The Indian Ocean is a unique geographical region that differs from other ocean basins due to its northern boundary being surrounded by landmass. The air-sea interaction processes within the Indian Ocean directly impact precipitation patterns in the adjacent regions, which are home to billions of people across national boundaries. The objective of this research is to comprehend the different coupled atmospheric-oceanic interaction mechanisms that are expected to occur in the Indian Ocean under future climate change. Specifically, the study examines (1) the factors governing the projected slowdown of the Indian Ocean Walker circulation, (2) formation mechanism of future Indian Ocean warming patterns, and (3) changes in rainfall over India in response to a warming climate.
Firstly, the projected weakening of zonal atmospheric circulation in the Pacific Ocean is a widely accepted to be a commonality with projections of climate change; however, little attention is paid to the changes in the Indian Ocean Walker circulation (IWC) in response to global warming. Previous studies attribute the projected IWC weakening is linked to a positive Indian Ocean Dipole (pIOD)-like warming pattern. But these pIOD-like warming patterns in the future climate could be induced by the slowdown of the IWC. As a result, establishing a clear cause-and-effect relationship becomes challenging, rendering the IWC projected changes poorly constrained and highly uncertain. This study employs a comprehensive set of coupled climate model simulations to investigate the IWC changes in the latter half of the 21st century under a high-emission scenario. Anomalous ascending motion over the western Indian Ocean and anomalous descending motion over the eastern Indian Ocean, signifying the slowdown of the IWC, is supported by an ensemble of climate model simulations. Furthermore, it is found that the weakening of the IWC is associated with not only a pIOD-like warming pattern but also the anomalous local meridional circulation that reinforces anomalous descending motion over the eastern Indian Ocean, contributing to the overall weakening in the strength of the IWC in the future climate. Moreover, further analysis based on atmosphere-only experiments indicates that the anomalous local meridional circulation is unlikely to result from a pIOD-like warming pattern, implying that the IWC slowdown is partly associated with the anomalous local meridional circulation. These findings highlight the significance of considering the dynamics of the local meridional circulation changes in understanding the projected IWC changes.
Secondly, the majority of future projections from coupled climate models indicate an inhomogeneous warming pattern in the Indian Ocean, with enhanced warming in the Arabian Sea (AS) and southeastern Indian Ocean (SEIO) during the latter half of the 21st century. Although the corresponding spatial temperature gradients are associated with large-scale atmospheric circulation anomalies and rainfall trends with far-reaching societal implications, little is known about the underlying physical drivers. This study employs an analysis from the extensive CESM2-LE simulations to identify the formation mechanism of non-uniform Indian Ocean warming patterns. The results show that strong negative air-sea interactions over the eastern Indian Ocean lead to a weakening of the equatorial zonal sea surface temperature gradient (east minus west) under future climate change, which, in turn, results in IWC slowdown and the generation of southeasterly wind anomalies over the AS. Consequently, these contribute to an enhanced anomalous northward ocean heat transport, reduced evaporative cooling, weakened upper ocean vertical mixing, and an overall enhanced future warming over the AS. In contrast, the projected warming pattern in the SEIO is primarily attributed to cloud-shortwave radiative flux changes, i.e., reduction in low-cloud cover leads to enhanced shortwave cloud forcing, thereby enhancing the amount of shortwave radiation reaching the ocean surface. This study shows the importance of different coupled atmospheric-oceanic interaction mechanisms in controlling the future Indian Ocean warming patterns.
Finally, I investigated the different factors contributing to the rainfall changes over India. Although coupled general circulation models project a substantial increase in rainfall over India by the end of the 21st century, little is known about the underlying physical drivers of rainfall changes over India beyond 2100. Here, I analyze the CESM2-LE extension run simulations under historical/SSP3-7.0, forcing over 1850-2500 to elucidate the drivers of rainfall changes over India. The results indicate that the primary factors responsible for rainfall changes (interhemispheric pressure gradient, AS wind changes, and tropical easterly jet) show a pronounced weakening after the 21st century, indicating the role of other processes in governing rainfall changes over India. Moreover, the lower-level land-sea thermal contrast is reduced due to enhanced sea surface warming over the southern Indian Ocean after 2100. In contrast, the land-sea thermal contrast at the upper level is enhanced, which increases the atmospheric stability, weakens the large-scale monsoonal circulation, and causes a northward shift of lower-level winds over the AS after the 21st century. Consequently, the northward shift of lower-level winds leads to the increased and northward shift of the moisture transport over the Indian region, ultimately leading to increased summer monsoon rainfall over India beyond 2100.
In conclusion, these findings provide valuable insights into the complex interplay of the different physical processes governing the Indian Ocean response to future climate change, which have significant implications for both society and ecosystems and implement effective strategies to mitigating the impacts of climate change in this region and beyond.