Influence of Soil Moisture and Surface Albedo Changes over the African Tropical Rain Forest on Summer Climate

Investigated with the MRI⋅GCM-I

Abstract

The influence of a change in soil characteristics due to a deforestation of the tropical rain forest on climate is studied by the use of an atmospheric general circulation model (MRI⋅GCM-I). Four experiments are run for four months, beginning on May 1 00Z and time averages for June through August are analyzed. The control run (C) has a spatially uniform soil moisture capacity and a seasonally varying surface albedo compiled by Matthews. The second is a soil moisture experiment (W) where the soil moisture capacity is reduced between 0° and 12°N over Africa (the anomaly region). The third is an albedo experiment (A) where the surface albedo is increased to 0.30 corresponding to a desert condition instead of the realistic one in the anomaly region. The final experiment (AW) changes both the soil moisture capacity and the surface albedo in the anomaly region.
The experiments show that the changes in soil moisture and surface albedo have very different effects on the local climate. In W, the decrease of evapotranspiration does not always result in a decrease of precipitation since there is an accompanying increase in the horizontal moisture flux convergence. This is accomplished by an increased surface temperature, greater diabatic heating in the lower troposphere by the increased sensible heat flux resulting in an increased convergence in the moisture flux. In A, the decrease of precipitation is greater than that of evapotranspiration, so the updraft is much suppressed and the net moisture flux divergence occurs. Since the surface heat energy nearly balances between the reduced absorption of solar radiation due to the increased surface albedo and the resulting reduced evapotranspiration, the decrease of surface air temperature is slight. Local responses obtained by AW are almost equal to the sum of those obtained in W and A.
The local Hadley circulation is weakened in AW. Heating anomalies over the anomaly region are negative in the upper and middle troposphere due to the reduced precipitation, and positive in the lower troposphere due to the increase in the sensible heat flux. The reverse is the case in the Sahel region where precipitation increased. The differences in the zonal wind over West Africa between AW and C resemble those observed during the wet and dry periods in the Sahel region. The east-west circulation in AW, consisting of the lower tropospheric westerlies (African monsoon flow) and updrafts, is strengthened by the increased heating associated with a greater sensible heat flux convergence. Clear changes occur in the east-west circulation between the African continent and the Indian Ocean, suggesting a westward displacement of the Indian monsoon precipitation maximum. The zonally symmetric circulation anomalies are also found. Anticyclonic circulations in the upper troposphere subtropics of both hemispheres are strengthened in AW. Superposed on these circulations are three well-defined quasi-stationary wave trains which are accompanied by precipitation and temperature anomalies.