Forced ocean response during the Frontal Air‐Sea Interaction Experiment

Abstract

A central hypothesis of the Frontal Air‐Sea Interaction Experiment (FASINEX) was that the strong gradients in the upper ocean associated with ocean fronts would spatially modulate the response of the ocean to local atmospheric forcing. To test this hypothesis, measurements of near‐surface meteorology and upper ocean variability were made for 5 months from an array of moorings and, for one of those 5 months, from two ships. A number of fronts passed through the moored array, and the moored meteorological and current meter data were used to examine the response of the upper ocean to atmospheric forcing at subinertial and near‐inertial frequencies. At low frequencies an Ekmanlike, frictionally driven component in the mixed layer and a more deeply penetrating component thought to be forced by Ekman pumping were found. Strong near‐inertial response was found in the mixed layer. The near‐inertial motion showed considerable spatial variability in the seasonal thermocline; here it was evident that the structure of the front influenced the downward propagating, near‐inertial internal waves. Data were also obtained by profiling instruments deployed along cross‐frontal sections. These data were used to examine the spatial and temporal variability of the internal waves, microstructure, and dissipation, and the relation of that variability to oceanic fronts and atmospheric events. The spectral densities of the components of the internal waves that rotated clockwise with increasing depth were higher in regions of negative relative vorticity than those predicted by Garrett and Munk (Munk, 1981), and estimates of clockwise and total shear were highest in regions of negative relative vorticity. In the strong shear of the ocean front, significant anisotropy of the internal wave field was observed and kinetic energy dissipation levels relative to the rate of work by the wind were higher than previously reported.