PNNL

Abstract

The population of the Sahel region of West Africa has approximately doubled in the past 50 years, and could potentially double again by the middle of this century. This has led to the northward expansion of agricultural areas at the expense of natural savanna, leading to widespread land use -land cover change (LULCC). Because there is strong evidence of significant surface-atmosphere coupling in this region, one of the main goals of the West African Monsoon Modeling and Evaluation (WAMME) Project Phase II is to provide basic understanding of LULCC on the regional climate, and to evaluate the sensitivity of the seasonal variability of the West African Monsoon (WAM) to LULCC. The prescribed LULCC is based on the recent 50-year period and represents a maximum feasible degradation scenario. It is applied to 5 state of the art global climate models (GCMs) over a 6-year period. Multiple GCMs are used because the magnitude of the impact of LULCC depends on model-dependent coupling strength between surface and overlying atmosphere, the magnitude of the surface biophysical changes, and how the key processes linking the surface with the atmosphere are parameterized within a particular model framework. Land cover maps and surface parameters may vary widely among models; we sought to impose consistent biogeophysical responses of surface parameters to LULCC using a simple experimental setup. The prescribed LULCC corresponds to degraded vegetation conditions, which mainly cause increases in the Bowen ratio and the surface net radiation, and result in a significant reduction in surface evaporation (upwards of 1 mm per day over a large part of the Sahel). This, in turn, leads to less moisture convergence, decreased convective heating of the atmosphere, and reduced cloud cover over the LULCC zone. The overall impact is a rainfall reduction, which ranges across models from 4-25 % averaged over the Sahel, and a southward shift of the rainfall maxima. The African Easterly Jet (AEJ) intensifies and shifts equator-ward, although the strength of this change varies considerably among the models. In most of the models, the main factor causing diabatic cooling of the upper troposphere and enhanced subsidence over the region of LULCC is the reduction of convective heating rates linked to reduced latent heat flux and moisture flux convergence. For most of the models, the LULCC evokes a precipitation dipole pattern which is consistent with the observed pattern for dry climate anomalies over the region of change. In broad agreement with previous studies, the impact of degradation on the regional climate is found to be vary among the different models, however, the signal is stronger and more consistent between the models here than in previous intercomparison projects. This is likely related to our emphasis on prioritizing a consistent impact of LULCC on the surface biophysical properties.