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 equatorward(1-3) while subtropical descent associated with the Hadley cell edge is predicted to expand poleward(4). These changes have important implications for regional climate(2,5-7), 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 weak(8). Regionally, this alters the East Asian summer mon-soon, shifting both the subtropical jet and Meiyu-Baiu rain-band equatorward. Among models in Phase 5 of the Coupled Model Intercomparison Project(9), 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.