Spatial patterns of northeast monsoon rainfall over sub-regions of southern peninsular India and Sri Lanka as revealed through empirical orthogonal function analysis

Abstract

The spatial variability of northeast monsoon (NEM) rainfall of peninsular India and Sri Lanka is studied using Empirical Orthogonal Function (EOF) analysis based on monthly / seasonal rainfall data for the months of October, November, December, January and for the season October-December (OND) for the 107 year period of 1900-2006 over nine sub-regions defined for the study based on climatology and geography. Monthly / seasonal rainfall series over these nine sub-regions are subjected to EOF analysis and 2-3 significant Principal Components (PCs) are identified for each case. Each PC is then linked to physical modes known to be associated with NEM using correlation and compositing techniques. For the OND season and for all the four individual months, the first PC explaining maximum variance of 49-64% in the spatial rainfall distribution is identified with the overall NEM strength. The second and the third PCs are identified with rainfall due to passage of synoptic scale systems such as cyclones and depressions (explaining 11-20% variation) and southwest monsoon (SWM) rainfall prior to onset of NEM in October (15% variance explained). PCs representing NEM strength and SWM contribution exhibit contrasting nature of relation with the major climate index Southern Oscillation Index. Relation between the PCs and important regional circulation features, viz., the subtropical ridge at 200 hPa level and the equatorial trough at 850 hPa are used to delineate the PCs associated with SWM and NEM rainfall during October. The study also reveals that the sub-region of north coastal Andhra Pradesh is not benefitted by the over NEM strength but, receives rainfall due to passage of cyclonic disturbances and SWM prior to NEM onset.