Near-shore transport processes induced by the wind

Water exchange between a semiclosed bay and the adjacent sea, and its relation to atmospheric forcing is examined from a field experiment undertaken in the Bay of Kastela (Adriatic Sea, Yugoslavia). Four ‘Aanderaa’ RCM4 current meters were attached on a single mooring in a 45 m deep inlet of the Bay for about two months. Water samples were taken for phytoplankton and bacteria determination at several additional stations. Salinity, temperature measurements, and determinations of the Secchi-disc depth were also made. Samples were taken approximately each week during the current measurement period so as to resolve the synoptic time scale. In the first part of the current measurement period the water was stratified, while in the second part the entire water column became vertically mixed after a strong wind event. During stratified conditions kinetic energy was at a maximum in the intermediate layer, while during homogeneous conditions the kinetic energy was trapped mainly in the bottom layer. Vertical modal structure of the current field showed that during homogeneous conditions wind induced surface flow was 180° out of phase with respect to the bottom flow. The current reversal appeared at the depth between 8 and 20 m. Under stratified conditions the surface flow was in phase with the bottom flow and water entered the Bay on one side of the inlet in the whole water column and left on the other side. These water exchange structures were reflected in the horizontal distribution of density, bacteria, Secchi-disc data and phytoplankton composition.