Strong near-inertial shear in the Gulf Stream front

Coherent bands of strong ageostrophic shear, characteristic of amplified near-inertial waves, were observed below the surface boundary layer in the highly baroclinic north wall of the Gulf Stream. These were observed under strong and variable wind forcing during the February-March 2012 LATeral MIXing (LATMIX) experiment. Lines of constant ageostrophic shear, nearly parallel to slanted isopycnals, propagate upwards with time and show a pronounced peak in energy at near-inertial frequencies. Vertical profiles of the shear show enhanced clockwise over counter-clockwise polarization with depth suggesting predominantly downward energy propagation. The gradient Richardson number, Ri, is also coherently banded and regions of low Ri coincide with maxima in ageostrophic shear. Within these bands there are areas about 1 km wide and 10 m deep where Ri < ¼ and hence the flow is susceptible to shear instability. We argue, using a theory appropriate for near-inertial waves propagating in submesoscale, geostrophic currents, that the strong ageostrophic shear is likely associated with the modification of the waves by both the baroclinicity and relative vorticity of the Gulf Stream front. This interaction preconditions the flow for shear instability. The amplified wave shear is superimposed on an already strong geostrophic shear and may enhance diapycnal mixing as well as isopycnal mixing—the latter through shear dispersion. To investigate this claim, we analyze the LATMIX data set, which includes high spatial and temporal resolution observations of velocity, hydrography, and microstructure made with a variety of shipboard, towed, and glider-mounted instruments collected in a nearly Lagrangian frame of reference.