Recent Strengthening of the Western Pacific Hadley Circulation Driven by Tropical Inter‐Basin Sea Surface Temperature Gradients

Abstract The Western Pacific Hadley Circulation (WPHC), the strongest regional Hadley circulation (HC), plays a crucial role in regional and global climate variability. Observations since 1979 reveal a pronounced strengthening of the boreal spring WPHC in the Northern Hemisphere, but the contributions of internal variability versus external forcing remain unclear. Using large ensemble simulations, we find that approximately 71% of the recent strengthening is attributed to phase transitions in three key sea surface temperature (SST) gradients—tropical Western Pacific (TWP)‐Western North Pacific, TWP‐Tropical Eastern Pacific, and TWP‐Tropical Indian Ocean, indicating an internal variability mode linked to the inter‐basin SST gradient. Constraining future projections with skillful ensemble members based on these gradient phases reduces projection uncertainty by nearly 49% and suggests a likely weakening in coming decades. These results highlight the dominant role of internal variability and the importance of accurately representing tropical SST gradients for near‐term regional HC projections.