Dependence of energy flux from the wind to surface inertial currents on the scale of atmospheric motions

Atmospheric features such as translating cold fronts and small lows with horizontal scales of about 100 km are traditionally thought to be most important in exciting near-inertial motions in the ocean. However, recent studies suggest that a significant fraction of energy flux from the wind to surface inertial currents may be supplied by atmospheric systems of larger scales. Here, the dependence of this energy flux on the scale of atmospheric motions is investigated using a high-resolution atmosphere reanalysis product and a slab model. It is found that mesoscale atmospheric systems with scales less than 1000 km are responsible for almost all the energy flux from the wind to near-inertial motions in the midlatitude North Atlantic and North Pacific. Transient atmospheric features with scales of ~100 km contribute significantly to this wind energy flux, but they are not as dominant as traditionally thought. Owing to the nonlinear nature of the stress law, energy flux from mesoscale atmospheric systems depends critically on the existence of the background, larger-scale wind field. Finally, accounting for relative motions in the stress calculation reduces the net wind energy flux to near-inertial motions by about one-fifth. Mesoscale atmospheric systems are found to be responsible for the majority of this relative wind damping effect.