On factors responsible for recent secular trend in the onset phase of monsoon intraseasonal oscillations

The timing of the dramatic 'onset' and subsequent northward progression of the Indian summer monsoon ( ISM) are unique features of the monsoon annual cycle and are strongly linked to planning of agricultural activity and water management. Here, we show that the northward propagation of the first episode of intraseasonal oscillation (ISO) associated with the onset phase of ISM has slowed down significantly during the past decade compared with prior decades. The slowdown is due to systematic weakening of two regional climatic features responsible for northward progression of the tropical convergence zone (TCZ) namely, the vertical easterly wind shear and the north-south gradient of the mean low-level humidity around the equator. Eastward shift of the Walker circulation associated with increasing strength of El Ninos appears to be responsible for weakening of the vertical wind shear over Indian monsoon region. The slow northward propagation during recent decades leads to a climatological sub-seasonal oscillation representing a predictable component of the onset phase of the ISM. Copyright (C) 2009 Royal Meteorological Society