The structure of incipient coastal counter currents in South Portugal as indicator of their forcing agents (original) (raw)
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Current variability and momentum balance in the along-shore flow for the Catalan inner-shelf
2012
We examine the circulation over the inner-shelf of the Catalan Sea using observations of currents obtained from three Acoustic Doppler Current Profilers (two at 24 m and one at 50 m) during March-April 2011. The along-shelf current fluctuations during that period are mainly controlled by local wind stress on short time scales and by remote pressure gradients on synoptic time scales. Different forcing mechanisms are involved in the along-shelf momentum balance. During storm conditions, wind stress, sea level gradients and the nonlinear terms dominate the balance. During weak wind conditions, the momentum balance is controlled by the pressure gradient, while during periods of moderate wind in the presence of considerable stratification, the balance is established between the Coriolis and wind stress terms. Vertical variations of velocity are affected by the strong observed density gradient. The increased vertical shear is accompanied by the development of stratified conditions due to local heating when the wind is not able to counteract (and break) stratification. The occasional influence of the Besòs River plume is observed in time scales of hours to days in a limited area in near the city of Barcelona. The area affected by the plume depends on the vertical extent of the fresher layer, the fast river discharge peak, and the relaxation of cross-shore velocities after northeast storm events. This contribution provides a first interpretation of the inner-shelf dynamics in the Catalan Sea.
Characterisation of coastal counter-currents on the inner shelf of the Gulf of Cadiz
Journal of Marine Systems
At the Gulf of Cadiz (GoC), poleward currents leaning along the coast alternate with coastal upwelling jets of opposite direction. Here the patterns of these coastal countercurrents (CCCs) are derived from ADCP data collected during 7 deployments at a single location on the inner shelf. The multiyear (2008-2014) time-series, constituting ~18 months of hourly records, are further analysed together with wind data from several sources representing local and basin-scale conditions. During one deployment, temperature sensors were also installed near the mooring site to examine the vertical thermal stratification associated with periods of poleward flow. These observations indicate that the coastal circulation is mainly alongshore and barotropic. However, a baroclinic flow is often observed shortly at the time of flow inversion to poleward. CCCs develop all year-round and exclusively control the occurrence of warm coastal water during the upwelling season. On average, one poleward flow lasting 3 days was observed every week, corresponding to CCCs during ~40% of the time without seasonal variability. Thus, the studied region is distinct from typical upwelling systems where equatorward coastal upwelling jets largely predominate. CCCs often start to develop near the bed and are frequently associated with 2-layer cross-shore flows characteristic of downwelling conditions (offshore near the bed). In general, the action of alongshore wind stress alone does not justify the development of CCCs. The coastal circulation is best correlated and shows the highest coherence with south-eastward wind in the basin that proceed from the rotation of southward wind at the West coast of Portugal, hence suggesting a dominant control of large-scale wind conditions. In agreement, wavelet analyses indicate that CCCs are best correlated with alongshore wind occurring in a band period characteristic of the upwelling system (8-32 days). Furthermore, in the absence of wind coastal currents tend to be poleward during summer. This set of observations supports that CCCs develop in response to the unbalance of an alongshore pressure gradient during the relaxation of (system-scale) upwelling-favourable winds, oriented southeastward in the basin. The relaxation periods defined based on this wind direction show a good correspondence with the periods of poleward flow.
The vertical structure of coastal currents
Deep Sea Research and Oceanographic Abstracts, 1976
A vertical array of four equidistant current meters was used to measure horizontal currents in 18m of water. The instruments resolved frequencies up to 15cph for a period of 33 days in late summer 1974. Onshore (EW) and longshore INS) currents were essentially uncorrelated at all depths. Longshore currents exhibit significant coherence with the surface tide but not at frequencies higher than the tidal frequencies. The effect of a southerly wind lasting over 3 days was evident as a northbound current that was most intense near the surface. The spectrum of onshore currents exhibits a peak at the semidiurnal frequency corresponding to internal tides, and there is a second lower, but broader, peak at frequencies between 1 cph and the buoyancy frequency. Onshore current amplitudes corresponding to the internal tides are on the order of 20cms-k while the higher frequency fluctuations have amplitudes on the order of 3 cm s-~. There is significant coherence between all measurements of onshore current with a phase shift of n between surface and bottom currents. The onshore current measurements are consistent with the mode 1 oscillations of a three-layer model consisting of homogeneous surface and bottom layers separated by a layer having constant buoyancy frequency.
