead to stable (unstable) atmospheric conditions over West Africa that block (generate) the development of convective systems.

Global monsoon: Concept and dynamic response to anthropogenic warming

Monsoon has been studied for centuries, yet only recently have regional monsoons been recognized as a global system. This paper begins with a review of the concept of Global Monsoon and related debating issues. We argue that GM drives annual cycles of Hadley circulation, Intertropical Convergence Zone, and subtropical high and dry climate regions. Land monsoon rainfall (LMR) provides water resources for about 70% of the world’s population. Here we review the climate sensitivity of global and regional LMR to anthropogenic warming projected by models participating in phase six of the Coupled Model Intercomparison Project (CMIP6), focusing on critical physical processes responsible for projected changes. In theory, regional mean LMR changes can be approximated by the changes in the product of the mid-tropospheric ascent and 850-hPa specific humidity, plus moderate contribution from evaporation. The greenhouse gases (GHGs) forcing increases moisture content but stabilizes the atmosphere; the two thermodynamic effects offset each other, resulting in a moderate thermodynamic impact on LMR. The GHGs-induced horizontally differential warming results in robust ‘‘northern hemisphere (NH)-warmer than- southern hemisphere (SH)’’, ‘‘land-warmer-than-ocean’’, and an El Nino–like warming pattern. The enhanced NH–SH thermal contrast will increase NH monsoon rainfall and reduce SH monsoon rainfall. The enhanced land–ocean thermal contrast will increase monsoon rainfall over the Asian–northern African monsoon regions. The projected eastern Pacific warming will reduce the North American monsoon. The Inter-model spread analysis suggests that the GHGs-induced circulation changes (dynamic effects) are primarily responsible for the regional differences. The last section discusses conceivable ways forward.

The Impact of the Eastern Pacific Fresh and Warm Pools on the Bimodal Seasonality of Barrier Layers

AbstractObservations of Argo profiles and TAO/TRITON array confirm the significant seasonality of the barrier layer (BL) and temperature inversion (TI) in the northeastern tropical Pacific (NETP). Statistical result of the occurrence based on the Argo profiles reveals a bimodal variability of the BL, with two peaks in July and October. This bimodal seasonality of BL is attributed to the out‐of‐phase variations of the eastern Pacific fresh and warm pools. The fresh and warm pools both expand westward from May to July, when the Inter‐Tropical Convergence Zone (ITCZ) becomes intense and broad. Heavy rainfall is the dominant contributor to the extension of the fresh and warm pools, leading to a high frequency of thick BL (40%). This frequent thick BL provides a precondition for its another development after August. The fresh pool is stable from August to November, while the warm pool contracts sharply. The cold tongue becomes active due to a prevailing trade wind and horizontal advection transports surface cold water to the northeastern warm pool. This cold advection deepens the isothermal layer and contributes to a frequent TI (30%) and thick BL (46%). The results suggest that the ITCZ rainfall and northward cold advection from equator dominate the upper layer stratification of NETP in summer and autumn, respectively.

Increasing Precipitation Efficiency Amplifies Climate Sensitivity by Enhancing Tropical Circulation Slowdown and Eastern Pacific Warming Pattern

AbstractThe role of precipitation efficiency (PE)—the fraction of column‐integrated condensate that reaches the surface as rain—in the global temperature response to CO2 rise is yet to be quantified. Here we employ 36 limited‐domain cloud resolving models (CRMs) from the Radiative‐Convective Equilibrium Model Intercomparison Project and find that they strongly imply higher PE at warmer temperatures. We then analyze 35 general circulation models (GCMs) from the Coupled Model Intercomparison Project Phase 6 and find that increasing PE is associated with tropical circulation slowdown and greater eastern equatorial Pacific warming. These changes trigger pan‐tropical positive cloud feedback through stratiform anvil cloud reduction and stratocumulus suppression, resulting in higher Effective Climate Sensitivity (ECS). We find that in 24 of 35 GCMs matching the CRMs in simulating increasing PE with greenhouse warming, mean ECS is 1 K higher than in PE‐decreasing GCMs. Thus, further constraining PE sensitivity to temperature could reduce uncertainty over future climate projections.

