A Simple Mechanism for ENSO Residuals and Asymmetry

Abstract

A simple mechanism is offered that accounts for a change in the long-term (decadal scale) mean of ocean temperatures as the El Niño?Southern Oscillation (ENSO) amplitude changes. It is intended as an illustration of a kinematic effect of oscillating a nonlinear temperature profile with finite-amplitude excursions that will cause the Eulerian time mean temperature to rise (fall) where the curvature of the temperature is positive (negative) as the amplitude of the oscillations increases. This mechanism is found to be able to mimic observed changes in the mean sea surface temperatures in the Pacific between the 1920s, 1960s, and 1990s due to the changing ENSO amplitude. The effects alter both the calculated mean surface temperatures and the time mean temperatures at depth. It also results in a skewness of the temperature distribution that shares many properties with the observed SST. In this model, the time-local gradients of temperature never change if referenced to a single isotherm (i.e., the Lagrangian description is one of DT/Dt = 0). This implies that changes in the amplitude of ENSO will have no influence on the stability of the underlying system, and that the simple Eulerian decadal mean temperature structure has no predictive value. This is in direct contrast to recent work that ascribes a change in ENSO statistics as due to a change in the background state.