Distinct internal tropical Pacific sea surface temperature patterns drive similar Hadley circulation trend uncertainty

The Hadley circulation critically influences tropical climate by redistributing heat and moisture between tropics and higher latitudes. Although global warming is expected to weaken the Hadley circulation following thermodynamic constraints, we observe a strengthening trend, especially in the Southern Hemisphere. Climate models also poorly agree on sign and magnitude of the historical trend, and sources of this uncertainty remain unclear. Here we demonstrate using model simulations that internal tropical Pacific Sea Surface Temperature variability modulates Southern Hemisphere Hadley circulation on multidecadal timescales, contributing to the historical trend uncertainty. Both internal warming and cooling in the tropical Pacific can drive similar circulation trends. Hemispherically symmetric tropical Pacific warming strengthens the circulation through direct thermal forcing, whereas asymmetric Southern Hemisphere cooling does so through indirect cross-equatorial energy transport. Therefore, low-frequency tropical Pacific temperature patterns are important when attributing the Southern Hemisphere Hadley circulation trend to external forcing over recent and coming decades.