Abstract
The Indian monsoon intensity index suggested by Webster and Yang (WY index) is optimized and improved in this paper. At first, the area (40 degrees E–110 degrees E, 0 degrees –20 degrees N) for calculating the zonal wind-shear between 850hPa and 200hPa in the WY index is optimized and adjusted according to the significance test of differences of wind fields. Then the regionally averaged zonal wind is computed over the optimized area. Finally, the optimal linear combination of the zonal winds at the two levels is performed using the regression method, thus defining a new broad-scale circulation index for the interannual variability of the south Asian summer monsoon, i.e. the improved south Asian summer monsoon index. Results indicate that the improved south Asian summer monsoon index has two advantages: its correlation with the All Indian Summer Monsoon Rainfall index is higher than that of WY index and its computational domain of circulation is also larger than that of WY index. The computational results based on the 1948–98 NCAR/NCEP wind data indicate that the correlation of the improved SASM index with the All Indian Summer Monsoon Rainfall index is higher than that of WY index by 0.27.
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