Predictability of the 1997 and 1998 south Asian summer monsoon low-level winds

The predictability of the 1997 and 1998 south Asian summer monsoon winds is examined from an ensemble of 10 atmospheric general circulation model simu lations with prescribed sea surface temperatures (SSTs) and soil moisture. The simulations have no memory of atmospheric initial conditions for the pe riods of interest. The model simulations show that the 1998 monsoon is considerably more predi ctable than the 1997 monsoon. During May and June of 1998 the predictabilit y of the low-level wind anomalies is largely associated with a local respon se to anomalously warm Indian Ocean SSTs. Predictability increases late in the season (July and August) as a result of the strengthening of the anomal ous Walker circulation and the associated development of easterly low-level wind anomalies that extend westward across India and the Arabian Sea. Duri ng these months the model is also the most skillful, with the analyses show ing a similar late-season westward extension of the easterly wind anomalies . The model shows little predictability or skill in the monthly mean low-leve l winds over Southeast Asia during 1997. Predictable wind anomalies do occu r over the western Indian Ocean and Indonesia; however, over the Indian Oce an the predictability is artificial, because the model is responding to SST anomalies that were wind driven. The reduced predictability in the low-lev el winds during 1997 appears to be the result of a weaker (as compared with 1998) simulated anomalous Walker circulation, and the reduced skill is ass ociated with pronounced intraseasonal activity that is not captured well by the model. It is remarkable that the model does produce an ensemble mean M adden-Julian oscillation (MJO) response, though it is approximately in quad rature with, and much weaker than, the observed MJO anomalies during 1997.