Dynamics and Energy Balance of the Hadley Circulation and the Tropical Precipitation Zones: Significance of the Distribution of Evaporation

Abstract

A series of numerical experiments is performed using a general circulation model with an idealistic ocean-covered boundary condition. The meridional structure of the Hadley circulation system, which is a combined structure of the Hadley circulation and the tropical precipitation zone, is examined from the standpoints of the water vapor and the energy budgets. Although the prescribed SST distribution has a broad peak centered at the equator, the distribution of the precipitation has two peaks straddling the equator. The distribution of the evaporation rate is revealed to be an important factor in the formation of this structure. The evaporation rate is smaller near the equator than in the subtropics because of its dependence upon the wind speed. If this dependence is removed from the parameterization of the evaporation, the latitudinal distribution of evaporation becomes flat and the precipitation concentrates at the equator to form a single band structure. Qualitatively similar results are obtained in the experiments with axisymmetric two-dimensional model without the effect of disturbances. The net supply of the moist static energy, which is the difference between the supply of energy mainly through evaporation and the loss of energy through radiative cooling, must balance with the divergence of the meridional transport of moist static energy. It is considered to be the quantity controlling the meridional structure of the system. Since the latent energy supply and the radiative loss tend to cancel each other, a small fractional change in the evaporation rate causes a large change in the net energy supply and thus results in a strong modification of the meridional structure of the system. In other words, a strong positive feedback between the strength of the Hadley circulation and the latitudinal concentration of the precipitation provides the high sensitivity of the Hadley circulation system to the distribution of the evaporation, which is the principal energy source of the system. A set of comparative experiments with different cumulus parameterization schemes is performed. It is shown that the difference in the vertical stratification significantly modifies the energy budget of the Hadley circulation system and causes a large difference in the distribution of precipitation.