Local meridional circulation changes contribute to a projected slowdown of the Indian Ocean Walker circulation

Sahil Sharma,Wenju Cai,Tamás Bódai,Eui-Seok Chung,Kyung-Ja Ha

doi:10.1038/s41612-022-00242-w

Abstract

The weakening of zonal atmospheric circulation, a widely accepted projection of climate change in response to global warming, features a weakening of the Indian Ocean Walker circulation (IWC), with an anomalous ascending motion over the western and anomalous descending motion over the eastern Indian Ocean. The projected IWC weakening has previously been attributed to slower warming in the east than the west, that is, to a positive Indian Ocean Dipole (IOD)-like warming pattern. However, such a warming pattern can also be induced by IWC weakening. As a result, the cause-and-effect relationship cannot be easily determined, and the projected change is poorly constrained and highly uncertain. Here, using a suite of coupled climate model simulations under a high-emission scenario, we find that the IWC slowdown is accompanied by not only a positive IOD-like warming pattern but also anomalous meridional circulation that is associated with anomalous descending motion over the eastern Indian Ocean. We further show that the anomalous local meridional circulation is closely linked to enhanced land-sea thermal contrast and is unlikely to result from the positive IOD-like warming pattern, suggesting that the IWC weakening is in part driven by the anomalous local meridional circulation. Our findings underscore the important role of local meridional circulation changes in modulating future IWC changes.

Highlights

Predicting changes in the pattern of atmospheric circulation and associated atmospheric teleconnections over the tropics is critical for agriculture, water resources, and ecosystem management
To determine the seasonal changes. This shows that the slowdown of Indian Ocean (IO) Walker circulation (IWC) is due to a positive IOD (pIOD)
Model simulations indicate comparatively less pronounced weakening in the descending branch in the western IO (WIO)

Summary

Introduction

Predicting changes in the pattern of atmospheric circulation and associated atmospheric teleconnections over the tropics is critical for agriculture, water resources, and ecosystem management. The. Walker circulation consists of the zonal atmospheric overturning cells over the tropics with surface westerlies (easterlies) over the equatorial Indian (Pacific) Ocean[1]. Variations in the structure of the Walker circulations are closely tied to precipitation and ocean temperature patterns[2–5] and to fishery stocks through variations in nutrient supply caused by the upwelling of cold water[6]. Recent studies of the Walker circulation over the IndoPacific region have conflictingly suggested either a weakening[6–16] or strengthening[1,3,4,17–24] pattern in response to global warming. Model-simulated weakening of the Walker circulation in response to increasing greenhouse gas concentrations has been attributed to a differential increasing rate between precipitation (~3%/K) and atmospheric water vapor (~7%/K)[9,10]. No existing study addresses the contribution of meridional circulation to zonal circulation changes over the Indian Ocean (IO).

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Conclusion