A new index to describe the tropical Asian summer monsoon

We define a new monsoon index (MV) as the product of relative vorticity and equivalent potential temperature using the long-term NCEP/NCAR reanalysis data. The MV index provides new insights into the intraseasonal and interannual variabilities of the broad-scale tropical Asian summer monsoon (TASM), including the South Asian summer monsoon (SASM) and the South China Sea summer monsoon (SCSSM). On the intraseasonal timescale, the pentad-to-pentad MV index bears a close relationship to the broad-scale rainfall in the TASM regions. Among 29 summers from 1979 to 2007, in 23/27 summers the correlation coefficients are higher than 0.7 in the SASM/SCSSM region. However, in fewer than 9 summers, the correlations between the broad-scale rainfall and the existing circulation indices are higher than 0.7. On the interannual timescale, various existing SASM circulation indices are moderately or well correlated with all-India summer monsoon rainfall, whereas their correlations with broad-scale SASM rainfall are weak. In contrast, the summer mean MV index correlates well with the broad-scale SASM rainfall and all-India summer monsoon rainfall (correlation of 0.73 and 0.65, respectively). In the SCSSM region, the summer mean MV index also bears a close relationship to the SCSSM rainfall, although some discrepancies exist during certain years. The composite strong TASM shows a stronger low-tropospheric low pressure in association with the enhanced westerly winds and moisture transfer, stronger convection, and upper-tropospheric easterly winds, which indicate that the MV index can well capture the features of TASM. Supported by National Basic Research Program of China (Grant No. 2006CB400500), China Postdoctoral Science Foundation (Grant No. 20070410133), Open Foundation of Jiangsu Key Laboratory of Meteorological Disaster (Grant No. KLME0704)