The present state of the water exchange through the Strait of Gibraltar

The present two-way exchange through the Strait of Gibraltar stems from the fresh water deficit of the Mediterranean basin. The size of the exchanged flows is the outcome of the climatology over the basin and the morphology and internal hydraulics of the Strait. Its highly constraining topography imposes an upper bound to the flows that cannot be exceeded, a limit known as maximal exchange, which is associated with the minimum salinity difference (density, in fact) between the inflowing (Atlantic) and outflowing (Mediterranean) waters. A relevant question is whether or not the present exchange has achieved this limit. A simple steady-state model of the Mediterranean Sea and Strait of Gibraltar system in which the deep water formation rate is balanced by the Mediterranean outflow, strongly suggests that the exchange is quite close to (or, indeed, it is) maximal. The Strait topography requires two hydraulic control sections for this limit to be achieved, which could be thought as if one of them controls the outflow (the section of the main sill of Camarinal) and the second one the inflow (the narrowest section or Tarifa Narrows, to the east of the main sill). Oceanwards of Camarinal, the outflow accelerates and settles as one of the fastest and brisk bottom currents of the world ocean. East of Tarifa Narrows the inflow becomes a swift jet, the Atlantic Jet, which enters the Mediterranean Sea and sets up the basic features of the Alboran Sea surface circulation. The steady exchange so far depicted is not realistic. The flows undergo large fluctuations at different time scales, which modify the former description drastically. Of special importance are the tides and associated tidal flows, which can be up to 5 times greater than the long-term averaged flows. Under such huge fluctuations, the hydraulic control in Camarinal sill is periodically lost about two hours before high water, during the rising tide, when the tidal currents still head toward the Atlantic Ocean. The flooding of the control is revealed by the release of a vast internal hydraulic jump that progresses towards the Mediterranean Sea while evolving into a regularly shaped train of great amplitude internal waves that carry away part of the tidal energy accumulated nearby the sill during the rising tide. Whenever the hydraulic control at Camarinal is lost, a second sill located to the west (Espartel sill, the westernmost gateway of the Strait) takes over the control of the Mediterranean outflow. The hydraulic control at Espartel is very seldom flooded, which is a fundamental fact for understanding the internal dynamics of the exchange under large barotropic fluctuations. Without this sill, the Strait dynamics would be very different. It is behind the powerful, but periodically inverting, bottom currents nearby Camarinal sill and the also strong, but already unidirectional, bottom currents in Espartel sill and beyond, a reason for which this sill might well be considered as the proper source of the Mediterranean plume in the Atlantic Ocean.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Is the Western Mediterranean Transition showing up in the Strait of Gibraltar?

Physical and chemical properties (temperature, salinity, pH) of the Mediterranean outflow near the seafloor at Espartel sill, at the westernmost section of the Strait of Gibraltar, have been measured by a monitoring station aimed at following the changes that are taking place in the Mediterranean Sea. Presently, temperature and salinity series are longer than 13 years and they show a slight positive trend (warmer and saltier) that has increased after year 2013. The possibility of this increase being the result of the arrival of the so-called Western Mediterranean Transition (WMT), which started in year 2005 after the huge production of new and denser-than-usual Western Mediterranean Deep Water (WMDW), has been considered. The hypothesis that the water of the WMT is now showing up in the Strait has been put forward by some authors and gets strong support from the pH time series recorded at the station, which shows a sudden and sustained decrease (acidification) starting in early 2016. This decrease is consistent with the fact that the water now being observed flowing out is younger, i.e. has been in contact with the atmosphere more recently, than the WMDWit used to flow out previously to this year. It was only after the large new WMDW production of the harsh winters of 2012 and 2013 and the expected uplift of older layers of WMDW they caused that WMT waters were made available to flow out the Mediterranean Sea.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Hydrodynamic changes in Guadalquivir estuary (Spain) due to a hypothetical tidal marsh restoration project

Tidal marsh restoration will be an important management issue in Guadalquivir Estuary in the near future. The Guadalquivir Estuary has been anthropologically modified several times, the river is bounded by embankments to protect rice and cotton plantations from tidal inundation, the meanders have been transformed into straight sections, about 80 % of the original marsh surface has been lost and approximately one-fourth of the total surface of the estuary is now part of two protected areas, one of them is a UNESCO, MAB Biosphere Reserve. A hypothetical restoration of tidal marshes will benefits different species and functions based on location, elevation, adjacent habitats, degree of hydrodynamic connectivity and would reduce the present high levels of turbidity. The restoration projects should be accompanied by studies for a better understanding of the environmental changes expected. A Large-scale construction of tidal marsh will change tidal dynamics, modify the tidal inundation regime and the freshwater flow over the surrounding areas. A hydrodynamic model has been developed to study changes in the tidal propagation and the flow regime due to a hypothetical marsh restoration. The model has been calibrated and the output has been validated with in situ water elevation and good agreement between modelled and real measurements have been obtained. A sensitivity test changing the size, locations from the estuary mouth and depth has been carried out to highlights impacts over the tidal propagation, flow regime, salinity intrusion and the tidal inundation regime. Our preliminary results show that the tidal elevation and the current speed will be considerably affected in the estuary. The model open the possibility to study interactions among the tidal marsh restoration project, Seville port operations, fisheries and agricultural activities. The numerical model will be a powerful tool in restoration projects in a complex socio-ecological system.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Autoridad Portuaria de Sevill

