El Niño Southern Oscillation

Principal Investigators:
Chunzai Wang
David B. Enfield

INTRODUCTION

Observations show that ENSO displays western Pacific anomaly patterns in addition to eastern Pacific anomaly patterns (Fig. 1). During the warm phase of ENSO, warm SST and low SLP anomalies in the equatorial eastern Pacific and low OLR anomalies in the equatorial central Pacific are accompanied by cold SST and high SLP anomalies in the off-equatorial western Pacific and high OLR anomalies in the off-equatorial western Pacific. The off-equatorial anomalous anticyclones in the western Pacific initiate equatorial easterly wind anomalies over the western Pacific. So, while the zonal wind anomalies over the equatorial central Pacific are westerly, those over the equatorial western Pacific are easterly. Despite the potential roles of the western Pacific in ENSO, interannual variability in the western Pacific has not been well studied, probably because the SST anomalies there are smaller than those in the east. However, due to mean convergence associated with the western Pacific warm pool, these small SST anomalies in the western Pacific can induce atmospheric responses of comparable magnitude to those in the eastern Pacific. The western Pacific is an important region for ENSO.

The El Niño events in the last five decades originate and develop differently (Fig. 2). For the El Niño events between 1950 and 1976, the warm SST anomalies start over the South American coast and peak there in the boreal spring of the El Niño year. Then, the warm SST anomalies expand westward and the whole eastern and central equatorial Pacific reaches its maximum warming during the boreal winter of the El Niño year. For the El Niño events between 1977 and 1996, no warmings are observed over the South American coast in the boreal spring of the El Niño year, and the initial warmings occur in the equatorial western and central Pacific and propagate eastward. For the 1997-98 El Niño, the initial warming occurs in both the equatorial central Pacific and the South American coast. The 2002-03 El Niño originates and develops in the equatorial central Pacific. Why the ENSO warm events originate and develop differently in the last five decades is not understood yet.

This project examines and investigates (1) the ENSO western Pacific patterns, their roles in the evolution of ENSO, and ENSO-related climate variability in the tropical North Atlantic; (2) why the El Niño events originate and develop differently in the last five decades.

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