Impact of the SST-wind stress coupling on the dynamics and stability of ocean current inside and outside SST frontal zones

The different impacts of the coupling are further illustrated analytically by comparing the normal mode growth rates for the SG and QG Eady models. The comparisons show that ocean current outside the SST frontal zone is stable under any normal mode disturbances when the coupling factor σQG<−U¯s where U¯s is the basic zonal flow at the surface, but it is unstable under certain normal modes when σQG>−U¯s. However the ocean current inside the frontal zone is always unstable under the influence of the SST-wind coupling or nonlinear Ekman effect. It is found that the negative coupling (when surface wind blows from cold water to warm water) tends to generate strong instability. In addition, the growing/decaying normal modes are non-dispersive and propagate only in one direction (eastward) outside the frontal zone, but they are dispersive and can propagate in both directions inside frontal zone.