Reassessing the role of stochastic forcing in the 1997–1998 El Niño

We explore the extent to which stochastic atmospheric variability was fundamental to development of extreme sea surface temperature anomalies (SSTAs) during the 1997–8 El Niño. The observed western equatorial Pacific westerly zonal stress anomalies (τ a x ), which appeared between Nov. 1996 and May 1997 as a series of episodic bursts, were largely reproducible by an atmospheric general circulation model (AGCM) ensemble forced with observed SST. Retrospective forecasts using a hybrid coupled model (HCM) indicate that coupling only the part of τ a x linearly related to large‐scale tropical Pacific SSTA is insufficient to capture the observed 1997 warming; but, accounting in the HCM for all the τ a x that was connected to SST, recovers most of the strong SSTA warming. The AGCM‐estimated range of stochastic τ a x forcing induces substantial dispersion in the forecasts, but does not obscure the large‐scale warming in most HCM ensemble members.