Dynamical predictability of seasonal monsoon rainfall: problems and prospects

The conceptual basis and the current status of dynamical prediction of the seasonal mean monsoon rainfall are reviewed. The monsoon may be viewed as a combination of a planetary-scale wavenumber-one circulation and a regional fluctuating Hadley circulation. The planetary-scale component is governed primarily by the sea surface temperature in the Pacific, and is well simulated by general circulation models. These models, however, suffer from systematic errors in the simulation of the regional component, which seems to be sensitive to a number of physical processes. The simulation of the mean rainfall over the Indian region is quite sensitive to the initial conditions, whereas simulations over the Pacific or the Sahel are not. It is hypothesised that the internal dynamics play an important role in determining even the low-frequency variability over the Indian region. Evidence is presented that there are strong intraseasonal oscillations over the Indian region during the northern hemisphere summer, and that these oscillations are the result of the internal dynamic