Abstract
Analysis of 17 years (1980-1996) of tropical Pacific thermocline variations indicates that short period (about 18 months) cycles in the early 1990s exhibit a relatively small meridional scale. The long period (4-5 year) cycles in the 1980s, associated with major basic-wide warming/cooling events, have much wider meridional scales. The dominant mode of interannual variation of thermocline depth can be approximately described by a basin-scale east-west seesaw oscillation, with an eastward propagation of thermocline depth anomalies occurring in the equatorial waveguide during the transition phases.
The long-period ENSO cycles associated with the major basic-wide warming/cooling events involve a substantial change of heat storage in the western North Pacific (WNP) (5-15°N, 130-170°E). The tendency of thermocline displacement and local wind stress curl in the WNP exhibit a coherent, broad spectral peak on a 8-20 month time scale. The deepening (rising) of the thermocline occurs in phase with the local anticyclonic (cyclonic) wind stress forcing, suggesting the essential roles of the in situ wind forcing in thermocline adjustment.
The surface wind variation in the western Pacific is found to play a critical role in the phase transition of ENSO cycles. During the mature phases of major warm episodes, there is a rapid establishment of an anomalous anticyclonic wind stress curl over the WNP. The anticyclonic wind stress deepens the WNP thermocline and starts a recharge of heat content in the warm pool. Meanwhile, the easterly anomalies to the south of the anticyclonic center elevate the thermocline in the equatorial western Pacific, and trigger an eastward migration of the rising thermocline along the equator, leading to cooling in the east. The transition from cooling to warming during the mature phases of cold episodes involves a similar process, but with opposite anomalies.
