Abstract

The mechanisms for decadal and longer timescale variability in the El Niño/Southern Oscillation (ENSO) phenomenon are examined. Decadal variability may be caused by factors internal to the tropical climate system, such as synoptic variability, or by those external to it, such as the so-called North Pacific gyre mode. Model evidence for these mechanisms is carefully considered.

Longer term variability of ENSO caused by such factors as global warming is also examined. Modeling evidence to date suggests little future change in ENSO due to this change in forcing. The limitations of models making such null projections are critically analyzed.

Introduction

The past fifteen years has seen significant advances in our understanding of the interannual aspects of El Niño/Southern Oscillation (ENSO). This has been achieved mainly as a result of the immensely successful Tropical Ocean-Global Atmosphere (TOGA) program. These advances have been accompanied by great progress in recent years in the development of realistic coupled ocean-atmosphere models, which are fundamental to increasing our understanding of the ENSO-coupled climate phenomenon.

While the dominant timescale usually associated with ENSO is interannual (roughly 2 to 7 years), it is quite clear from observational studies that the spectral peak is broad and has power at decadal and longer time scales. Although an understanding of this low-frequency signal is more limited, significant progress has been made in the past five years. This effort has been built upon the foundations of our knowledge regarding interannual timescales of climatic variability.