Mid‐latitude mesoscale variability

Neither homogeneous quasi‐geostrophic turbulence nor the effect of eddies on the interior ocean circulation remains the major focus of mesoscale research as was the case for much of the preceding 20 years following identification of the phenomena. Concentration on basin‐wide geographical exploration of eddy properties was rather well established by 1978. Over the last few years, a promising zero‐order description and rationalization of the mesoscale field has become available. That is, eddy energy is largest near strong flows (intensification not only in western boundary currents, but along their mid‐latitude open‐ocean extension), roughly as indicated by gyre‐scale numerical models where eddies develop as a result of instability processes in the mid‐latitude jet and recirculation regimes, and then propagate into the ocean interior. Our point of view is that mesoscale eddies drive recirculations but do not play a crucial role in the dynamics of the mean flow in the ocean interior [in the North Atlantic, for example, the idea is that the downstream increase in transport of the Gulf Stream is eddy‐driven but not the flow through the Florida Straits]. In addition, the fluctuating response to forcing by variable winds may now be widely perceived as forming the horizontally homogeneous background signal in subtropical gyres. The time‐dependent field associated with thermohaline flows at abyssal depths also has a special character.