Mixed instabilities in the Gulf Stream over the continental slope

A numerical model study is presented of the unstable normal modes of oscillation of a boundary current. The model background current approximates the Gulf Stream south of Cape Hatteras, North Carolina. Observations in the Gulf Stream south of Cape Hatteras have revealed a persistent wave-like meander pattern in the Stream with a period of 708 days. This wave form propagates in the downstream direction with a phase speed of about 40 km day super(-1) and is uncorrelated with local wind forcing. An 8-day wave also appears as an eigenmode in the model, and the perturbation velocity and buoyancy fields are consistent with observations. The instability mechanism of the model wave is of the mixed barotropic-baroclinic type, with the majority (about 80%) of the perturbation energy derived from the potential energy of the background flow.