More-frequent extreme northward shifts of eastern Indian Ocean tropical convergence under greenhouse warming

Abstract

The Intertropical Convergence Zone (ITCZ) in the tropical eastern Indian Ocean exhibits strong interannual variability, often co-occurring with positive Indian Ocean Dipole (pIOD) events. During what we identify as an extreme ITCZ event, a drastic northward shift of atmospheric convection coincides with an anomalously strong north-minus-south sea surface temperature (SST) gradient over the eastern equatorial Indian Ocean. Such shifts lead to severe droughts over the maritime continent and surrounding islands but also devastating floods in southern parts of the Indian subcontinent. Understanding future changes of the ITCZ is therefore of major scientific and socioeconomic interest. Here we find a more-than-doubling in the frequency of extreme ITCZ events under greenhouse warming, estimated from climate models participating in the Coupled Model Intercomparison Project phase 5 that are able to simulate such events. The increase is due to a mean state change with an enhanced north-minus-south SST gradient and a weakened Walker Circulation, facilitating smaller perturbations to shift the ITCZ northwards.