TROPICAL AND MONSOON
RESEARCH
Modelling the Impact of the West Pacific and Equatorial Indian Ocean Sea
Surface Temperature anomalies on South Asian Summer Monsoon
An ensemble approach (6 members) was taken to study the impact of idealized SST anomalies over the west Pacific and equatorial Indian Ocean on the Asian summer monsoon using the U.K. Universities' Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (GCM).The simulated ensemble mean 850 hPa wind anomalies and precipitation anomalies in JJAS are shown in Figure 1 and Figure 2. Positive SST anomalies over the west Pacific result in stronger southwesterlies over the northern Arabian Sea, stronger westerlies over the Indian subcontinent, the Bay of Bengal and southeast Asia. This enhanced monsoon circulation is associated with enhanced precipitation over India. The result is consistent with the study of Soman and Slingo (1997) with more realistic SST anomalies associated with the La Niña event, and supports the hypothesis of Ju and Slingo (1995) that warm anomalies in the west Pacific, even when small, can have a direct effect on the south Asian summer monsoon. In the simulation with negative SST anomalies, the significant changes in the lower tropospheric circulation are more confined to the region with imposed SST anomalies while there is hardly any significant change in precipitation over India. These clearly indicate the nonlinearity of the monsoon circulation response to the sign of SST anomalies over the west Pacific, with positive SST anomalies more effective. Positive SST anomalies over the equatorial Indian Ocean lead to a decrease in southwesterlies over the northern Arabian Sea, the Indian subcontinent and the Bay of Bengal, while negative SST anomalies lead to the opposite changes. However, the changes in precipitation over the Indian subcontinent in these two cases are not significant due to the fact that there is large spread in regional monsoon precipitation among the ensembles. The analysis of intraensemble variability indicates that the changes in the large scale monsoon circulation is mainly controlled by the SST anomalies with the initial conditions having little effect, implying higher predictability in large scale monsoon circulation. However, the local precipitation changes over Asian monsoon region associated with SST anomalies are quite sensitive to the initial conditions, implying lower predictability in regional monsoon precipitation. With respect to the model response to SST anomalies over the tropical Indian Ocean, the results in this study indicate that simulated precipitation changes over south Asia are not well coupled to the simulated changes in large scale monsoon circulation. This weak coupling between the large scale circulation changes and local precipitation changes may be due to the model systematic error.
Publications
Dong, B.-W. and P. J. Valdes, 1998: Simulations of the Last Glacial Maximum climates using a general circulation model: Prescribed versus computed sea surface temperatures. Clim. Dyn., 14, 571-591.
Dong, B.-W. and Valdes, P. J. 1998 Modelling Asian summer monsoon rainfall and Eurasian winter/spring snow mass. Quart J. Roy. Met. Soc., 124, 2567-2596.
Buwen Dong and Paul J. Valdes
University of Reading
swsdong@met.rdg.ac.uk
