Westerly Wind Events and the 1997/98 El Niño Event in the ECMWF Seasonal Forecasting System: A Case Study

Abstract

The 1997/98 El Niño was one of the strongest on record. Its onset was predicted by several numerical models, though none fully captured its intensity. This was the case for the ECMWF seasonal forecasting system that underestimated the intensification during the June?July 1997 period by more than 1 K. Several strong westerly wind events developed during the onset of the 1997/98 El Niño suggesting that westerly wind events played a key role in the intensification of this El Niño. The present paper quantifies the impact of westerly wind events on the 1997/98 El Niño in the ECMWF seasonal forecasting system, through a series of experiments in which various modifications are made to convective parameterization and wind forcing to increase wind variability in the western Pacific. The ECMWF model does not produce significant westerly wind events. A set of experiments suggests that if the model were able to simulate the May?June westerly wind event, it would have predicted significantly warmer Niño-3 (5°S?5°N, 90°?150°W) sea surface temperatures (SSTs). Increasing the convective available potential energy threshold of the cumulus parameterization significantly improves the simulation of westerly wind events and as a consequence the prediction of Niño-3 SSTs. The response of the coupled model to the wind perturbation is smaller than that in forced mode, probably due to the strong damping effect of the induced heat flux. The different ocean mean state does not seem to be responsible for the weak coupled response in the Niño-3 region.