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How and to what extent the intraseasonal oscillations (ISOs) influence the seasonal mean and its interannual variability of the Indian summer monsoon is investigated using 42-yr (1956-97) daily circulation data from National Centers for Environmental Prediction-National Center for Atmospheric Research 40-Year Reanalysis and satellite-derived outgoing longwave radiation data for the period of 1974-97. Based on zonal winds at 850 hPa over the Bay of Bengal, a criterion is devised to define ''active'' and ''break'' monsoon conditions. The underlying spatial structure of a typical ISO cycle in circulation and convection that is invariant over the years is constructed using a composite technique. A typical ISO has large-scale horizontal structure similar to the seasonal mean and intensifies (weakens) the mean flow during its active (break) phase. A typical active (break) phase is also associated with enhanced (decreased) cyclonic low-level vorticity and convection and anomalous upward (downward) motion in the northern position of the tropical convergence zone (TCZ) and decreased (increased) convection and anomalous downward (upward) motion in the southern position of the TCZ. The cycle evolves with a northward propagation of the TCZ and convection from the southern to the northern position of the TCZ.
Goswami et al. (Thu,) studied this question.