Thursday, 17 January 2002: 9:14 AM
Interannual variability of the West African monsoon in the NCEP SFM and reanalysis
Gridded precipitation data, NCEP Seasonal Forecast Model (SFM) simulated rainfall and monsoon circulation, and NCEP-NCAR Reanalysis (R1) and NCEP-DOE Reanalysis (R2) are used to evaluate the performance of the NCEP model to simulate the annual cycle, the base state, and the interannual variability of the West African monsoon system. The paper also compares R1 with R2 which was designed to improve upon R1 by including a simple rainfall assimilation scheme and by updating the parameterization of physical processes. Results show that the general features of the observed climatological mean circulation are reasonably well captured by the NCEP SFM model. However, the model produces stronger winds in the low-level southwesterly flow, a stronger African Easterly Jet (AEJ), and a stronger Tropical Easterly Jet (TEJ) compared to observations. With respect to precipitation, the climatological mean rainfall simulated by the model features higher intensity of rainfall over the Guinean highlands than observed. This is also the case for R2 which exhibits a rainfall pattern over the Guinean highlands quite similar to the OLR pattern. Rainfall intensity in R1 over the Guinean highlands appears to be reasonably well simulated. The model's annual cycle depicts reasonably well the onset of the West African monsoon, but with lower rainfall amounts over both the Sahel and the Gulf of Guinea compared to observations. Thus, the model depicts a lag in the northward progression of the rains and an early withdrawal throughout the seasonal cycle. However, the model results are quite encouraging in depicting the base state and the interannual variability.