Glacial Warming in the Eastern Pacific Warm Pool

The Eastern Pacific Warm Pool (EPWP) modulates global climate through its connection with tropical Pacific circulation, but sparse paleoceanographic data from this region limits our understanding of its role in past climate variability. We present a 144 kyr alkenone-sea surface temperature (SST) reconstruction from core NH22P, located in the northern EPWP, that shows local warming occurred during periods of global cooling. Climate model simulations of the Last Glacial Maximum indicate that both ice sheet and greenhouse gas forcing slowed wind speeds over the EPWP, which attenuated glacial cooling of local SST via the wind-evaporation-SST feedback. Spectral analysis further suggests precessional pacing of the warming spikes. Vernal equinox insolation could explain this pacing as direct shortwave heating during boreal spring would have contributed to the early seasonal intensification of the EPWP. This work provides crucial constraints on tropical Pacific glacial climate variability and highlights the unique response of the EPWP to global climate forcings.

Climatic Drivers of Deglacial SST Variability in the Eastern Pacific

AbstractWe explore the response of northeastern Pacific sea surface temperature (SST) to deglacial (16–7 ka) climate variability as recorded in ‐based SST reconstructions spanning 65°N to 10°S. Included in the analysis is a new 23 kyr SST record from core NH8P from the northwest Mexican Margin. We isolate spatiotemporal patterns in regional SSTs with trend empirical orthogonal function (TEOF) analysis. The dominant TEOF mode reflects deglacial warming associated with rising . Tropical and subtropical SSTs correlated most strongly with this mode, suggesting that the thermodynamic response of the tropical eastern Pacific to greenhouse gas forcing was the dominant driver of regional SST change during deglaciation. The second TEOF mode reflects millennial‐scale variability and is most strongly expressed in subpolar SSTs. The synchronous timing between North Pacific and North Atlantic SST oscillations is evidence for the rapid transmission of millennial‐scale climate perturbations between the basins, likely through an atmospheric teleconnection. SSTs at NH8P have no correlation with either leading TEOF mode as there is minimal change in SST at this site after 20 ka. A model simulation of the LGM indicates that glacial cooling was muted in much of the Eastern Pacific Warm Pool (EPWP), in which NH8P lies, due to reductions in latent heat flux. This suggests that the wind‐evaporation‐SST feedback was responsible for the attenuation of EPWP cooling. Overall, this study highlights the distinct latitudinal trends in the Pacific's response to deglaciation.

Robust Asymmetry of the Future Arctic Polar Vortex Is Driven by Tropical Pacific Warming

AbstractThe future stratosphere is globally dominated by a strong radiative cooling due to the increase in greenhouse gases. However, we find that over North America, the Arctic stratospheric cooling is suppressed or rather warming occurs, whereas over Eurasia stratospheric cooling is most pronounced, leading to an asymmetric polar vortex, based on 21st century climate model simulations. There are many causes that drive polar vortex variability, which make future projections highly uncertain. Our model simulations demonstrate that tropical warming induces the asymmetric polar vortex. The eastern equatorial Pacific warming causes eastward‐shifted teleconnection with a deepened Aleutian low, which strengthens the polar vortex over Eurasia and weakens over North America by enhancing the vertical wave propagation into the stratosphere. The asymmetric polar vortex is projected to markedly develop in the 2030s, and so could affect winter surface climate over mid‐ to high‐latitudes of Eurasia in the near future.

