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 variabilit y is investigated using long records of daily circulation data (1956-1997) and outgoing long wave radiation (OLR) data (1974-1997), The underlying spa tial structure of a typical. ISO cycle that is invariant from event to even t 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 mon soon. Two-dimensional probability density function estimates of the ISOs sh ow that 'strong' ('weak') monsoon years are indeed associated with higher p robability of occurrence of 'active' ('break') conditions. For the first ti me, these results show that the frequency of chaotic ISO regimes determine the seasonal mean monsoon, thereby setting a limit on monsoon predictabilit y.