On the dynamics of the Hadley circulation and subtropical drying

Changes in subtropical precipitation and the Hadley circulation (HC) are inextricably linked. The original Halley–Hadley model cannot explain certain aspects of the Earth’s meridional circulation in the tropics and is of limited use in understanding regional changes in precipitation. Here, we expand on previous work on the regional and seasonal aspects of the HC, in particular how land–sea temperature contrasts contribute to the strength and width of the HC. We show that the Earth’s HC is well described by three regionally distinct cells along the eastern edges of the major ocean basins with opposite circulations elsewhere. Moreover, comparable summertime hemisphere cells emerge in each region. While it has been recognized that continents modify the meridional pressure gradient, we demonstrate that a substantial part of the Earth’s HC is driven by zonal pressure gradients (ZPGs) that only exist due to continental heating and air–sea interaction. Projected changes in land–sea temperature contrasts in a warming climate due to the relatively low thermal capacity of land will also affect ZPGs and thus HC strength and width, with implications for extremes in hydroclimate and freshwater resources across the increasingly vulnerable subtropics.