Three anticyclonic slope water oceanic eDDIES (SWODDIES) in the Southern Bay of Biscay in 1990

Abstract

Jet-like extensions of the slope current off northern Spain and France in the southern Bay of Biscay in the winter develop into anticyclonic eddies with an upper core of slope water. Three eddies were observed to develop in the winter of 1989/1990, subsequently named F90a, F90b and O90. F90a was an anticclonic eddy of radius 50–60 km, with a central core of mixed water of slope origin (from the slopes in the vicinity of Cap Ferret Canyon) and of volume ∼400 k³. In the eddt centre, the mixed core extended from ∼70 to ∼280 m. Measurements at sea showed minimal changes in the core characteristics of F90a (potential temperature, 12.95°C, salinity, 35.74 psu) with time, and remote sensing studies demonstrated that these slope water core eddies can maintain their identities for about a year. Rotation rates in the central core of F90a were about 3 days and values of normalized relative vorticity were about −0.5. Maximummean azimuthal velocities were about 30 cm s⁻¹ at a radius of 30 km. Hydrographic data showed that the presence of the core was felt to a depth of ∼1500m, resulting in an azimuthal transport of about 8 Sv. Both F90a and O90 moved westward across the deep (4800 m) Bay of Biscay, and the westward migration speed (2 cm s⁻¹) seems in excess of simple theoretical estimates for the β-induced westward propagation speed (0.4 cm s⁻¹) of anticyclonic eddies. If the latter figure is used for self-advection and the effects of topography and mutual influence are neglected, the observed westward movement suggests a clockwise mean circulation −2 cm s⁻¹) for the oceanic water in the Bay of Biscay. By contrast, F90b remained nearly stationary near 4°W. Remote sensing studies indicate that the occurence of a 4°W eddy in the summer of any year miight be attributed to warm slope water inflow along the northern Spanish slope in the previous winter.