Reassessing the Role of Stochastic Forcing in the 1997-8 El Niņo

Andrew T. Wittenberg, Gabriel A. Vecchi, and Anthony Rosati

2006 Joint Assembly, Baltimore, Maryland, 23-26 May 2006

Abstract: 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 Niņo.  The observed western equatorial Pacific westerly zonal stress anomalies (τxa), which appeared between November 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 τxa linearly related to large-scale tropical Pacific SSTA is insufficient to capture the observed 1997 warming; but, accounting in the HCM for all the τxa that was connected to SST, recovers most of the strong SSTA warming. The AGCM-estimated range of stochastic τxa forcing induces substantial dispersion in the forecasts, but does not obscure the large-scale warming in most HCM ensemble members.

Slides from the 12-minute talk (PDF, 0.4 MB)