Enhanced equatorial warming causes deep-tropical contraction and subtropical monsoon shift

Under anthropogenic warming, deep-tropical ascent of the intertropical convergence zone (ITCZ) is projected to contract equatorward1–3 while subtropical descent associated with the Hadley cell edge is predicted to expand poleward4. These changes have important implications for regional climate2,5–7, but their mechanisms are not well understood. Here we reveal a key role of enhanced equatorial surface warming (EEW) in driving the deep-tropical contraction and modulating the Hadley expansion. By shifting the seasonally warmed sea surface temperature equatorward, EEW reduces the meridional migration of the seasonal ITCZ and causes an annual-mean deep-tropical contraction. This process further contracts the subtropical circulation, as seen during El Nino, and counteracts the Hadley expansion caused by the global-scale warming. The EEW-induced contraction even dominates in the Northern Hemisphere early summer (June–July), when atmospheric circulation responses to the global-scale warming are weak8. Regionally, this alters the East Asian summer monsoon, shifting both the subtropical jet and Meiyu–Baiu rainband equatorward. Among models in Phase 5 of the Coupled Model Intercomparison Project9, the degrees of the equatorward shift in the ITCZ, the early-summer subtropical circulation and the East Asian summer monsoon are correlated with EEW. Our results suggest that a better constraint on EEW is critical for accurate projection of tropical and subtropical climate change. Global warming projections exhibit a contracted intertropical convergence zone (ITCZ) and an expanded Hadley cell. Here, equatorial Pacific warming is shown to contract seasonal ITCZ migration and counteract Hadley cell expansion, leading to an equatorward shift in the Asian subtropical monsoon.