An empirical adjusted ENSO ocean energetics framework based on observational wind power in the tropical Pacific

Abstract

According to El Nino–Southern Oscillation (ENSO) energetics theory, the work done by the wind on the ocean, known as wind power, has a strong forcing relationship with ENSO sea surface temperature (SST) changes, wherein negative (positive) tropical Pacific wind power anomalies contribute to warm (cold) ENSO events. The ENSO energetics framework assumes a mean state characterized by easterly winds, westward currents and a thermocline shoaling from west to east such that positive zonal wind power anomalies will induce a La Nina state and negative anomalies will induce an El Nino state. In this study, tropical Pacific wind power is computed using satellite data and its correlations with Nino 3.4 SST anomalies evaluated and compared to the conventional dynamical predictors, namely warm water volume and wind stress. Analysis of the spatial and temporal structure of climatological wind power and its variability during individual ENSO events reveals sign inconsistencies in which certain wind power anomalies (e.g. those associated with westerly wind events in the far west) are positive despite forcing the system towards an El Nino state. These results show that the conventional ENSO energetics framework makes assumptions about the climatological state that are not always consistent with observations. We apply sign adjustments to the computation of a tropical Pacific wind power index that include the directional aspect of the wind power perturbations and find that these adjustments greatly enhance the lead correlations between wind power and Nino3.4 SST anomalies, which are now comparable to conventional dynamical predictors.