Vorticity and Vertical Circulation at an Ocean Front

Abstract

Density and velocity data with 4-km horizontal resolution from a survey of a front during FASINEX (Frontal Air-Sea Interaction Experiment) are combined to describe the structure of the top 300 m of the ocean. The geostrophic velocity field is derived and is used to examine the relative importance of stratification, relative vorticity, and twisting terms in Ertel's potential vorticity Q. Tenfold isopycnic changes in Q are found across a horizontal scale of only 10 km. These changes are confined to isopycnals that outcrop from the seasonal thermocline into the mixed layer. The ageostrophic velocity field is quantified by solution of the omega equation, and vertical velocities of up to 40 m day?1 are found. Small (40 km) surface-trapped (top 200 m) eddies are found to play a crucial role in the transport and effective diffusion of properties across the thermocline out of the mixed layer.