A common spatial mode for intra-seasonal and inter-annual variation and predictability of the Indian summer monsoon

How and to what extent the intra-seasonal oscillations (ISOs) of the Indian summer monsoon influence the seasonal mean and its inter-annual variability is investigated using long records of daily circulation data (1956-1997) and outgoing long wave radiation (OLR) data (1974-1997). The underlying spatial structure of a typical ISO cycle that is invariant from event to event and year to year is brought out. It is shown that the intra-seasonal and interannual variations are governed by a common mode of spatial variability. A higher frequency of occurrence of 'active' ('break') conditions within a monsoon season, therefore, could result in a 'strong' ('weak') summer monsoon. Two-dimensional probability density function estimates of the ISOs show that 'strong' ('weak') monsoon years are indeed associated with higher probability of occurrence of 'active' ('break') conditions. For the first time, these results show that the frequency of chaotic ISO regimes determine the seasonal mean monsoon, thereby setting a limit on monsoon predictability.