Development and Application of a Mesoscale Climate Model for the Tropics: Influence of SST Anomalies on the West African Monsoon

A mesoscale climate model (MCM) is developed from the Pennsylvania State University/ National Center for Atmospheric Research (PSU/ NCAR) MM5 model to simulate the West African summer monsoon. Results from the MCM are compared to observations, a reanalysis, and GCM output to show that the MCM reasonably simulates the West African monsoon climate and its variability, and improves on many shortcomings of the GCM simulations. The MCM's ability to correctly capture processes that cause interannual variability and, thereby allow us to improve our physical understanding of that variability, is investigated by examining the influence of Gulf of Guinea SSTAs on the West African monsoon. Similar to observations, precipitation decreases over the Sahel and increases along the Guinean Coast in response to anomalously warm SSTs in the Gulf. The increase in rainfall along the Guinean Coast is associated with an increase in lower-tropospheric water vapor content due to enhanced evaporation over the warm SSTAs and northward moisture advection in the monsoon flow. This stronger precipitation on the coast occurs despite a decrease in the land/sea temperature gradient, which is the fundamental driver of the monsoon. The decrease in rainfall over the southern Sahel is associated with lower-tropospheric subsidence replacing rising vertical motions as the monsoon circulation is shifted equatorward. Dynamically, the subsidence over the southern Sahel is associated with shrinking of both planetary and relative vorticity. The former is related to the equatorward shift of the monsoon, which results in an expansion of the equatorward flow from the Sahel to the northern Guinean Coast. The latter occurs because the equatorward flow over the southern Sahel occurs lower in the troposphere (i.e., 850 mb) than in the control (i.e., 700 mb) where the meridional relative vorticity gradient is opposite to the gradient at 700 mb.