An improvement of the Plateau monsoon index based on seasonal variation of wind and its comparison with other indexes

When considering the Tibetan Plateau (TP) monsoon, different TP monsoon indexes have been defined from various perspectives, such as the near surface layer thermal low, the cyclonic circulation shear, the vorticity and divergence of wind field, etc. However, these existing indexes are more concerned of the spatial contrast of the variable and have not considered the characteristics of wind field change between the summer and winter. To address this issue, a new TP monsoon index has been proposed based on seasonal variation of winds in the author's previous work. The new index is further improved and simplified in the present paper. The ERA-interim reanalysis data is used to calculate six different TP monsoon indexes. Annual and inter-annual variations of the six indexes and their correlation coefficients with summer precipitation are compared. The results show that the algorithm for the calculation of the new TP monsoon index is simple but with clear physical meaning. Due to differences in the physics considered, the new TP monsoon index peaks in August, which is different to other indexes that peak in June. In general, characteristics of annual variations of different monsoon indexes and precipitation agree well with each other. The correlation coefficient between the new TP monsoon index and summer precipitation shows opposite signs between the western and eastern TP, while a consistent positive correlation coefficient distribution can be found on the TP between other TP monsoon indexes and summer precipitation. Above all, the new TP monsoon index shows a satisfactory result especially in the southeastern part of TP, which is a relatively densely populated area. Thereby, this new index has potential application value in the summer precipitation prediction over the TP.