Recent strengthening of the Western Pacific Hadley Circulation driven by tropical inter-basin sea surface temperature gradients

The Western Pacific Hadley Circulation (WPHC), the strongest regional Hadley circulation, plays a crucial role in regional and global climate variability. Observations since 1979 indicate a significant strengthening of the boreal spring WPHC in the Northern Hemisphere; however, the relative roles of internal climate variability and external forcing remain unclear. Here, using large ensemble climate simulations together with observational constraints, we quantify the drivers of recent WPHC changes and provide near-term future projections.

We show that approximately 71% of the observed strengthening is attributable to internal variability associated with phase transitions in three key tropical inter-basin sea surface temperature (SST) gradients—tropical Western Pacific (TWP)-Western North Pacific, TWP-Tropical Eastern Pacific, and TWP-Tropical Indian Ocean. By constraining future projections using ensemble members that better reproduce the historical evolution of these SST gradients, we reduce projection uncertainty by nearly 49%. The constrained projections consistently indicate a likely weakening of the WPHC in the coming decades. 

Our results highlight the critical importance of tropical inter-basin SST gradients in shaping regional Hadley circulation variability and underscore their value for improving the reliability of near-term regional climate projections.