On the termination of the 2002-03 El Nino event

Every new El Nino presents an opportunity to revisit our understanding of El Nino characteristics and processes. We examine the extent to which the termination of the 2002-03 El Nino followed the scenario of Harrison and Vecchi [1999], in which (1) there is a late-year southward shift of near-equatorial westerly wind anomalies, and (2) subsequent eastern equatorial Pacific cold tongue thermocline shallowing is driven by the wind shift. There was a prominent late-year southward shift in the low-frequency west Pacific zonal wind anomalies in 2002-03. Ocean general circulation model experiments establish the shift as the primary cause of cold tongue thermocline shallowing. Reflected equatorial waves and local wind anomaly changes are much less important. Successful theories and models of El Nino should address the processes that cause the year-end southward wind shift. Interactions between anomalous El Nino conditions and the seasonal cycle of solar insolation may provide such a process.