Climate change is a major global concern today. The world is experiencing a drastic increase in temperatures, both over land and oceans. Although this warming is global, some areas are warming faster than others. This temperature variability, especially over oceans, has far-reaching consequences for the wind and weather systems that will affect societies and ecosystems across continents. For instance, the Arctic amplification patterns and the east–west temperature difference in the equatorial Pacific can impact regions far beyond the tropics.
Similarly, changes in sea surface temperatures over the Indian Ocean (IO) could influence climate and rainfall trends across Asia, Africa, and Australia. In fact, IO warming variations can also impact North Atlantic temperature patterns and the Atlantic meridional overturning circulation, an essential component of the global climate system. Many climate projections using coupled general circulation models reveal a non-uniform warming of the IO, with warming hotspots in the Arabian Sea (AS) and the southeastern IO (SEIO). However, there is little understanding of the mechanisms that can lead to such warming patterns and subsequently affect the weather in adjacent land areas.
To bridge this knowledge gap, a team of researchers from Korea and Japan led by Professor Kyung-Ja Ha from Pusan National University (PNU), Korea, has recently investigated the IO warming patterns using large ensemble simulations. In this study, authored by Ph.D. student Sahil Sharma from PNU and published online in Nature Communications on 30 March 2023, the researchers performed an ocean–atmosphere-based analysis using the Community Earth System Model 2 (CESM2), identifying the physical mechanisms underlying the future non-uniform IO warming.
"Instead of running the climate model simulation only once, we performed one hundred simulations on CESM2, which represented the different realizations of the variability in the IO climate system. This new modeling resource has been instrumental in identifying the complex ocean–atmosphere interactions responsible for modulating the IO circulation and warming patterns," explains Mr. Sharma.
The researchers identified the air–sea interactions in the Eastern IO (EIO)-somewhere near western Indonesia- as a key driver for the warming in the AS. Here, colder deep water upwells to the surface, causing the EIO to be relatively cooler than other IO waters with weak regional warming predicted in the region. This reduced regional EIO warming is also accompanied by enhanced sea-level pressure and strong winds blowing towards the AS, which significantly alter the IO Walker circulation. These changes will cause the warmer tropical waters to move towards the AS, resulting in its warming. In addition, this will induce wetter conditions in the region due to more rainfall and less evaporation, resulting in more stable near-surface-water stratification. This development, in turn, will prevent the upwelling of deeper, cooler waters, further enhancing AS warming.
In contrast, the researchers found that the projected warming in the SEIO will be amplified by a reduction in the total low cloud cover over the region, resulting in more shortwave radiation hitting the ocean surface and causing it to warm. Therefore, the drivers for the future warming hotspots in the AS and the SEIO are quite distinct from each other, with ocean circulations modulating the former and cloud-induced heat fluxes controlling the latter phenomenon.
Based on these results, the researchers concluded that regional air–sea interactions can trigger large-scale variations in atmospheric circulations with widespread impacts even in regions outside the IO. Highlighting the implications of the present findings, Prof. Ha says: "Unless carbon dioxide emissions are drastically reduced, global warming will continue to enhance the warming in the AS with dire consequences for regional climate, marine biogeochemistry, and the African and Indian monsoon systems. This research could facilitate better marine resource management and information dissemination to fisheries and also guide future research on the effect of the projected non-uniform IO warming on regional rainfall patterns."
Source: https://www.pusan.ac.kr/eng/