ENSO theory

Abstract

Abstract. Beginning from the hypothesis by Bjerknes [1969] that oceanatmosphere interaction was essential to the E1 Nifio-Southern Oscillation (ENSO) phenomenon, the Tropical Ocean-Global Atmosphere (TOGA) decade has not only confirmed this but has supplied detailed theory for mechanisms setting the underlying period and possible mechanisms responsible for the irregularity of ENSO. Essentials of the theory of ocean dynamical adjustment are reviewed from an ENSO perspective. Approaches to simple atmospheric modeling greatly aided development of theory for ENSO atmospheric feedbacks but are critically reviewed for current stumbling blocks for applications beyond ENSO. ENSO theory has benefitted from an unusually complete hierarchy of coupled models of various levels of complexity. Most of the progress during the ENSO decade came from models of intermediate complexity, which are sufficiently detailed to compare to observations and to use in prediction but are less complex than coupled general circulation models. ENSO theory in simple models lagged behind ENSO simulation in intermediate models but has provided a useful role in uniting seemingly diverse viewpoints. The process of boiling ENSO theory down to a single consensus model of all aspects of the phenomenon is still a rapidly progressing area, and theoretical limits to ENSO predictability are still in debate, but a thorough foundation for the discussion has been established in the TOGA decade.

Coherent Tropical Indo-Pacific Interannual Climate Variability

“…The ocean response can lead to a positive (Bjerknes) feedback on the SST anomalies, and the ocean adjustment due to equatorial wave processes provides a delayed negative feedback (Neelin et al 1998;Jin 1997a,b). Meinen and McPhaden (2000) showed that a sufficiently high Pacific warm water volume (WWV; the volume of water warmer than 208C) is relevant for the occurrence of El Niño events because in order to warm the eastern Pacific Ocean (PO), sufficient heat should be present in the western PO, which is then discharged during El Niño.…”

The “normality” of El Niño

“…However, it is far from clear how much non-linearity there is in the coupled Pacific ocean-atmosphere system [Neelin et al, 1998]. If the system were only very weakly non-linear, then one would expect the cumulative response to weather noise to be close to normally distributed.…”

The response of an ENSO Model to climate noise, weather noise and intraseasonal forcing

“…Now that a basic understanding of the oscillatory nature of ENSO has been obtained (for review see Neelin et al [1998]), more attention is being paid to the effect that stochastic forcing has on the system [Kleeman and Power, 1994; Moore and Kleeman, 1996; Kleeman and Moore, 1997; Blanke, 1997]. The role of the Madden-Julian intraseasonal atmospheric oscillation (MJO) on the interannual ENSO system has been of particular interest [Zebiak, 1989;Moore and Kleeman, 1999].…”