Role of Continents in Driving the Hadley Cells

GCM simulations are used to investigate how forcing that originates over land surfaces influences the Hadley circulation. The presence of continental surfaces is found to approximately double the intensity of the winter hemisphere Hadley cell and to halve the intensity of the summer cell. The strengthening of the winter cell occurs because the increase in surface friction associated with land enhances the angular momentum flux into the atmosphere. The development of strong monsoon circulations in the Northern Hemisphere summer and the convergence zones of the Southern Hemisphere (South Pacific, South Atlantic, and South Indian convergence zones) shifts mass out of the subtropics, lowers the zonal mean subtropical highs, and weakens the summer cells. The responses of the summer and winter cells are different signs and occur by different processes, because heating of the land surfaces in the summer is effectively communicated through the depth of the atmosphere, whereas cooling in the winter is not. Also, the higher surface wind speeds and shears of the winter hemisphere trade winds make the winter cell more sensitive to surface friction than the summer cell. These results suggest that the axisymmetric models that have provided a theoretical basis for the understanding of the Hadley circulation do not capture some of the important physical processes that determine the intensity of the mean meridional circulation.