GCM simulations of the interannual and low frequency intraseasonal variability of the Indian summer monsoon during 1987 and 1988 are analyzed and compared with observations. Ensemble integrations with a T30 L18 version of the Center for Ocean-Land-Atmosphere Studies (COLA) GCM provide somewhat realistic simulations of the mean Indian summer monsoon as well as its interannual variability are also simulated including active and break periods, and differences in the occurrence of these periods between 1987 and 1988. Composites of simulated active/break monsoon rainfall and upper air circulation patterns are in qualitative agreement with historical composites of observations.
An attempt is made to understand the role of Pacific SST anomalies in modulating the active and break cycles and to identify the associated dynamical mechanisms. Significantly more active (break) days occur during 1988 (1987) in both the simulations and observations. The observed and simulated equatorial MJO was stronger (weaker) during 1987 (1988). However, the observed and simulated northward propagating mode (NPM) over the Indian monsoon region was weaker (stronger) during 1987 (1988). An increase in the period of both the MJO and the NPM was simulated in 1987 relative to that simulated in 1988. Previous studies have linked increases in the period of the MJO to the occurence of EL Nino. Assuming that the MJO interacts with the NPM in the Indian monsoon region, it is plausible that a relative increase in the period of the MJO during 1987 is at least partially responsible for the increase in the period of the NPM. A longer period NPM could possibly lead to longer monsoon break periods, as observed and simulated during 1987.
Complete copies of this report are available from:
Center for Ocean-Land-Atmosphere Studies
last update: 1 July 1996
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