Home (English)   Home (Chinese) 

West African Summer Monsoon Index (WASMI)

The WASM index data

The West African summer monsoon index (WASMI) is defined as an area-averaged seasonally (JJAS) dynamical normalized seasonality (DNS)  at 850 hPa within the West African monsoon domain (5°-17.5°N, 20°W-40°E) (Li and Zeng, 2002, 2003, 2005). A remarkable feature of the West African summer monsoon (WASM) different from other monsoons is that the WASMI is dominated by variability on the decadal time-scale. 

The West African Summer Monsoon Index (WASMI)

The normalized time series of WASMI (1948-2010).

The summer here is JJAS. The thick solid lines indicate 9-year Gaussian-type filtered values.

The WASMI occurred a decadal abrupt decrease in 1967 which may has the crucial role in reducing rainfall over the Sahel. The WASMI shows a very high correlation of 0.76 with the Sahel summer (JJAS) rainfall for the period of 1958-98. Moreover, a broad region of significantly positive correlations between the WASMI and summer rainfall over the African monsoon regime is just found in the Sahel. Li and Zeng (2002) pointed out that during the persistent weak African monsoon period (1967-98) the 925 hPa flux of moisture from the Atlantic into the Sahel region is decreased by about 20-50% of the climatological values (1958-98) and by about 40-80% of the average values for the strong WASM period (1958-66). As a result the supply of moisture for precipitation over there is insufficient, and thereby producing less latent heating there. The facts imply that the persistent weak WASM can provide a coherent change in the Sahel rainfall, mainly through the local monsoon effect; that is, the persistent decreased southwesterly monsoon leads to a persistent reduction in moisture flux from the Atlantic into the Sahel.