Intraseasonal oscillations and interannual variability of the Indian summer monsoon

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. It is shown that the intraseasonal and interannual variations are governed by a common mode of spatial variability. The spatial pattern of standard deviation of intraseasonal and interannual variability of low-level vorticity is shown to be similar. The spatial pattern of the dominant mode of ISO variability of the low-level winds is also shown to be similar to that of the interannual variability of the seasonal mean winds. The similarity between the spatial patterns of the two variabilities indicates that higher frequency of occurrence of active (break) conditions would result in "stronger'' ("weaker'') than normal seasonal mean. This possibility is tested by calculating the two-dimensional probability density function (PDF) of the ISO activity in the low-level vorticity. The PDF estimates for "strong'' and "weak'' monsoon years are shown to be asymmetric in both the cases. It is seen that the strong (weak) monsoon years are associated with higher probability of occurrence of active (break) conditions. This result is further supported by the calculation of PDF of ISO activity from combined vorticity and outgoing longwave radiation. This clear signal indicates that the frequency of intraseasonal pattern determines the seasonal mean. Because the ISOs are essentially chaotic, it raises an important question on predictability of the Indian summer monsoon.