The influence of wind-driven currents on the circulation and bay dynamics of a semi-enclosed reefal bay, Wreck Bay, Jamaica

Abstract

Investigations into the descriptive hydrodynamics of a semi-enclosed reefal bay showed that inner bay waters re-circled the reef and particular forcing conditions of wind and tide retarded or enhanced this circulation. Fringing reef parabola, partially enclosing bays, are generally accepted as the limit of the bay dynamics. The dynamics displayed at Wreck Bay, on the southeast coast of Jamaica, however, showed the reef as the heart of main circulation and therefore the centre of the functioning bay. Results from field measurements of the spatial and temporal variability in circulation revealed that inner bay water surging from the back-reef exited quickly through a gap in the reef (channel flow, up to 60 cm s −1), diverged on the fore-reef, then either re-entered the back-reef to become closed circum-reef circulation (CRC), or continued along the fore-reef as open CRC. Variation in the wind regime controlled the relative importance of wind or tides to driving circulation and bay emanation. Wind speeds above 5 m s −1 were found to be the dominant forcing factor and, combined with tides, accounted for 60% of the variability in CRC when the sea-breeze developed during early summer and again when the land-breeze strengthened during the winter deployment. Open CRC was extended seawards when channel flow currents displayed occasional surges associated with strong wind-driven currents combined with similar incident wave approach. This led to change in bay dynamics by extreme expansion in bay characteristics. The opposite scenario occurred on rare occasions of low tide with wind calms, when channel flow reversed into the inner bay and flow over the emergent reef was reduced, leading to the most extreme contraction of bay characteristics.