The evolving structure of an upwelling filament

Abstract

The evolution of an upwelling filament was studied over a 2‐week period by using satellite infrared images, and its thermohaline structure was mapped in situ. The surface velocity field consisted of a large meander extending offshore for at least 300 km. The northern branch was ∼40 km wide, flowing offshore at a peak velocity of 0.55 m/s; the southern branch was flowing inshore at 0.35 m/s. The offshore transport was more than 10 6 m 3 s −1 , larger than the Ekman transport. The meander was unstable to barotropic instabilities at a scale of ∼15 km. From a succession of images a surface convergence γ ≈ 8·10 −6 s −1 over 20 km was observed near the sharp front limiting the filament to the south. The ∼350 m width of the front indicates a separation of scales between the large‐scale strain field and the mixed‐layer turbulence parameterized with an eddy diffusion coefficient K H ≈ 0.25 m 2 s −1 . Thermohaline layers that originated at the convergence near the sharp front suggest a secondary circulation subducting denser waters to the south underneath the lighter northern water.