ALBERT

Description: India Meteorological Department (IMD) reported strong sub-seasonal rainfall variability in the Indian subcontinent during the 2021 summer monsoon season. During the month of August 2021 rainfall recorded a 19% deficit, lowest since 2002 according to IMD. The percentage departures of all India averaged rainfall from IMD (GPCP) for June, July, August and September respectively are 14(10), -5(1), -19(-20), 35(29) for year 2021. At the same time, abrupt changes in all India rainfall from the month of August to September is noticed with rainfall percentage departure changed from -19 to 35 i.e., total change of 54%. Such a huge increase of rainfall in September was not anticipated as the August rainfall was largely deficient. It is found that the tropical-midlatitude circulation interaction played an important role in altering the sub-seasonal rainfall over India in summer 2021. The Southward displacement of the Asian jet and Pacific Japan pattern caused moisture divergence over the monsoon region in August and caused deficit rainfall. On the other hand, changes in upper level mid-latitude wave propagation are associated with the Silk Road pattern responsible for high rainfall over northwest India in September 2021. Understanding the dynamics and physical mechanisms that are responsible for such unusual changes in monthly/sub-seasonal rainfall are useful for its skillful prediction. Mechanisms and dynamics underlying for deficient (positive) rainfall in August (September) 2021 is analyzed further in detail in observations and CFSv2 hindcasts.