Observations and models of the energy flux from the wind to mixed-layer inertial currents

Observations of wind stress, upper-ocean currents, and stratification from a mooring at 10∘∘N in the Pacific are used along with two different one-dimensional mixed-layer models to investigate the horizontal kinetic energy (KE) balance for inertial motions in the mixed-layer. Results from four other sites spanning 25∘∘N to 60∘∘N in the Atlantic also are presented. Determining the work done by the wind on the ocean during strong resonant forcing events is of particular interest. The damped-slab mixed-layer model used in previous studies is appealing for this purpose, since it is possible to estimate the wind work using only the wind stress, prescribed mixed-layer depth, and an empirical damping constant. However, it is found that this model, which does not allow for interaction between the mixed-layer and the stratified water column below, does not reproduce the observed KE balance for strong forcing events and systematically over-estimates the wind work. A slightly more sophisticated model, which includes a shear instability mechanism to facilitate momentum transfer between the mixed-layer and a transition layer below, reproduces the observed KE balance and the cumulative wind work much more accurately. Mixed-layer inertial currents unrelated to wind forcing (e.g., due to upward propagating internal waves) are found in the observations, but are not accounted for in either model. When wind forcing is relatively weak, these background currents may be of comparable amplitude to wind-forced currents, and can have a significant impact on the inertial response.