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.