An Idealized Study of West African Monsoon Variability: An Energetic Perspective

The Sahel region of Africa exhibits great interannual and decadal variability in precipitation, largely because of variations in the strength of the summer monsoon. Variations in Sahel precipitation have been shown to be associated with changes in sea surface temperature (SST) in the Atlantic and Indian Oceans (e.g. Giannini et. al 2003), as well as with changes in the strength of the Sahara Heat Low (e.g. Biasutti et. al 2008). However, the mechanisms responsible for these correlations remain unclear.

To investigate these mechanisms, we conduct a study with an idealized model. We use the Weather Research and Forecasting (WRF) model in a 3D moist configuration at 15 km horizontal resolution, with zonally symmetric lower boundary conditions on an equatorial β-plane. The domain is chosen to have a zonal extent of 20° longitude and a meridional extent of 140° latitude. Boundaries and land surface properties were chosen to be representative of western Africa during the summer season. In particular, an ocean with prescribed SST is located south of 5°N and north of 32°N. Between 5°N and 32°N, the land consists of tropical grassland south of a subtropical desert. The simulated mean state provides a reasonably accurate representation of the African easterly jet, the Saharan heat low, and associated transient eddy activity.

We apply energetic perturbations to this simulated mean state, such as changes in desert land surface albedo and SST, which produce changes in the monsoonal ITCZ. The ITCZ response is generally consistent with both observations and previous model studies, in that the ITCZ shifts toward a positive anomalous energy source. We construct a column integrated moist static energy (MSE) budget to better understand the mechanism of these changes. Changes in desert albedo produce an anomalous energy source that is compensated for mostly by meridional energy transports accomplished by an anomalous time-mean, deep-tropospheric meridional circulation. Although there are large changes in the strength of the desert heat low, its associated shallow meridional circulation, and eddy activity in the Sahel, those changes have a small effect on the MSE budget. This suggests that the mechanism for variations in Sahel rainfall might be understood in terms of first-baroclinic mode theory for the tropical atmosphere, despite the involvement of shallow circulations that project onto higher baroclinic modes. We also present the MSE budget response to changes in SST that are local to and remote from the monsoonal ITCZ, and discuss possible connections to observed interannual variability.