The impact of ocean initial conditions on ENSO forecasting with a coupled model

Abstract

A coupled atmosphere-ocean GCM (general circulation model) has been developed for climate predictions on seasonal to interannual timescales. The atmosphere model is a global spectral GCM T30L18 and the ocean model is global on a 1° grid. Initial conditions for the atmosphere were obtained from National Meteorological Center (now known as the National Centers for Environmental Prediction) analyses, while those for the ocean came from three ocean data assimilation (DA) systems. One system is a four-dimensional DA scheme that uses conventional SST observations and vertical temperature profiles inserted into the ocean model and is forced from winds from an operational analysis. The other two initialization schemes are based on the coupled model, both nudging the surface temperature toward observed SSTs and one nudging surface winds from an operational analysis. All three systems were run from 1979 to 1988, saving the state of the ocean every month, thus initial conditions may be obtained for any month during this period. The ocean heat content from the three systems was examined, and it was found that a strong lag correlation between Nino-3 SST anomalies and equatorial thermocline displacements exists. This suggests that, based on subsurface temperature field only, eastern tropical Pacific SST changes are possibly predictable at lead times of a year or more. It is this "memory" that is the physical basis for ENSO predictions. Using the coupled GCM, 13-month forecasts were made for seven January and seven July cases, focusing on ENSO (El Niño-Southern Oscillation) prediction. The forecasts, whose ocean initial conditions contained subsurface thermal data, were successful in predicting the two El Niño and two La Niña events during the decade, whereas the forecasts that utilized ocean initial conditions from the coupled model that were nudged toward surface wind fields and SST only, failed to predict the events. Despite the coupled model's poor simulation of the annual cycle in the tropical Pacific, the ENSO forecasts from the full DA were remarkably good.