Local versus remote wind effects on the coastal circulation of a microtidal bay in the Mediterranean Sea

Coastal currents in microtidal bays are assumed to be weak and random, promptly responding to variations in wind forcing. However wind effects can act at scales that vary from local to large scales. With the aim of examining the response of coastal flow to local and regional wind forcing and their relative importance as drivers of coastal circulation, current data were collected during five months during the summer 2009 in Palma Bay (western Mediterranean Sea). The data set is jointly analyzed with a primitive-equation model to characterize the circulation in the bay. Moored acoustic Doppler current profiler (ADCP) data shows that currents in the bay fluctuate widely in direction resulting in an almost negligible mean current (< 1.5 cm/s). This strong variability is mainly attributed to two major wind forced mechanisms interacting in the bay: island trapped waves (ITWs) propagating at an island scale and locally wind-induced mass balance. Wind forced oscillations at periods of 60 and 24 h dominate current variability. The fluctuations at 60 h are associated with the first radial and first azimuthal mode of ITWs generated by remote wind. Weak ITWs are associated with currents flowing parallel to the coast, whereas more intense ITWs form an anticyclonic gyre over the southeastern part of the bay due to flow separation at Enderrocat Cape. At the period of 24 h, the wind has two effects: the first radial and second azimuthal mode of ITWs and a local wind-induced mass balance in response to the sea breeze. Variations in wind and consequent changes in coastal circulation are a major contributor to coastal water renewal, and hence critically influence key environmental aspects such as water quality or marine organism dispersal in Palma Bay.

► We analyze the response of coastal flow to wind using ADCP data and a numerical model.
► Wind-forced island trapped waves oscillate at periods of 60 and 24 h.
► Currents flow parallel to the coast for weak island trapped waves.
► Intense island trapped waves form an anticyclonic gyre due to flow separation.
► Sea breeze induces a locally wind-induced mass balance.