A Linear Stability Analysis of Coupled Tropical Atlantic Variability

Abstract

A linear stability analysis of an intermediate coupled ocean?atmosphere model reveals that the tropical Atlantic has two types of coupled modes: a meridional mode at the decadal time scale and a zonal mode at the interannual time scale. The meridional mode, which manifests itself as an interhemispheric SST fluctuation, is controlled by the thermodynamical feedback between winds, latent heat flux, and SST, further modified by ocean heat transport. The zonal mode, which manifests itself as an SST fluctuation in the eastern equatorial basin, is dominated by the dynamical feedback between winds, thermocline, upwelling, and SST. The relative strength of thermodynamical versus dynamical feedback determines the behavior of the coupled system. When the thermodynamical feedback dominates, the meridional mode is the leading coupled mode; when the dynamical feedback dominates, the zonal mode leads all other coupled modes. Interestingly, a nonoscillatory regime exists for the leading mode when both feedbacks are comparable in strength, suggesting a destructive interference between the meridional and zonal modes.