MIXING AT CAMARINAL SILL IN THE STRAIT OF GIBRALTAR

During the Gibraltar Experiment in 1985-1986, observations of shear, s tratification, microstructure, and acoustic backscatter revealed a com plex mixing regime at Camarinal Sill in the Strait of Gibraltar. The m ixing is forced by strong semidiurnal tidal currents and weaker, but m ore steady, baroclinic flows. Flows are characterized as outflow or in flow, nearly equivalent to westward or eastward. Rising water at Gibra ltar coincides with outflow at all depths. We observed two modes of ou tflow at the sill. In May 1986 the transition layer separating Atlanti c and Mediterranean water was between 0.75 MPa and 1 MPa when we sampl ed east of the sill near neap tide. The transition turned down just be fore reaching the sill crest, where the flow became critical. Increasi ng shear soon destabilized the transition, producing billows which gre w to 30-75 m on the west flank of the sill. The overturning generated intense turbulence, with peak dissipation rates, epsilon, exceeding 10 (-2) W kg-1. This turbulence rapidly thickened the transition from DEL TAz = 30-50 m east of the sill to DELTAz = 130-150 m within 1-2 km wes t of the sill. From the rate of thickening, we estimate the rate of fo rmation of new transitional water during neap tide as (0.8-1.2) x 10(5 ) m 3 s-1. This outflow mode did not release eastward-propagating inte rnal bores when outflow turned to inflow, but the inflow did appear to arrest the deep outflow. In May, the largest average dissipation rate over and just west of the sill had the same magnitude as estimates of the net energy lost by adjustment of velocity and mass across the sil l during outflow. Other averages are smaller than the energy loss, pre sumably because the limited sampling severely underestimates the avera ge dissipation rate. During outflow, strong shears were observed to ri se to 100 times those in the background internal wave field. However, probably owing to spatial offsets between the ship's acoustic Doppler current profiler and our tethered free-fall profiler, the only statist ically significant correlations were found between overturning scales measured directly and those estimated from epsilon in the same profile . We observed the other outflow mode in October 1985, when we profiled over the sill near spring tide and the transition was east of the sil l, centered near 1.5 MPa. After encountering the sill, the transition rose sharply and remained within 10-20 m of the surface across the sil l, plunging steeply downward just past the west edge of the sill. This was accompanied by intense turbulence and at least one rebound of the transition. We did not observe release of eastward propagating bores when inflow turned to outflow, but observations by Armi and Farmer (19 88) in April 1986 found bores released at spring tide in regimes simil ar to the one we observed in October 1985.