Dramatic Weakening of the Tropical Easterly Jet Projected by CMIP6 Models

AbstractThe upper-level tropical easterly jet (TEJ) is a crucial component of the summer monsoon system and tropical general circulation. The simulation and projection of the TEJ, however, have not been assessed. Here we evaluate models’ fidelity and assess the future change of the TEJ by utilizing 16 models that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). Most of the models can reproduce the TEJ reasonably well in terms of climatology, seasonal evolution, and interannual variability. Nevertheless, underestimation of the TEJ’s intensity and extent is identified, with the maximum bias occurring in the jet centers over the tropical Indian Ocean (IO) and the tropical eastern Pacific (EP). Under the shared socioeconomic pathway 5–8.5, the multimodel ensemble projects a remarkable reduction in the central TEJ intensity by about 18% over the IO and 77% over the EP toward the end of the twenty-first century. The mean intensity of TEJ will weaken by about 11%, and the extent will reduce by 6%, suggesting a significantly weakened upper-level monsoon circulation in the future climate. The projected El Niño–like warming pattern over the tropical Pacific may play a critical role in the future weakening of the TEJ via inducing suppressed rainfall over the tropical eastern IO and Central America. The model uncertainties in the projected TEJ changes may arise from the uncertainties in the models’ projected tropical EP warming. The sensitivity of future projections to model selection is also examined. Results show that the selection of models based on different physical considerations does not yield a significantly different projection.

The role of oceanic feedbacks in the 2014–2016 El Niño events as derived from ocean reanalysis data

Why did the predicted “super El Nino” fade out in the summer 2014 and the following year develop into one of the three strongest El Nino on record? Although some hypotheses have been proposed in previous studies, the quantitative contribution of oceanic processes to these events remains unclear. We investigated the role of various oceanic feedbacks, especially in response to intra-seasonal westerly wind busts, in the evolution of the 2014–2016 El Nino events, through a detailed heat budget analysis using high temporal resolution Estimating the Circulation and Climate of the Ocean—Phase II (ECCO2) simulation outputs and satellite-based observations. Results show that the Ekman feedback and zonal advective feedback were the two dominant oceanic processes in the developing phase of the warm event in the spring of 2014 and its decay in June. In the 2015–2016 super El Nino event, the zonal advective feedback and thermocline feedback played a significant role in the eastern Pacific warming. Moreover, the thermocline feedback tended to weaken in the central Pacific where the zonal advection feedback became the dominant positive feedback.

Strong remote control of future equatorial warming by off-equatorial forcing

The tropical climate response to GHG forcing is spatially non-uniform1–3. Even though enhanced equatorial and eastern Pacific warming is simulated by most climate models, the underlying mechanisms—including the relative roles of atmospheric and oceanic feedbacks—remain debated. Here, we use a climate model with idealized CO2-radiative forcing patterns to show that off-equatorial radiative forcing and corresponding coupled circulation/cloud adjustments are responsible for much of equatorial warming in response to global CO2 forcing. For equatorial forcing, the atmosphere responds by enhancing atmospheric heat export to the extra-tropics, an associated strengthening of the ascending Hadley circulation branch and strong negative equatorial cloud feedbacks. These processes together greatly dampen equatorial surface warming. Intensification of the oceanic subtropical cells and increased cold subsurface water upwelling in the eastern tropical Pacific provide an additional negative feedback for surface temperatures. In contrast, applying off-equatorial forcing, the atmosphere responds by exporting less heat from the tropics, Hadley circulation weakening and weaker negative equatorial cloud feedbacks, while the subtropical cells slow down in the ocean. Our results demonstrate a delicate balance in the coupled climate system between remote circulation adjustments and regional feedbacks that create the patterns of future climate change. The relative roles of local and remote processes in determining equatorial warming are still debated. Model simulations show that coupled feedbacks strongly damp the equatorial surface temperature response to local equatorial forcing, while amplifying the response to remote off-equatorial forcing.

