ON LARGE-SCALE CIRCULATIONS IN CONVECTING ATMOSPHERES

The dominant thinking about the interaction between large-scale atmosp heric circulations and moist convection holds that convection acts as a heat source for the large-scale circulations, while the latter suppl y water vapour to the convection. We show that this idea has led to fu ndamental misconceptions about this interaction, and offer an alternat ive paradigm, based on the idea that convection is nearly in statistic al equilibrium with its environment. According to the alternative para digm, the vertical temperature profile itself, rather than the heating , is controlled by the convection, which ties the temperature directly to the subcloud-layer entropy. The understanding of large-scale circu lations in convecting atmospheres can, therefore, be regarded as a pro blem of understanding the distribution in space and time of the subclo ud-layer entropy. We show that the subcloud-layer entropy is controlle d by the sea surface temperature, the surface wind speed, and the larg e-scale vertical velocity in the convecting layer, and demonstrate how the recognition of this control leads to a simple, physically consist ent view of large-scale flows, ranging from the Hadley and Walker circ ulations to the 30-50-day oscillation. In particular, we argue that th e direct effect of convection on large-scale circulations is to reduce by roughly an order of magnitude the effective static stability felt by such circulations, and to damp all of them.