On the instability problem in simple air-sea coupled models with an oceanic surface boundary layer

Abstract

Simple air-sea coupled models have been proposed to explain the El Niño phenomena. The stability in an air-sea coupled model with an oceanic surface boundary layer introduced by Zebiak and Cane (1987) was investigated by solving the eigenvalue problem. This model is also associated with the eastward propagating unstable modes similar to the mode found by Hirst (1986). The stability was found to depend on the depth of mixed layer. The unstable mode does not appear in the western Pacific which has a deep mixed layer, even if the air-sea coupling coefficient (the heating parameter) is increased. However, the unstable mode readily appears in the central Pacific. It was found that, in addition to the Rossby mode, the Kelvin mode also becomes unstable at the limit of the advection in the model.