The Response of a Baroclinic Anticyclonic Mesoscale Eddy to Relative Wind Stress Forcing

Including the ocean surface current in relative wind stress is known to damp mesoscale eddies through a negative wind power input. This is thought to have potential ramifications for eddy longevity. Here, we study the spin-down of a baroclinic anticyclonic eddy subject to absolute and relative wind stress forcing by employing an idealised high-resolution numerical model. To assess the effect of relative wind stress on the eddy, we examine wind-induced vertical motions and energetics. Results from this study show that relative wind stress damps eddy kinetic energy (EKE) at the eddy’s surface. However, relative wind stress also induces additional vertical motions, in the form of Ekman pumping, that increases baroclinic conversion i.e., a conversion of potential to kinetic energy. When horizontally integrated, this additional baroclinic conversion by relative wind stress is positive throughout the eddy water column. The positive baroclinic conversion in the lower depths of the eddy leads to an increase in deep EKE, relative to the absolute wind stress case. In fact, over the eddy volume, the damping of EKE by relative wind stress is offset by this conversion of energy. Moreover, this conversion turns out to dominate any damping by wind during the later stages of eddy lifetime. A scaling analysis of relative wind stress-induced baroclinic conversion and relative wind stress damping also confirms these numerical findings, showing that energy conversion is greater than wind damping. Overall, this highlights the complexities of ocean-atmosphere interactions at the mesoscale, and points to the need for further study in this area.