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.