Diurnal–inertial motions and diapycnal mixing on the Portuguese shelf

Abstract

This work presents observations of diurnal–inertial motions forced by a sea–land breeze regime near critical latitudes ( 30 ∘ ± 10 ∘ N / S ) for resonance on the Portuguese shelf. Wind conditions were determined with an Aanderaa meteorological buoy placed in the Bay of Setúbal during the Maritime Rapid Environmental Assessment 2004 (MREA04) sea trial. Currents data for this period were recorded from two Barny ADCP (acoustic Doppler current profiler) deployments located on the north and south sides of the bay, respectively. Both ADCPs covered almost the full depth of water ( ∼ 112 m ) with 24 bins of 4 m size at both sites. Dominant motions exhibited a two-layer configuration in response to energetic out-of-phase oscillations driven by diurnal–inertial currents. Hence, simultaneous CTD (conductivity, temperature, depth) data were considered to assess the strength of observed motions in promoting diapycnal mixing. With this aim, ADCP and CTD data were used to analyse the vertical shear ( S 2), squared buoyancy frequency ( N 2) and gradient Richardson number ( Ri) of the water column in terms of two main layers with highly stratified and vertically sheared flows during diurnal–inertial events. Observations show a strong variation in the vertical density field near both ADCP deployments that involved high vertical shears distributed into two layers associated with diurnal–inertial currents. The results of our analysis highlight the role of wind-forced diurnal–inertial motions in shelf areas near the critical latitudes, where they can greatly contribute to triggering diapycnal mixing at the scale of a few days, owing to sea–land breeze forcing.