INTENSIVE SURVEYS OF THE AZORES FRONT .1. TRACERS AND DYNAMICS

The hypothesis that fronts are sites of active. subduction is examined using density, temperature, salinity, and horizontal velocity data fr om a trio of surveys of the Azores Front done in May 1991 and March 19 92. These surveys were made using a SeaSoar equipped with a conductivi ty-temperature-depth profiler and a shipboard acoustic Doppler current profiler. The potential density and potential vorticity indicate that dense water from the north side of the front may be sliding down bene ath the surface outcrop. This apparently subducting isopycnal has a gr eat deal of temperature and salinity variability. Horizontal velocity is neatly parallel to isopycnals, indicating that the time rate of cha nge and vertical advection must be small. The thermal wind balance is observed to be valid, especially in the region of the largest horizont al density gradients. Shear at the base of the mixed layer is likely d ue to near-inertial motions. The potential vorticity is dominated by t he planetary vorticity, except at the front, where vertical shears (th e tilting term) become large. The tilting term acts to reduce the magn itude of the potential vorticity at the front, in agreement with simpl e theoretical models. The magnitude of the tilting term is similar to the total vorticity in the seasonal thermocline.