The relationship between the evolution of the Asian summer monsoon and equatorial sea-surface-temperature anomalies has been studied using results from an integration with the UK Universities' Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM). The integration was performed as part of the Atmospheric Model Intercomparison Project and thus used the observed sea surface temperatures (SSTs) for the decade January 1979 to December 1988. The mean evolution of the Asian summer monsoon has been successfully simulated in terms of many aspects of the rapid transition of the large-scale circulation during the boreal spring and summer. However, the results for individual years showed considerable interannual variability, both in the strength of the monsoon and in the time of onset. A relationship has been identified between the evolution of the monsoon now and the phase of the Fl Nino Southern Oscillation (ENSO). In agreement with observed results, years with warm SST anomalies in the equatorial central and east Pacific Ocean (El Nino) have a weaker monsoon circulation and a delayed onset. An opposite behaviour is noted for those years with cold Pacific SST anomalies (La Nina). Diagnostics from analyses from the National Meteorological Center and the European Centre for Medium-range Weather Forecasts, and from data on the outgoing long-wave radiation observed by the Advanced Very High Resolution Radiometer, have been used to verify the model results. A description of the mechanism by which the phase of ENSO remotely influences the dynamics of the Asian summer monsoon has been developed involving changes in the heating patterns over Indonesia and the west Pacific in the preceding spring.
