A systematic approximation of the SST anomaly equation for ENSO

A simplified SST equation for the El Nino-Southern Oscillation (ENSO) is derived based on the budget analysis of the SST anomalies. The analysis is done using both advective and flux forms of the SST anomaly equation and ocean reanalysis data set from the National Centers for Environmental Prediction. The main contributing terms to the SST variation are the zonal advection by anomalous current and the anomalous meridional and vertical heat flux divergences by climatological-mean currents. Note that the anomalous upwelling term associated with the Bjerknes' coupled instability is relatively small in most of the tropical Pacific. It is also shown that in the equatorial Pacific, the vertical heat flux term can be parameterized in terms of the thermocline depth anomaly and the meridional flux term can be effectively included in the Newtonian cooling term. The meridional flux term plays an important role to drain the heat in the ocean surface layer from the equator into the off-equator and as a result, to shape the meridional structure of SST anomaly. It is suggested that the narrow meridional structure of ENSO SST anomalies simulated by current coupled GCMs can be related to the weak simulated mean meridional current.