Interaction of a Coastal Kelvin Wave with the Mean State in the Gulf Stream Separation Area

Abstract

The interaction of a coastal Kelvin wave with the mean state in the Gulf Stream separation area is studied using a hierarchy of numerical models including both low- and high-resolution 2.5-layer models and a coarse-resolution ocean general circulation model (OGCM) in a simple configuration. When the Kelvin front reaches the separation area in the low-resolution 2.5-layer model, an anomaly of opposite sign emerges and remains in the upper layer of the separation area. The mechanism leading to its buildup is the following: the variations of thickness in the active midlayer due to the propagation of the Kelvin wave induce current variations that act as a source of thickness anomalies for the upper layer (negative feedback). This source term is proportional to the mean vorticity gradient. The latter therefore must be large enough to obtain a significant response, which explains why this phenomenon occurs only in the separation area. The anomaly remains in the separation area because the advection by the mean zonal current is balanced by the current anomalies due to the variations of thickness in the surface layer. A very similar response is obtained in the high-resolution case and with the OGCM; thus, the mechanism leading to this evolution seems also largely independent of the model. However, in the OGCM, the surface current is sufficiently strong to advect the anomaly in the surface layers (above 200-m depth). Note finally that these anomalies do not prevent the Kelvin waves from pursuing their travel southward and then eastward toward the eastern boundary where Rossby waves are radiated. Numerous recent results based on this adjustment mechanism therefore would be robust.