Recent advances in monsoon research with particular reference to the Indian monsoon

The major advances in the understanding of the structure of the Indian summer monsoon and its intraseasonal and interannual variations have been discussed. The large-scale monsoon rainfall has been shown to be associated with a tropical convergence zone (TCZ) with the dynamical characteristics of the ITCZ discussed by Charney (1969). The intraseasonal and interannual variations of the monsoon rainfall arise from the space-time variations of the TCZ. The prominent scales of intraseasonal variations between active spells and breaks have been identified as the 15-day scale (associated with westward propagation of synoptic-scale disturbances) and the 40-day scale (associated with northward propagation of the TCZ). Detailed analysis of satellite imagery has revealed that the most prominent feature of the intraseasonal variation over the Indian longitudes is the northward propagation of the TCZ from the equatorial Indian ocean onto the heated continent. So far such poleward propagations have not been reported for any other part of the tropics. Simple models capable of simulating the seasonal transitions as well as the intraseasonal fluctuations of the monsoon have been developed. However, the underlying mechanisms are yet to be fully understood. The structure of the variations on the interannual scale turns out to be markedly similar to that on the intraseasonal scale. Thus analysis of the inter-relationship between the continental TCZ and those over the Indian ocean and the Pacific on the intraseasonal scale, should provide insight into the interannual variations as well. On the interannual scale, links between the Asian monsoon and conditions over the Pacific as manifested by the association between the El Nino and Indian droughts have been well established. The relationship between SST and cloudiness over the tropics has been shown to be rather complex, with a threshold of 28°C for organised convection. With deeper understanding of the dynamics of the tropical convergence zones, more light is bound to be shed on the intraseasonal and interannual variation of the monsoon In the near future.