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Most state-of-art models project a reduced equatorial Pacific east-west temperature gradient and a weakened Walker circulation under global warming. However, the causes of this robust projection remain elusive. Here, we devise a series of slab ocean model experiments to diagnostically decompose the global warming response into the contributions from the direct carbon dioxide (CO2) forcing, sea ice changes, and regional ocean heat uptake. The CO2 forcing dominates the Walker circulation slowdown through enhancing the tropical tropospheric stability. Antarctic sea ice changes and local ocean heat release are the dominant drivers for reduced zonal temperature gradient over the equatorial Pacific, while the Southern Ocean heat uptake opposes this change. Corroborating our model experiments, multimodel analysis shows that the models with greater Southern Ocean heat uptake exhibit less reduction in the temperature gradient and less weakening of the Walker circulation. Therefore, constraining the tropical Pacific projection requires a better insight into Southern Ocean processes.

Walker circulation variability and associated zonal shifts in the heating of the tropical atmosphere have far-reaching global impacts well into high latitudes. Yet the reversed high latitude-to-Walker circulation teleconnection is not fully understood. Here, we reveal the dynamical pathways of this teleconnection across different components of the climate system using a hierarchy of climate model simulations. In the fully coupled system with ocean circulation adjustments, the Walker circulation strengthens in response to extratropical radiative cooling of either hemisphere, associated with the upwelling of colder subsurface water in the eastern equatorial Pacific. By contrast, in the absence of ocean circulation adjustments, the Walker circulation response is sensitive to the forcing hemisphere, due to the blocking effect of the northward-displaced climatological intertropical convergence zone and shortwave cloud radiative effects. Our study implies that energy biases in the extratropics can cause pronounced changes of tropical climate patterns.

Walker circulation variability and associated zonal shifts in the heating of the tropical atmosphere have far-reaching global impacts well into high latitudes. Yet the reversed high latitude-to-Walker circulation teleconnection is not fully understood. Here, we reveal the dynamical pathways of this teleconnection across different components of the climate system using a hierarchy of climate model simulations. In the fully coupled system with ocean circulation adjustments, the Walker circulation strengthens in response to extratropical radiative cooling of either hemisphere, associated with the upwelling of colder subsurface water in the eastern equatorial Pacific. By contrast, in the absence of ocean circulation adjustments, the Walker circulation response is sensitive to the forcing hemisphere, due to the blocking effect of the northward-displaced climatological intertropical convergence zone and shortwave cloud radiative effects. Our study implies that energy biases in the extratropics can cause pronounced changes of tropical climate patterns.

Excessive precipitation over the southeastern tropical Pacific is a major common bias that persists through generations of global climate models. While recent studies suggest an overly warm Southern Ocean as the cause, models disagree on the quantitative importance of this remote mechanism in light of ocean circulation feedback. Here, using a multimodel experiment in which the Southern Ocean is radiatively cooled, we show a teleconnection from the Southern Ocean to the tropical Pacific that is mediated by a shortwave subtropical cloud feedback. Cooling the Southern Ocean preferentially cools the southeastern tropical Pacific, thereby shifting the eastern tropical Pacific rainbelt northward with the reduced precipitation bias. Regional cloud locking experiments confirm that the teleconnection efficiency depends on subtropical stratocumulus cloud feedback. This subtropical cloud feedback is too weak in most climate models, suggesting that teleconnections from the Southern Ocean to the tropical Pacific are stronger than widely thought.

The El Niño-Southern Oscillation (ENSO), which originates in the Pacific, is the strongest and most well-known mode of tropical climate variability. Its reach is global, and it can force climate variations of the tropical Atlantic and Indian Oceans by perturbing the global atmospheric circulation. Less appreciated is how the tropical Atlantic and Indian Oceans affect the Pacific. Especially noteworthy is the multidecadal Atlantic warming that began in the late 1990s, because recent research suggests that it has influenced Indo-Pacific climate, the character of the ENSO cycle, and the hiatus in global surface warming. Discovery of these pantropical interactions provides a pathway forward for improving predictions of climate variability in the current climate and for refining projections of future climate under different anthropogenic forcing scenarios.

