Seasonal Dependence of the Projected Indian Ocean Walker Circulation Uncertainty under Greenhouse Warming

Abstract Climate models exhibit a weakening of the Indian Ocean Walker circulation (IWC) under global warming. However, the model projections are highly uncertain. As the ocean and atmosphere are fully coupled in the tropical Indian Ocean, it is difficult to attribute this circulation uncertainty to the sea surface temperature warming pattern through local dynamics. This study investigates the projected IWC uncertainty from the perspective of interbasin and extratropical processes. The sources of intermodel uncertainty vary across seasons. In boreal winter, tropical processes are the primary factors contributing to the IWC uncertainty, as the convection anchored over the Maritime Continent significantly modulates changes in the tropical atmospheric circulations. Particularly, the Pacific teleconnection plays a key role because the IWC and Pacific Walker circulation covary and are both regulated by warm pool convections. In boreal summer, warm pool convection is insufficient to dominate the IWC uncertainty as the precipitation maximum has migrated northward to the monsoon region. Extratropical factors such as changes in the Atlantic meridional overturning circulation and sea ice, which potentially contribute to the interhemispheric warming contrast, are suggested to dominate the IWC uncertainty. Our findings reveal the distinct sources of intermodel uncertainty in the IWC change in different seasons and advance the understanding of the relative importance of tropical and extratropical processes in shaping the Indian Ocean climate change. Significance Statement Under global warming, the Indian Ocean Walker circulation (IWC) is suggested to weaken based on climate model simulations. However, the model projections exhibit large disagreements. This study aims to identify the reasons why climate models disagree with each other. We find that the sources of the discrepancy vary seasonally. Tropical processes such as warm pool convection and Pacific teleconnection are considered to be the primary factors in boreal winter. In contrast, extratropical processes including interhemispheric thermal contrasts dominate in boreal summer. These results have implications for understanding the roles of complex interactions that contribute to climate change in the Indian Ocean.