Index of activity of the monsoon trough over India

Abstract

Measures for the daily, monthly and seasonal activity of the Indian monsoon trough are developed; and their variability and relationships with Indian rainfall during the summer monsoon season are examined.

The daily monsoon troug activity index is the anomaly of the daily pressure within the trough from the long-period mean. The frequency of the normalized daily monsoon trough activity index < -1.0 is higher in years of good Indian monsoon than that in years of deficient Indian monsoon, at a level of significance of above 0: 1%. For days in September, the daily monsoon trough activity index is appreciably higher (i.e., trough appreciably weaker) in years of deficient Indian monsoon than that in years of good Indian monsoon.

The monthly measures of the monsoon trough activity, derived from the daily monsoon trough activity index show significant relationship with monthly rainfall for all the monsoon months except June. The relationship for  September is highly significant and stable.

Amongst the seasonal measures of the monsoon trough activity, the seasonal frequency of days with active  monsoon trough is significantly (at I %) and stably related to Indian monsoon rainfall.

Low pressure systems are found to add largely to the activity of the monsoon trough.

The seasonal frequency of days with active monsoon trough is related directly and significantly (at 1 %) to April 500 mb ridge location over India, inversely and significantly (at 1 %) to the eastern equatorial Pacific SST anomaly tendency from MAM to JJA. The April 500 mb ridge thus appears to be a good indicator of the  monsoon trough activity over India during the monsoon season.