Precursors of the El Niño-Southern Oscillation (ENSO) are
atmospheric or oceanic phenomena that often occur before
the onset of ENSO events and offer the potential to predict ENSO
events with significant lead times. Most of the well-known ENSO
precursors identified, so far, occur within the tropical Pacific, such
as the build-up of subsurface ocean heat content anomalies in the
tropical western Pacific (e.g., Wyrtki 1985; Meinen and McPhaden
2000) and the appearance of westerly wind bursts in the tropical
western-to-central Pacific (e.g., McPhaden 1999; Vecchi and
Harrison 2000; Zhang and Gottschalk 2002). These precursors
have been suggested to affect ENSO onset through fluctuations in
thermocline depths in the equatorial Pacific, which are recognized
as a central element of the ENSO generation mechanism.