Coupled atmosphere - Tropical Pacific model: Forecasting the 1997-1998 El Nino southern oscillation

An intermediate coupled tropical Pacific Ocean-atmosphere model is used to study the sensitivity of 1997-1998 El Nino forecasts to the vertical struct ure of the ocean. The coupled model consists of a tropical Pacific Ocean mo del taking into account three baroclinic modes and the Gill tropical atmosp here model [1]. Experiments based on a model using the observed wind stress are initially considered. The sea-level and surface-current fields obtaine d in the experiment based on a multimode ocean model are in better agreemen t with observational data than the experimental results obtained by a singl e-mode ocean model. A series of forecasting experiments with model integrat ion over two years during the 1970-1998 period are conducted. These forecas ts are used to obtain statistical estimates for the predictive ability of t he model. In order to assess the feasibility of employing altimetric data t o improve the quality of the initial conditions, the model is tested for it s sensitivity to the vertical structure of the ocean in 1996-1998. The test s show that the consideration of the second baroclinic mode in the ocean mo del is necessary for the reproduction of an anomalous increase in temperatu re in April 1997 and also for a correct estimation of the amplitude of this phenomenon in its culmination. On the other hand, the first baroclinic mod e is more important for the reproduction of cool ocean conditions formed si nce early January 1998.