Response of the tropical Indian Ocean to past AMOC weakening and implications for the future

Abrupt changes in the Atlantic meridional overturning circulation (AMOC) can cause dramatic global climate changes, but their impacts on tropical hydroclimate remain uncertain. Heinrich Stadial 1 (HS1, ~18 to 14.5 thousand years ago) involves the largest AMOC reduction in recent geological time, providing a unique opportunity to investigate the influence of AMOC reduction on tropical hydroclimate. Our proxy data-model simulation synthesis reveals a zonal hydroclimate mode characterized by widespread drought in tropical East Africa and generally wet but spatially heterogeneous conditions in the Maritime Continent, analogous to the modern negative phase of the Indian Ocean Dipole. We propose that North Atlantic cooling associated with a weakened AMOC drives millennial-scale tropical Indian Ocean hydroclimate variations by affecting both the latitudinal position of the ITCZ, and the strength of the Indian Ocean Walker circulation.

In addition, we conducted new sensitivity experiments using iCESM1.3 that show glacial boundary conditions, especially changes in sea level and the exposure of Sunda and Sahul shelves, strongly modulate the tropical Indian Ocean response to North Atlantic cooling by altering the background state and interannual SST variability in the tropical Indian Ocean. Furthermore, we explored the response of the tropical Indian Ocean to a potential AMOC weakening under a global warming scenario and its relationship to interannual variability over the Indian Ocean to assess the implications for future climate change and extreme events in this region.