The Pacific cold tongue and the ENSO mode: A unified theory within the Zebiak-Cane model

The equatorial tropical Pacific climate system is a delicate coupled system in which winds driven by gradients of sea surface temperature (SST) within the basin interact with the ocean circulation to maintain SST gradients. T his results in a time mean state having a strong zonal temperature contrast along the equator with an eastern cold tongue and a western warm pool. By the same coupled processes, interannual variability, known as the El Nino-S outhern Oscillation (ENSO), is present in the Pacific. This variability can be attributed to an oscillatory coupled mode, the ENSO mode, in the equato rial ocean-atmosphere system. Using a Zebiak-Cane-type intermediate coupled model. the coexistence of an eastern cold tongue in the annual mean state and ENSO in the Pacific climate system is investigated. The ENSO mode arise s as a robust oscillatory mode on a coupled mean state and becomes unstable if the cold tongue of the mean,rate is sufficiently strong. The origin of this mode, its propagation mechanism, its sensitivity to parameters, and it s relation to the spatial structure of the annual mean state are considered .