The velocity structure of the upper ocean in the presence of surface forcing and mesoscale oceanic eddies

Abstract
One goal of the Joint Air-Sea Interaction Experiment (JASIN) was to investigate the structure of the near-surface velocity field and to attempt to quantify what fraction of that field was related to the local wind. Toward that end, in the late summer of 1978, two moorings were deployed in the northern Rockall Trough with oceanographic instrumentation concentrated in the upper 100 m of the ocean. Simultaneous observations were made of the surface winds at each mooring and, adjacent to one of the moorings, of the velocity field at depths from 79 to 1000 m. Energetic, eddy-like circulation dominated the velocity field in the JASIN area at depths shallower than approximately 800 m. However, both the velocity and the vertical shear of horizontal velocity showed variability that increased with proximity to the surface. Empirical orthogonal functions, computed to separate the velocity data into uncorrelated modes of variability, showed that over 97 % of the variability in the upper 300 m was distributed among only three vertical modes. The first function had little depth dependence; the second had strong depth-independent flow in the depth range of the mixed layer and weak flow in the opposite direction at all depths below; and the third had strong flow near the surface, strong flow in the opposite direction just below the base of the mixed layer, and weaker flow at all other depths. Function 1 alone provided a near-complete description of the velocity variability below 85 m, where the flows associated with the eddy-like circulation and the barotropic semidiurnal tide were the dominant components. At 85 m and above all three functions were necessary to provide a complete description. Temporal variability of function 2 was coherent with the local wind stress at the inertial frequency, but, at lower frequencies, resulted in transport in the mixed layer to the southeast that was not coherent with the local wind. Low frequency temporal variability of function 3 was coherent with the local wind stress; at these frequencies the velocity vector of function 3 nearest the surface was directed to the right of the wind stress vector and the velocity vector just below the base of the mixed layer was directed to the left of the wind stress. Thus, forcing by the local wind can account for some but not all of the increased variability found near the surface.

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