Unusual circulation pattern during Indian summer monsoon failure in July 2002 and June 2009

Abstract

The circulation patterns over the Indian Ocean and the surrounding continents have been studied during June 2009 and July 2002 to explain the failure of Indian summer monsoon (ISM) rainfall. This study presents evidences that the failure of the ISM during these 2 months was probably due to the development of cyclonic circulation anomaly over the Western Asia and anticyclonic circulation anomalies downstream of Eastern Asia. These circulation anomalies were associated with the equatorward advection of cold air up to 10°N. This may be due to the equatorward intrusion of midlatitude Rossby waves. We hypothesize that the intrusion of midlatitude Rossby wave is responsible for breaking the east–west circulation cell over the Indian region into two cells and weakening it. The weak east–west cell reduces the strength of the easterly wind field usually present over the monsoonal region, thus reducing the cross-equatorial moisture transport into the Indian subcontinent and decreasing monsoon rainfall.