An Intermediate Model of the Tropical Pacific Ocean

Abstract

An intermediate tropical Pacific Ocean model is developed to bridge the gap between anomaly models of El Niño and ocean general circulation models. The model contains essential physics for reproducing both the annual and interannual variations of sea surface temperature (SST). A new parameterization scheme for entrained water temperature is shown to work satisfactorily in both the cold tongues and warm pools. This scheme combines the Cane-Zebiak (CZ) model's dynamic framework and mixed layer physics, giving a more realistic description of the active tropical ocean. Incorporation of the Niiler-Kraus scheme for turbulent entrainment enables the model to better simulate El Niño-Southern Oscillation in the central equatorial Pacific where the CZ model considerably underestimates observed SST variations. It also improves the model's performance on the seasonal cycle, especially in the central-eastern equatorial Pacific and the intertropical convergence zone (ITCZ). The potential energy generation induced by penetrative solar radiation tends to reduce entrainment in the central equatorial Pacific but to enhance mixing in the far eastern equatorial Pacific. Without this process, the model central (eastern) Pacific would be excessively cold (warm). In response to an idealized sequential westerly burst located in the western equatorial Pacific, the CZ model produces SST oscillations in the eastern equatorial Pacific due to the thermocline oscillation associated with passages of Kelvin waves. In the present model, however, SST variation in the eastern Pacific is insignificant because local entrainment transcends the influence of thermocline oscillation; on the other hand, positive SST anomalies slowly amplify near the date line due to the reduction in wind-induced mixing and surface evaporation. The annual variations of the oceanic momentum and heat transports associated with the annual march of the ITCZ are shown to have significant impacts on the annual mean state. On the other hand, including an annual mean heat flux correction in the present model does not strongly influence the amplitudes of annual and interannual SST variations. However, it does improve the phase structure of the annual cycle by providing a more accurate annual mean state.