West African Summer Monsoon Index (WASMI)

 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 (1948-2023).

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.

 Correlation maps between the seasonal (June-September) rainfall and WASMI (1979-97). The shaded areas indicate significant at the 95% confidence level.

Correlation between precipitation and the WASMI (shading) and regression of 850-mb winds against the monsoon index (vectors) for June, July, August and September from 1979 to 2006, respectively. The shaded areas and vectors indicate significant at the 95% confidence level.

Related References

• Li, J. P., J. Feng, and Y. Li, 2011: A possible cause of decreasing summer rainfall in northeast Australia. Int. J. Climatol., 31, doi: 10.1002/joc.2328. PDF
• Li, J. P., Z. W. Wu, Z. H. Jiang, and J. H. He, 2010: Can global warming strengthen the East Asian summer monsoon?. J. Climate, 23, 6696-6705. PDF
• Feng, J., J. P. Li, and Y. Li, 2010: A monsoon-like Southwest Australian circulation and its relation with rainfall in Southwest Western Australia. J. Climate, 23: 1334-1353. PDF
• Wang, B., Z. W. Wu, J. P. Li, J. Liu, C. P. Chang, Y. H. Ding, and G. X. Wu, 2008: How to measure the strength of the East Asian summer monsoon. J. Climate, 21, 4449-4462. PDF
• Li, J. P., and Q. C. Zeng, 2005: A new monsoon index, its interannual variability and relation with monsoon precipitation. Climatic and Environmental Research, 10(3): 351-365. PDF　
• Li, J. P., and Q. C. Zeng, 2003: A new monsoon index and the geographical distribution of the global monsoons. Adv. Atmos. Sci., 20, 299-302. PDF 　
• Li, J. P., and Q. C. Zeng, 2002: A unified monsoon index. Geophysical Research Letters, 29(8), 1274, doi:10.1029/2001GL013874. PDF 
• Zeng, Q. C., and J. P. Li, 2002: On the Interaction between Northern and Southern Hemispheric Atmospheres and the Essence of Tropical Monsoon. Chinese Journal of Atmospheric Sciences, 26, 207-226. PDF
• Li, J. P., and Q. C. Zeng, 2000: Significance of the normalized seasonality of wind field and its rationality for characterizing the monsoon. Science in China (D), 43(6): 647-653. PDF  

Subroutine for Dynamical Normalized Seasonality (DNS): seasonality(mx,my,v1,v2,v3,v4,fai0,grdy,suv)