Spatial variability of intra-seasonal oscillations during extreme Indian monsoons

The intra-seasonal variability of the Indian monsoon rainfall plays a dominant role in deciding the seasonal strength of the monsoon. The Indian monsoon is known to exhibit two dominant periodicities of intra-seasonal oscillations (ISOs) 30-60 days and 10-20 days on the spatial scale of entire landmass. In this paper the nature and intensity of these dominant periodicities have been studied on smaller spatial domains during all-India deficient and excess monsoons. The high resolution data 1 degrees x 1 degrees latitude/longitude over Indian domain for the recent period of 1951-2003 provided by India Meteorological Department have been used in this analysis. The multi-taper spectrum analysis (MTMA) method has been applied to identify the periodicities over four representative regions. The 30-60 days periodicity is dominant over west coast and South-east India during deficient monsoons while the excess monsoons are characterized by high frequency oscillations. The time variation of these periodicities is Studied by applying wavelet transforms (WTs). During deficient Indian monsoons coherent regions with dominant oscillations Lire observed. Empirical Orthogonal Function (EOF) analysis reveals that both the oscillations have weakened after mid 1970s. Implications of this on forecasting Indian monsoon are discussed. Copyright (C) 2009 Royal Meteorological Society