Effects of the meridional extent of mean equatorial upwelling on atmosphere-ocean coupled instabilities over an unbounded ocean

The large scale instabilities of a coupled atmosphere-ocean system on an unbounded equatorial beta plane are investigated by solving the eigenvalue problem. Special attention is paid to the effect of the basic state inhomogeneity due to the meridional extent of the mean equatorial Upwelling. It is found that if the upwelling region near the equator is meridionally wide, an eastward-propagating mode becomes unstable, while if it is narrow, the second symmetric Rossby mode becomes unstable. In the narrow case, when the atmosphere-ocean coupling is strong, both modes can be unstable. Seeming inconsistencies involving the characteristics of anomaly propagation in some previously published results of atmosphere-ocean coupled models of the ENSO phenomenon may arise from this sensitivity to the meridional width of the basic state upwelling.