SUBMESOSCALE DYNAMICS NEAR A SEAMOUNT .1. MEASUREMENTS OF ERTEL VORTICITY

The prevailing view that submesoscale fluctuations (horizontal wavelen gths less than a few kilometers and vertical wavelengths less than a f ew hundred meters) are dominated by internal gravity waves is tested b y measuring Ertel's potential vorticity, II = (f + del X del).del B, w here the buoyancy B = -g delta rho/rho(o). Unlike geostrophic or nonli near Ertel vorticity-carrying motions, internal waves have no Ertel vo rticity fluctuations. Velocity and temperature profile surveys beside Ampere Seamount reveal appreciable Ertel enstrophy, and thus a signifi cant non-internal-wave component, on horizontal wavelengths of 6-15 km and vertical wavelengths of 50-380 m. The twisting terms are negligib le and the relative vorticities less than 0.2f, so the anomalies are i n geostrophic balance. It is unlikely that the anomalies arise from st irring of the large-scale isopycnal gradients of stretching and planet ary Ertel vorticity as this would require stirring lengths of thousand s of kilometers. The most likely source appears to be forcing at the s eamount, but generation by (i) dissipative 3D turbulence in the pycnoc line or (ii) detrainment of the winter mixed layer cannot be absolutel y ruled out. It remains to determine whether the coexistence of intern al wave and Ertel vorticity-carrying fluctuations characterizes smalle r scales (lambda less than or equal to 50 m, lambda(H) less than or eq ual to 5 km) in the deep ocean away from topography as well.