Regime shift in Indian summer monsoon climatological intraseasonal oscillations

Abstract

Using a high resolution daily rainfall data over Indian continent between 1951 and 2004, summer monsoon climatological intraseasonal oscillations (CISO) and its regime shift around mid seventies is investigated. Explaining 20% -40% of intraseasonal oscillation (ISO) amplitude, the summer monsoon CISO represents a significant predictable component of the monsoon ISOs. Indian monsoon CISO is characterized by a dominant northward propagating episode in both pre-seventies (1951-75, pre75) and in the post-seventies (1979-04, post79). The dominant episode starts in the beginning of July during pre75 in contrast to beginning of June during post79 period. We find that the IAV of the initial phase of both the first and the second ISO episodes worked in tandem with the changes in the northward propagation speed in producing the phase locking for the second episode during pre75 while for the first episode during post79. Change in northward propagation speed is shown to be consistent with changes in easterly vertical shear and meridional gradient of low level humidity during the two climate regimes.