Active and Break Spells of the Indian Summer Monsoon
Sub-seasonal rainfall fluctuations, characterized as active and weak spells during the Indian monsoon season (June to September) is an important component of the variability of the Indian monsoon rainfall. In this paper, we suggest criteria for identification of active and break events of the Indian summer monsoon on the basis of the recently derived high resolution daily gridded rainfall data set over India (1951-2007). Active and break events are identified from the average rainfall data over a critical area, called the core monsoon zone within which the monsoon trough/ Continental Tropical Convergence Zone (CTCZ) normally fluctuates in the peak monsoon months of July and August. Active and break events are defined as periods in which the normalized anomaly of the rainfall over the monsoon zone exceeds 1 or is less than -1.0 respectively, provided the criterion is satisfied for at least three consecutive days. The active and break events using this criterion are found to be comparable to those identified by using the traditional IMD criteria based on low level pressure and wind patterns. The criteria suggested here can be used to monitor the active and break events of the Indian summer monsoon on a real time basis. On an average, there are 7 days of active and break events during the period July and August. Breaks tend to have a longer life-span than active spells. While, almost 80% of the active spells lasted 3-4 days only 40% of the break spells were of such short duration. A small fraction (9%) of active spells and 32% of break spells lasted for a week or longer, of these, almost 30% break spells persisted for more than 10 days. Active events are more common than breaks. While active events occurring almost every year, not a single break occurred in 26% of the years. There are no significant trends in either the days of active events or break events during the monsoon season. The space-time evolution of the rainfall composite patterns suggests that the revival from breaks seems to occur primarily from northward propagations of the convective cloud zone. We also show that there are important differences between the spatial patterns of the active/ break spells and those characteristic of interannual variation, particularly those associated with the link to ENSO. Hence the interannual variation of the Indian monsoon cannot be considered as primarily arising from the interannual variation of intraseasonal variation. However, the signature over the eastern equatorial Indian Oceans on intraseasonal time-scales is similar to that on the interannual time-scales. With the help of NCEP/NCAR reanalysis data, for the first time, the present study has elucidated the difference in the meridional circulation between the active spells with moist convection and intense break events with a heat trough type circulation.