Impacts of oceanic warming patterns versus CO2 radiative forcing on the Hadley Circulation

The Hadley circulation (hereafter HC) is one of the most prominent meridional overturning circulations in the climate system. In addition to maintaining energy balance and momentum exchange in tropics and extratropics, it can also shape the Intertropical Convergence Zone (ITCZ) and subtropical dry arid zones by regulating the hydrological cycle in tropical and extratropical regions. Weakening and expanding HC and narrowing of the ITCZ are projected with human greenhouse gas emissions. However, no consensus has been achieved regarding the relative importance of direct CO₂ radiative effect and indirect effects via SST changes in shaping the future HC changes. This limits our deep understanding of the climate impacts imposed by changes in the HC. Here we analyze a broad range of CMIP5 experiments and show that future changes in SST patterns play the leading role in the determining the future changes in HC and ITCZ. In addition, a series of individual basin perturbation experiments were conducted at 1.5°C, 2°C, and 3°C temperature thresholds to identify key basins that determine HC strength, edges, and ITCZ locations. Our work highlights the overwhelming role of future tropical Indian Ocean warming on the HC and ITCZ changes.