A TROPICAL PACIFIC PREDICTION SYSTEM OF INTERMEDIATE COMPLEXITY: ROLE OF THE VERTICAL STRUCTURE VARIABILITY
An intermediate ocean-atmosphere coupled model of the tropical Pacific is used to investigate the sensitivity of the seasonal forecasts to the configuration of the oceanic vertical structure. The models consist in a three baroclinic mode tropical Pacific Ocean model and a Gill (1980)'s tropical atmosphere. The predictive skill of the model using a simple nudging method for the initialization is first presented from 1970 and compared to the results of other prediction system of similar complexity, which emphasizes the modulation of the skill on decadal timescales. It is then demonstrated that the skill is critically dependant on the energy distribution on the baroclinic mode, higher-order mode contributions being favored at some period and not at others. Linear Green’s function are used to assimilate satellite observations (SST and wind) and derive the optimized set of parameters that determines the vertical structure in the model for a particular period of time. The scheme is first tested for the period prior to the development of the 1997 El Nino. It is shown that substantial improvement in forecasting the event is realized for an increase of the relative contribution of the higher-order modes through the model parameters setting. Assimilation experiments of satellite data for the September 2003-February 2004 period are carried for producing initial conditions for the coupled model. Results of forecasts runs for 2004 are presented and discussed.