The Community Climate Model (CCM3) Simulation of the West African Rainy Season. Implications for driving Regional Climate Models

The 1980’s represent one of the driest decades in West Africa during the 20^th Century. Climate model simulations with the CCM3 using climatological and observed sea surface temperatures have been undertaken to examine the model’s ability to capture the variability during this time period. These simulations are compared to the NCEP reanalysis. The CCM3 simulations have two significant biases in their mean states. (1) The 200 mb tropical easterly jet (TEJ) is located too far north and the 700 African Easterly Jet (AEJ) does not extend far enough to the east. Moreover, the observed TEJ is generally located equatorward of the AEJ but in the CCM3, they are located at nearly the same latitudes.

A more important concern is that the CCM3 does not show any significant easterly wave activity. This is vital since easterly waves are generally associated with rain and they may have played a critical role in the recent dry period. On large scales, the NCEP reanalysis show that the TEJ was weaker during every JJA period with the exception of 1988. The AEJ was also stronger and in some years was clearly displaced to lower latitudes. This pattern of a weaker TEJ is captured to some degree in the CCM3, but the 700 mb differences are not. Finally the mean states of a regional climate model simulation that is driven at the lateral boundaries by the CCM3 during the 1988 JJA period is presented and implications are shown.