Different Influences of Two El Niño Types on Low-Level Atmospheric Circulation over the Subtropical Western North Pacific

ABSTRACTThis study focuses on different evolutions of the low-level atmospheric circulations between eastern Pacific (EP) El Niño and central Pacific-II (CP-II) El Niño. The western North Pacific anomalous anticyclone (WNPAC) originates from the northern South China Sea for EP El Niño, and moves to the western North Pacific (WNP) afterward. Compared with EP El Niño, the origin of the WNPAC is farther west during CP-II El Niño, with the center over the Indochina Peninsula. Moreover, the WNPAC shows a weaker eastward shift. Such discrepancies are attributed to different evolutions of the cyclonic response over the WNP, which can suppress the convection in the western flank of the anomalous cyclone. The eastward retreat of the anomalous cyclone is significant for EP El Niño, but less evident for CP-II El Niño. These discrepancies are related to zonal evolutions of the increased precipitation over the equatorial Pacific. Following the southward migration of the intertropical convergence zone (ITCZ), the deep-convection region extends eastward along the equator, reinforcing the atmospheric response to the eastern Pacific warming in EP El Niño. For CP-II El Niño, the atmospheric response is insignificant over the eastern Pacific without warming. Moreover, the meridional migration of the ITCZ can modulate zonal variations of the easterly trade wind and specific humidity as well. Due to the combined effects of the climatological background and atmospheric anomalies, the specific humidity–induced and wind-induced moist enthalpy advection contribute to different shifts of the precipitation center.

Impact of Different Types of ENSO Years on Intensity Changes of Landfalling Tropical Cyclones over China

This study examines whether there are significant differences in intensity and destructiveness of landfalling tropical cyclones (TCs) over China in central Pacific warm (CPW), eastern Pacific warm (EPW) and La Niña (LA) years. By analyzing different seasons and locations of TCs making landfall over China, it was found that TCs in LA years generally had a larger power dissipation index (PDI) and may cause more disasters in China, while TCs in EPW years had a larger PDI over South China in autumn. A larger PDI of TCSC (landing location in Southern China) usually occurred in EPW years and a larger PDI of TCEC (landing location in Eastern China) occurred in LA years, compared with CPW years. The TCs in LA years were generally stronger, more frequent, and of longer duration over China, because of the positive relative humidity (RH) anomalies, the significant anomalous cyclone that occupied the South China Sea (SCS), and the easterly wind anomalies providing a beneficial steering flow for TCs making landfall. In EPW years, although TCs were less frequent, they had stronger intensity when making landfall and a longer lifetime over land which was mainly caused by a broad band of anomalous westerlies over the SCS giving rise to a belt of positive relative vorticity anomalies, as well as the slow translation speed of TCs before landfall supplying more energy for TCs to survive over land. Overall, we conclude that greater caution is warranted when TCs occur in LA and EPW years, as they may result in more serious disasters in China.

North Pacific temperature and precipitation response to El Niño-like equatorial heating: sensitivity to forcing location

This study investigates the sensitivity of oceanic and atmospheric response in the extra-tropics, especially in the North Pacific, to the position of equatorial El Nino-like heating within a slab-ocean climate model. In a suite of numerical experiments, we impose an idealized equatorial sea surface temperature (SST) anomaly in the Pacific and systematically vary its longitudinal position along the equator to mimic different “flavors” of El Nino. We find that regardless of the forcing location, the induced SST pattern closely resembles a positive phase of the Pacific Decadal Oscillation with a characteristic warming along the North American coast as part of an arc-shaped pattern, accompanied by wind anomalies around the Aleutian low. However, the extent and magnitude of the coastal warming vary nonmonotonically when the forcing shifts westward along the equator. The strongest response is found when the equatorial forcing is located in the central Pacific close to the Dateline. In contrast, precipitation response over Southern California is strongest for an eastern Pacific warming centered at 150°W, even though its magnitude is highly uncertain since the boundary between dry and wet precipitation anomalies passes through this region. We repeat the experiments for cold (i.e. La Nina-like) anomalies and observe a significant asymmetry in the SST and atmospheric response between the warm and cold cases. Finally, our experiments suggest that tropical heating (or cooling) over the Western Pacific warm pool generates the largest tropical rainfall response and hence the largest global-mean SST anomaly.