Active and break spells of the Indian summer monsoon

Abstract

In this paper, we suggest criteria for the identification of active and break events of the Indian summer monsoon on the basis of recently derived high resolution daily gridded rainfall dataset over India . Active and break events are defined as periods during the peak monsoon months of July and August, in which the normalized anomaly of the rainfall over a critical area, called the monsoon core zone exceeds 1 or is less than −1.0 respectively, provided the criterion is satisfied for at least three consecutive days. We elucidate the major features of these events. We consider very briefly the relationship of the intraseasonal fluctuations between these events and the interannual variation of the summer monsoon rainfall.We find that 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. While active events occurred almost every year, not a single break occurred in 26% of the years considered. On an average, there are 7 days of active and break events from July through August. There are no significant trends in either the days of active or break events. We have shown that there is a major difference between weak spells and long intense breaks. While weak spells are characterized by weak moist convective regimes, long intense break events have a heat trough type circulation which is similar to the circulation over the Indian subcontinent before the onset of the monsoon.The space-time evolution of the rainfall composite patterns suggests that the revival from breaks occurs primarily from northward propagations of the convective cloud zone. 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 Ocean on intraseasonal time scales is similar to that on the interannual time scales.

Absorbing aerosols facilitate transition of Indian monsoon breaks to active spells

“…Therefore, composites of winds and OLR for the BNFA cases were based on the seven events. While here we use an OLR index to identify active and break spells of Indian monsoon, they could also be identified by using a variety of other indices, such as a rainfall averaged over the monsoon trough zone (Rajeevan et al 2010) and a zonal wind index over Bay of Bengal south of the monsoon trough (Goswami and Ajaya Mohan 2001). It is found that breaks identified by the OLR index used here follow very closely those identified by the monsoon trough zone precipitation index (Rajeevan et al 2010).…”

Northeast monsoon over India: variability and prediction

“…Several studies have shown that during the southwest monsoon season (June to September) a substantial component of the variability of convection and rainfall over the Indian region arises from the fluctuation on the intraseasonal scale between active and weak or break spells (Ramamurthy, 1969;Goswami, 2005;Waliser, 2006;Rajeevan et al, 2010). Long intense breaks are known to have an impact on the seasonal monsoon rainfall over the country (Webster et al, 1998;Gadgil and Joseph, 2003).…”

“…Within the broad range of 10-90 day periods, two period ranges with periodicities between 10 and 20 days and 30 and 60 days respectively are particularly prominent (Goswami, 2005). Active and break spells and intraseasonal variations of the Indian summer monsoon have been extensively studied, particularly in the last decade (Yasunari, 1979;Krishnamurti and Ardanuy, 1980;Sikka and Gadgil, 1980;Webster et al, 1998;Krishnamurthy and Shukla, 2000;Annamalai and Slingo, 2001;Goswami and Ajayamohan, 2001;Lawrence and Webster, 2001;Rajeevan et al, 2010;Kulkarni et al, 2011 and the recent reviews by Goswami, 2005 andWaliser, 2006 and the references therein).…”

An inter‐comparison of Kalpana‐1 and Meteosat‐7 atmospheric motion vectors against radiosonde winds and NWP forecasts during monsoon 2011

“…Prasad and Hayashi (2005) defined the onset and withdrawal of the Indian summer monsoon in terms of zonal asymmetric temperature anomaly and vertical wind shear between 850 and 200 hPa respectively. The active (periods of high rainfall) and break (periods of low rainfall) phases of the monsoon are defined as periods in which the normalized anomaly of the rainfall over a critical area, called the monsoon core zone, exceeds 1.0 or is less than −1.0, respectively, provided the criterion is satisfied for at least three consecutive days (Rajeevan et al, 2010). The 'break' is a special feature that occurs in monsoon conditions (Ramamurthy, 1969).…”