Structure and Coherence of flow through a Tidally Energetic Channel measured by High Frequency Radar
This studypresents the application of a noveltechnique to investigatethe circulationin the Fair Isle Gap (FIG) by the High FrequencyRadar (HFR),deployed in the area during the Brahan Project (from 29thAugust 2013 to8thSeptember 2014). The performance of the FIGHFRsystem is assessed, permitting the estimate of the volume transport intotheNorth Sea(NS). This research focuses onusing the HFR data, together with the other available in situobservations, hydrodynamic model output and meteorological data, to better understand the vertical structure and coherence of the inflow into the North Sea. By combining the horizontal HFR surface current data with the other sources, this study seeks to reconstruct the nature of the flow by setting the HFR current data in the context of depth-resolving data. The mean transportwasbetween 0.12 and 0.36 Sv, persistently flowing into the NSthroughout the year. A markedreductionin the volume transportwasobserved between autumn and winter, with transport during the formerestimated bythe HFR at0.60 Sv, and during the latterat 0.20 Sv. The results show that the monthly volume transport into the NSwas broadlyassociated with meteorological forcing from the south-west sector. Both the maximum (0.75 Sv) and minimum (-0.20 Sv) occurred duringperiods when the water column wasvertically well-mixed (between December and January). There is evidenceof periodicity of ~14 and 100 days. The former is due to the topography and thestrongtidal currents(i.e. the spring-neaps tidal cycle)in the area, while the latter corresponds to the wind-driven circulation variability.
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