The Structure of Nearshore Currents Driven by Changes in Meteo-Marine Forcings
Coastal Engineering Proceedings
This work aims to better understand the physical processes governing the wave propagation in a vertically sheared current and the resulting nearshore circulation patterns. It is based on a high resolution hydro-morphodynamic field campaign, ROUSTY2014, collecting a comprehensive hydro-morphodynamical dataset during a full winter season. The overall analysis highlights three main circulation patterns, largely controled by the bathymetric features and by co-working or competing wind and waves forcings. Regarding the vertical structuration of the circulation, most of field observations shown seaward directed circulation with onshore component close to the air/sea surface whose intensity varies according to incoming wave conditions. Forth, vertical shear increase when breaking happen close to sensor location and orientation of mean circulation depend on wind direction.
2003
High-Frequency Doppler radars synoptically mapped surface currents over the inner-shelf off the North Carolina Outer Banks during the Duck94 experiment. The radar system sampled for the month of October, 1994 over a region 45 to 66 km southeast of the Chesapeake Bay mouth up to 34 km offshore. Concurrently, salinity and temperature were measured at locations 0.5, 1.6, 5.3 and 16.6 km offshore along a cross-shelf transect. Salinities over the shelf ranged from 26.6 to 32.7, with the lower salinities resulting from the intermittent presence of the coastal buoyancy current from the Chesapeake Bay. Three events were observed in which downwelling favorable winds persisted for at least three days, relaxed and then rotated to upwelling favorable. Downwelling favorable winds produced coastal sea level setup. When the buoyancy current was present, cross-shelf salinity gradients were established between fresher water near the coast and the more saline water offshore. The response of the buoyancy current was sensitive to the temporal evolution of the wind field from downwelling to upwelling favorable. Following downwelling favorable winds the wind typically shifted to an onshore direction and decreased in magnitude. When the alongshelf velocities decreased, the onshore Coriolis force also decreased and the buoyancy current exhibited significant alongshelf variability over scales less than 10 km. Offshore flowing jets and undulations in the flow developed that formed vortices with diameters of 5-10 km following two of the downwelling events. Offshore surface layer velocities were observed between 4 and 16 hours before the wind switched to upwelling favorable. Formation of the vortices was likely triggered by an imbalance in the crossshelf momentum due to nearshore water level setup and the decreased onshore Coriolis component.
Interactions between wind and tidally induced currents in coastal and shelf basins
Ocean Dynamics, 2017
This paper addresses the impact of atmospheric variability on ocean circulation in tidal and non-tidal basins. The data are generated by an unstructured-grid numerical model resolving the dynamics in the coastal area, as well as in the straits connecting the North Sea and Baltic Sea. The model response to atmospheric forcing in different frequency intervals is quantified. The results demonstrate that the effects of the two mechanical drivers, tides and wind, are not additive, yet non-linear interactions play an important role. There is a tendency for tidally and wind-driven circulations to be coupled, in particular in the coastal areas and straits. High-frequency atmospheric variability tends to amplify the mean circulation and modify the exchange between the North and the Baltic Sea. The ocean response to different frequency ranges in the wind forcing is areaselective depending on specific local dynamics. The work
Three-Dimensional Wind-Driven Coastal Circulation past a Headland
Journal of Physical Oceanography, 2006
Local velocities and the trajectories of fluid parcels forced by wind blowing over a continental shelf, in the vicinity of a headland, are described with a linear, steady, three-dimensional barotropic model. The dynamical balance that governs the transport is similar to the wind-driven general circulation, because the varying bottom depth acts in the same way as meridional variation in the rotation rate. Far from the headland the circulation is independent of alongshore position, and the transport is parallel to the coast. The alongshore pressure gradient is a significant term in the alongshore momentum balance. Near the headland, the amplitude of the circulation, including the vertical motion, is larger on the upwave side (the side toward which a Kelvin wave would travel) than on the downwave side. On the upwave side, the flow adjusts to the presence of the headland over a distance of order δEB*, where δE is the ratio of the Ekman depth to the maximum shelf depth and B* is the widt...
On Wind and Ocean-Velocity Correlations in a Coastal-Upwelling System
Journal of Physical Oceanography, 1983
This note studies the response of a simple linear baroclinic coastal-upwelling model to fluctuating longshore winds. Correlations between wind stress and velocities are computed explicitly. It is shown that these correlations depend primarily upon the wind-~ spectrum and that. for a realistic spectrum, the wind stress leads the longshore velocity by approximately one day. The computed correlations agree remarkably well with observations and dismiss the belief that time lags ought to be directly related to the local inertial period, i.e., some fraction of the local pendulum day.