Thermodynamic air/ocean feedback mechanisms in the equatorial Pacific

Abstract

The occurrence of the El Nino Southern Oscillation (ENSO) has been studied from numerous air-ocean interaction aspects, yielding theories implying a positive-only correlation between the time rate of change of sea surface temperature anomaly (@T's/@t) and the corresponding change in the depth of the ocean mixed layer (h'). However, an alternate proposal by Chu (1991a,b,1992) holds that there are both positive and negative correlations between @T's/@t and h' . Chu's proposal, based upon the modified Kraus-Turner ocean mixed layer (OML) thermodynamic model, goes further to say that the regions occupied by these positive and negatives correlation values correspond to those occupied by positive and negative values of a surface forcing function, P, that is an indication of the strength of the counteracting surface forcings of wind work and buoyant damping. The model generated fields of surface wind stress, net heat flux, sea surface temperature, and primary (mixed) layer depth are analyzed for a two year ENSO event, 1986-86, covering the region of the equatorial Pacific, 10N to 10S and 140E to80W, in order to determine the validity of these theories. The results shows a fairly uniform P-field over the entire period, consisting of a negative P-field (indicative of a shallowing regime) in most of the western Pacific and in the far eastern part of the eastern Pacific with a positive P-field (indicative of an entrainment regime) in between. The correlations between @T's/@t and h' show that there exists both positive and negative correlations between @T's/@t and h', however there is no appreciable similarity between the regions occupied by positive and negatives correlations and those occupied by positive and negative P-fields.