The Asian monsoon: Interdecadal variability

Abstract

Prediction of the seasonal mean monsoon at least one season in advance is one of the most important problems in tropical climate. However, it also happens to be one of most difficult problem. The Asian monsoon climate exhibits variability in a variety of time scales. The predictability of the seasonal mean monsoon depends on the nature of the interannual variability (IAV) of the monsoon. Extensive studies on IAV of different components of the Asian monsoon have led to better documentation and better understanding of physical mechanisms responsible for IAV of the monsoon. Various components of the Asian monsoon also exhibit significant interdecadal variability (Mooley and Parthasarathy, 1984; Kripalani et al., 1997; Mehta and Lau, 1997; Chang et al., 2001b, 2000; Parthasarathy et al., 1991; Wu and Wang, 2002). Modulation of IAV by the interdecadal variability influences predictability of the seasonal mean monsoon. One example of role of interdecadal variability on the predictability of the summer monsoon is seen in the change in usefulness of several predictors used in statistical prediction of the Indian summer monsoon precipitation (Gowarikar et al., 1989, 1991; Thapliyal and Rajeevan, 2003). The correlation between several of these predictors and the Indian summer monsoon precipitation has been found to undergo interdecadal variations (Kumar et al., 1999; Krishnamurthy and Goswami, 2000) forcing the India Meteorological Department to drop many of the original predictors in their recent statistical model (Rajeevan et al., 2004). A better understanding of the interdecadal variability may, therefore, be very important in improving the predictability of the seasonal monsoon climate. However, the space-time structure of the monsoon interdecadal variability is less well documented than the IAV and mechanisms responsible for it are poorly understood. This problem is largely related to the lack of availability of good quality data for a sufficiently long period. While the instrumented record of surface climate (e.g. temperature, surface pressure and precipitation) could be extended to about 150 years, upper air data is available for only about 50 years. In this review, we shall attempt to highlight the temporal and spatial characteristics of the surface climate associated with the dominant interdecadal variability of the Asian monsoon making use of long records of rainfall, sea surface temperature (SST) and sea level pressure (SLP). Further, we shall endeavor to unravel the three dimensional structure of the dominant mode of interdecadal variability from available upper air circulation data. Connections between interdecadal variability of the monsoon and that of other climate regimes around the globe will be established and indicated that the monsoon interdecadal variability may be manifestations of a global coupled ocean-atmosphere mode of interdecadal variability. A positive feedback mechanism involving air-sea interaction will be proposed for intensification of the oscillation. An attempt will be made to understand the observed relationship between the Indian monsoon and El Nino and Southern Oscillation (ENSO) on interdecadal time scale within the context of the global three dimensional structure of the dominant mode of monsoon interdecadal variability.