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

Dacha Gushchina, Boris Dewitte and Mikhail Petrossiants

An intermediate coupled tropical Pacific Ocean-atmosphere model is used to study the sensitivity of 1997-1998 El Niño
forecasts to the vertical structure of the ocean. The coupled model consists of a tropical Pacific Ocean model taking into
account three baroclinic modes and the Gill tropical atmosphere model [1]. Experiments based on a model using the observed
wind stress are initially considered. The sea-level and surface-current fields obtained in the experiment based on a multimode
ocean model are in better agreement with observational data than the experimental results obtained by a single-mode ocean
model. A series of forecasting experiments with model integration over two years during the 1970-1998 period are conducted.
These forecasts are used to obtain statistical estimates for the predictive ability of the model. In order to assess the feasibility
of employing altimetric data to improve the quality of the initial conditions, the model is tested for its sensitivity to the vertical
structure of the ocean in 1996-1998. The tests show that the consideration of the second baroclinic mode in the ocean model
is necessary for the reproduction of an anomalous increase in temperature 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 mode is more important for the
reproduction of cool ocean conditions formed since early January 1998.



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