Multiseasonal predictions with a coupled tropical ocean-global atmosphere system

The Center for Ocean-Land-Atmosphere Studies anomaly coupled prediction system, using a sophisticated dynamical model of the tropical Pacific Ocean and the global atmosphere, is described. The resolution of the component models is moderate, with the atmospheric spectral model truncated at triangular total wavenumber 30 and 18 vertical levels. The ocean model is a Pacific Basin model with 0.5 degrees latitude and 1.5 degrees longitude resolution in the waveguide and 20 vertical levels. The performance of the uncoupled component models motivates the anomaly coupling strategy and has led to the development of a simple empirical technique for converting the 850-mb zonal wind into a zonal surface stress that is used in the prediction experiments described here. In developing ocean initial conditions, an iterative procedure that assimilates the zonal wind stress based on the simulated sea surface temperature anomaly error is applied. Based on a sample of 78 18-month hindcasts, the predictions have useful skill in the Nino-3 region for at least 12 months. The systematic error of the predictions is shown to be relatively small because the ocean initial conditions are in reasonable equilibrium with the ocean model. Finally, composites of the hindcast warm El Nino-Southern Oscillation (ENSO) events indicate that the model simulates the basic features of ENSO, but there are errors in the horizontal structure of the sea surface temperature anomaly that potentially limit the predictability of the model.