Intensive surveys of the Azores Front: 2. Inferring the geostrophic and vertical velocity fields

The geostrophic and vertical velocity fields are inferred using density and horizontal velocity data from three SeaSoar/acoustic Doppler current profiler surveys of the Azores Front. The analysis is a two‐step procedure consisting of (1) objective analyses to reflect the observed length scales and (2) dynamical adjustments so that the density field is statically stable and the velocity field is in geostrophic balance. The vertical velocity is inferred using a version of the quasi‐geostrophic omega equation in which the stratification is allowed to vary in the horizontal. The resulting vertical velocity peaks at 2 × 10 −4 m s −1 at 200–300 m. There is a tendency for the denser water to the north of the front to be downwelled, while the warmer water to the south is upwelled. The implied heat flux exceeds 10 W m −2 near 100‐m depth, so the upper 100 m is warmed while the lower water column is cooled, tending to stratify the upper ocean. Ageostrophic horizontal flow is toward the dense side of the front at the surface. The dominant terms in the density balance are time rate of change and horizontal advection. The inferred circulation cells may be an important mechanism of subduction in the upper ocean.