The El Niño-Southern Oscillation (ENSO), which originates in the Pacific, is the strongest and most well-known mode of tropical climate variability. Its reach is global, and it can force climate variations of the tropical Atlantic and Indian Oceans by perturbing the global atmospheric circulation. Less appreciated is how the tropical Atlantic and Indian Oceans affect the Pacific. Especially noteworthy is the multidecadal Atlantic warming that began in the late 1990s, because recent research suggests that it has influenced Indo-Pacific climate, the character of the ENSO cycle, and the hiatus in global surface warming. Discovery of these pantropical interactions provides a pathway forward for improving predictions of climate variability in the current climate and for refining projections of future climate under different anthropogenic forcing scenarios.

Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. http://www.sciencemag.org/about/science-licenses-journal-article-reuse This is an article distributed under the terms of the Science Journals Default License., The El Niño–Southern Oscillation (ENSO), which originates in the Pacific, is the strongestand most well-known mode of tropical climate variability. Its reach is global, and it canforce climate variations of the tropical Atlantic and Indian Oceans by perturbing the globalatmospheric circulation. Less appreciated is how the tropical Atlantic and Indian Oceansaffect the Pacific. Especially noteworthy is the multidecadal Atlantic warming that began inthe late 1990s, because recent research suggests that it has influenced Indo-Pacificclimate, the character of the ENSO cycle, and the hiatus in global surface warming.Discovery of these pantropical interactions provides a pathway forward for improvingpredictions of climate variability in the current climate and for refining projections offuture climate under different anthropogenic forcing scenarios., W.C. is supportedby National Key R&D Program of China (2018YFA0605700).L.W., Xia.L., and B.G. are supported by the National NaturalScience Foundation of China (NSFC) projects (41490643,41490640, U1606402, and 41521091). W.C., G.W., B.N., and A.S.are supported by CSHOR and the Earth System and ClimateChange Hub of the Australian Government’sNationalEnvironment Science Program. CSHOR is a joint research Centrefor Southern Hemisphere Oceans Research between QNLM andCSIRO. M.L. is supported by the GOTHAM Belmont project(ANR-15-JCLI-0004-01) and the ARiSE ANR project. T.L. issupported by NSFC 41630423 and NSF AGS-1565653 grants.S.M. is supported by the Australian Research Council (ARC)through grant number FT160100162. J.-S.K. was supported bya National Research Foundation of Korea (NRF) grant fundedby the Korea government (MSIT) (NRF-2018R1A5A1024958).J.-Y.Y. is supported by NSF AGS-1505145 and AGS-1833075grants. M.F.S is supported by the Institute for Basic Science(project code IBS-R028-D1). Y.-G.H. is funded by the KoreaMeteorological AdministrationResearchandDevelopmentProgram under grant KMI2018-03214. M.J.M is supported byNOAA and PMEL contribution number 4838. Y.D. is supportedby the National Natural Science Foundation of China (41525019,41830538, and 41521005) and the State Oceanic Administrationof China (GASI-IPOVAI-02). D.D. is supported by ARC grantnumber CE170100023. M.M.-R. has been supported by theMORDICUS grant under contract ANR-13-762 SENV-0002-01and CGL2017-86415-R. Y.R.-R. is supported by an 800154-INADEC individual MSCA-IF-EF-ST grant. S.-P.X. is supportedby the NSF. J.B.K. was supported by the National EnvironmentResearch Council (NE/N005783/1).Author contributions:The manuscript was written as a group effort during two“Pantropical interbasin climate interactions”workshops heldat Xiamen University and Jeju Island. All authors contributedto the manuscript preparation, interpretations, and thediscussions that led to the final figure design. W.C. and L.W, Peer Reviewed, Postprint (author's final draft)

The El Niño–Southern Oscillation (ENSO), which originates in the Pacific, is the strongest and most well-known mode of tropical climate variability. Its reach is global, and it can force climate variations of the tropical Atlantic and Indian Oceans by perturbing the global atmospheric circulation. Less appreciated is how the tropical Atlantic and Indian Oceans affect the Pacific. Especially noteworthy is the multidecadal Atlantic warming that began in the late 1990s, because recent research suggests that it has influenced Indo-Pacific climate, the character of the ENSO cycle, and the hiatus in global surface warming. Discovery of these pantropical interactions provides a pathway forward for improving predictions of climate variability in the current climate and for refining projections of future climate under different anthropogenic forcing scenarios