A model based approach to understanding the phase locking of ENSO and the annual cycle

Abstract

The present study is an investigation into the physical underpinnings of the phase locking between ENSO and the annual cycle. An appreciable amount of work has been aimed at this and similar questions, particularly observational studies resulting from the TOGA decade. In contrast, relatively little modeling efforts have been directed at understanding why peak conditions of most El Nino events in recent decades have occurred in boreal winter. Current knowledge of the global effects of El Nino remains based on observations of El Nino impacting the Earth during boreal winter. Using an OGCM of the tropical Pacific Ocean and various in situ data, it is found that the first order explanation of the seasonal timing of ENSO events, simply that westerly wind bursts occur during that season, is far short of complete. Rather, the state of the ocean is itself better situated thermodynamically to respond to the wind anomalies that are believed to play an important role in the genesis of El Nino events.