The Energy Flux from the Wind to Near-Inertial Motions in the Surface Mixed Layer

Abstract

Time series of wind stress computed from long-term meteorological buoy data off North America are used to examine the forcing of surface inertial currents by the wind. A simple damped slab model of the mixed layer is used to compute 〈Π(H)〉, the average flux of energy from the wind to mixed layer inertial currents in a mixed layer of fixed depth H. The forcing of mixed layer inertial motions is highly intermittent. Most of the forcing occurs during the winter months, with a few dozen events accounting for typically over half the total energy flux. Major forcing events are usually associated with translating cold fronts or small lows with scales of about 100 km. The larger, synoptic scale features have little energy at the inertial frequency and thus result in only weak forcing of inertial currents. A strong seasonal signal exists in the inertial forcing. At OWS-P (50°N, 145°W), 〈Π(50 m)〉 is largest from October to February and is a factor of 12 above the June and July values. If seasonally varying mixed layer depths are used, 〈Π(H)〉 is largest in October, due to the combination of a shallow mixed layer and strong forcing. The forcing of inertial motions varies with location, although comprehensive geographical coverage is not obtained here. In these data the 1~ wintertime average 〈Π(50 m)〉 is found off the east coast of North America at about 35°N. The smallest value, in the Gulf of Mexico, is four times less. Although a strong correlation between 〈Π(50 m)〉 and 〈u*3〉 the parameterized flux of energy to mixed layer turbulence, is found at OWS-P, this correlation does not hold at all other locations. This suggests that climatological models that attempt to parameterize 〈Π〉 in terms of 〈u*3〉 will need to be tuned to local conditions.