2002 Annual

Monday, 14 January 2002
The CCSM Simulation of West African Climate: Performance in Simulating Factors Affecting the Development of African Easterly Waves
Evan M. Blaisdell, Penn State Univ., University Park, PA; and G. S. Jenkins
In past studies, the Community Climate Model Version 3.6 (CCM3) has been used to simulate the climate of West Africa. Now this model has been integrated into the Community Climate System Model (CCSM) in the hope of better simulating both the current mean climate state and that under climate change scenarios. The uncoupled CCM3 did a passable job simulating some features of the African Easterly Jet (AEJ) and the Tropical Easterly Jet (TEJ), but had difficulty resolving the rainfall associated with African Easterly Waves (AEWs).

In this study, we examine the simulation of the TEJ and the AEJ in the CCM3 in its coupled mode, now that the model functions as the atmospheric component of the CCSM. From this we may be able to infer characteristics of AEW activity, although individual waves are of a smaller time scale than that examined here. We investigate the performance of the model during three simulated decades (the 1980s, the 2030s and the 2080s) under the IPCC A1 scenario of future greenhouse gas concentrations. The 1980s are simulated to determine how well the model simulates a known climate state.

We find a general improvement in model performance during the simulated 1980s. The geographic distribution of rainfall is better simulated, especially off the west coast of Africa. Importantly, the bimodal distribution in West African monthly rainfall found in the uncoupled CCM3 has now been replaced by a more accurate single rainfall peak in August, although the totals are still less than those observed. This may indicate that AEWs are being resolved. The TEJ and AEJ (at 200 and 700 mb, respectively) are also better resolved, but still far from perfect. A potentially important result is that the AEJ strengthens by the 2030s but then weakens by the 2080s.

The behavior of AEWs are highly dependent on both the positions and strengths of the AEJ and TEJ, and instabilities in those jet streams. Because the simulated jet streams during the 1980s are still not being sufficiently well-captured, future investigations may study the modeling of factors that influence their development, such as meridional temperature gradients and soil moisture.

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