Observations of the structure of the upper ocean: wind-driven momentum budget

Abstract

The response of the surface layer to a sudden 0.3 N m -2 wind event is described in unusual detail by combining current data from instruments hung beneath a drifting spar buoy with temperature and salinity data from conductivity-temperature-depth sensors on an undulating vehicle towed 26 times around a square of side 7 km, maintaining position relative to the drifting spar. Horizontal advection is minimized by the Lagrangian experiment. The momentum equations for the wind-driven response of the surface layer cannot be balanced by wind stress, temporal rate of change and coriolis terms alone. Mixing rearranges vertical and horizontal density gradients, resulting in a mixed-layer depth that slopes horizontally at about 10 -3 (1 m km -1 ), which in turn modifies pressure gradient terms, causing thermal wind shear across the base of the mixing layer of up to 1 cms -1 m -1 . Rapid adjustment of the pressure gradient terms is the most probable cause of inertial oscillations 50% larger than can be accounted for by wind stress alone, but tidal-inertial interactions are also significant. The geostrophic, wind-driven mean and inertial currents are depth independent within 2 cm s -1 in the mixing layer, at least from the shallowest current meter at 14 down to 25 m.