OBSERVATIONS OF CURRENTS AND DENSITY STRUCTURE ACROSS A BUOYANT PLUMEFRONT

Observations of the Mobile Bay, Alabama, plume during a flood event in April 1991 reveal significant differences in the current field on eit her side of a front associated with the buoyant plume. During a strong southeasterly wind, turbid, low salinity water from Mobile Bay was pu shed through an opening in the west side of the ebb-tidal delta and mo ved parallel to the coast. A stable front developed between the low sa linity water of the buoyant plume (11 parts per thousand) and the high salinity coastal water (> 23 parts per thousand) that was being force d landward by the prevailing winds. Despite the shallow water depth of 6 m, measurements of currents, temperature, and salinity show large s hears and density gradients in both the vertical and the horizontal di rections. At a station outside of the buoyant plume, currents at 0.5 m and 1.5 m below the surface were in the same direction as the wind. I nside the plume, however, currents at 0.5 m below the surface were par allel to the coast, 45-degrees off the direction of the wind and the m agnitude was 45% larger than the magnitude of the surface currents out side the plume. Beneath the level of the plume, the currents were iden tical to the wind-driven currents in the ambient water south of the fr ont. Our observations suggest that the wind-driven surface currents of the ambient water converged with the buoyant plume at the front and w ere subducted beneath the plume. The motion of the ambient coastal sur face water was in the direction of the local wind stress, however, the motion of the plume had no northerly component of motion. The plume a lso did not show any flow toward the front, suggesting a balance betwe en the northerly component of wind stress and the southerly component of buoyant spreading. In addition, the motion of the plume did not app ear to affect the motion of the underlying ambient water, suggesting a lack of mixing between the two waters.