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

Abstract

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. This 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-Southern Oscillation (ENSO), is present in the Pacific. This variability can be attributed to an oscillatory coupled mode, the ENSO mode, in the equatorial 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 arises as a robust oscillatory mode on a coupled mean state and becomes unstable if the cold tongue of the mean state is sufficiently strong. The origin of this mode, its propagation mechanism, its sensitivity to parameters, and its relation to the spatial structure of the annual mean state are considered.