Modeling changes in the tidal propagation and its implication for vessel navigation in Guadalquivir estuary ( Spain)

The Port of Seville is an inland harbour located in the Guadalquivir Estuary some 80 km from the river mouth and is the unique Spanish inland port. Vessel traffic in the estuary is a relevant economic activity and a suitable trade-off between vessel draught and safety to prevent ship aground is required and to optimize the port operability. The Guadalquivir is a mesotidal estuary with tidal range of 2-3 m, an important fraction of the minimum depth of the navigation waterway (presently 6.5m). Upstream navigation is favoured around high water as the tide progresses at 12 knots, which is comparable to the vessel speed, thus allowing greater vessel draughts. Oceanwards navigation of heavy vessels, on the contrary, is hampered by the tide because a low water is unavoidably met when heading downstream. A 3D, high resolution hydrodynamic model has been implemented in the whole estuary to study the tidal propagation. The model is forced by the oceanic tide at the mouth and freshwater discharges controlled by an upstream dam at the head. It has been satisfactorily validated and predicts tidal oscillations with high accuracy (less than 4 cm in amplitude and 20 min in phase everywhere in the estuary). Based on the model outputs of tidal heights and currents and using present-day estuary bathymetry, a MATLAB application has been developed for shipping planning (Vessel Traffic Decision Support System, VTDSS). The application allows the final users to test different traffic scheduling scenarios in order to assess the effects on navigational patterns and explore possible management and policy scenarios under sea level rise and changes in tidal propagation. A description of the model and an overview of the VTDSS are presented here; the effectiveness as a decision support tool is demonstrated via the simulated navigation time of several vessels.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Autoridad Portuaria de Sevill

3D hydrodynamic model as a tool for more efficient port management and operations.

Ports have been attempting to increase their competitiveness by enhancing their productivity and operate in a more environmentally friendly way. The Port of Seville is located in the Guadalquivir River in the south of Spain and it is the unique Spanish inland port. The estuary has generated and is still generating conflicts of interests. The access channel to the port is being periodically dredged, the natural course has been anthropologically modified several times, original salt marshes have been transformed to grow rice and approximately one-fourth of the total surface of the estuary is now part of two protected areas, one of them is a UNESCO_MAB Biosphere Reserve. Despite its socio-economic and environmental significance there is a surprising lack of scientific and technical information about the environmental interactions between the port activities and the Guadalquivir estuary stakeholders. A 3D hydrodynamic model has been developed to study the tidal regime, water circulation, temperature and salinity distributions, flooding areas and the sediment dynamics in the estuary. The model output has been validated with in situ current speed, direction, water elevation and also with temperature and salinity measurements. Good agreement between modeled and real measurements have been obtained. Our preliminary results show that the vessel traffic management could be improved by using the tidal elevations and currents calculated by the model in the whole estuary. The interactions among the port activities (mainly due of changes in the sediments dynamics), the watershed management and the saline intrusion evolution will be studied in detail. 3D Hydrodynamic Modelling provide spatially explicit information on the key variables governing the dynamics of estuarine areas. The numerical model is a powerful tool to effectively guide the management and operations of ports located in a complex socio-ecological systems.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Lagrangian particle tracking for the assessment of the flushing efficiency of harbor structures: the case of the Port of the Bay of Algeciras, Strait of Gibraltar

The Bay of Algeciras, an inlet of about 9 x 11 km located at the eastern margin of the Strait of Gibraltar, opens to the south, where the Atlantic jet leaves the Strait and starts spreading into the Alboran Sea. In its western side, the Bay hosts one of the main ports in Europe, neuralgic base of the major traffic load from Europe to Africa and from Europa to the rest of the oversea countries. The massive transport of liquid and solid bulk or bunkering activity, daily carried out in the Bay, combined with the harsh weather conditions that often lash the zone, give the ideal scenario for an incipient ecological disaster. This high environmental risk motivates the interest for a deeper understanding of the small scale dynamics of the Bay and the role played by the port structures in case of oil spill or other surface contaminations. A series of Lagrangian particles tracking (LPT) experiments were carried out to investigate the flushing patterns of the Bay and 8 different docks inside the local port, under a representative variety of external conditions, such as tide phase and strength, and winds. A 2D LPT algorithm has been adapted to fully exploit the outputs of a very high resolution (∼30m) three-domain-nested hydrodynamic model, with the aim of resolving the complex circulation within the structures of the harbor. Winds are a clearly dominant factor, with westerlies featuring e-folding times one order of magnitude lower than easterlies. Fortnightly tidal modulation presents a counter-intuitive effect, with spring tides that, despite promoting higher ventilation in a first instance, end up providing higher accumulation of particles inside the docks and higher e-folding times than neap tides. Tide phase affects the current direction at the entrance of the docks during the first few hours of simulation and its effect is progressively masked throughout the experiment. In the 45% of the experiments, a significant quantity of particles flow out to the Alboran Sea (55% in the whole Bay experiment), confirming the importance of the Bay on the exportation of pollutant/properties to the nearby basin.Universidad de Málaga. Campus de Excelencia Internacional del Mar, CEIMAR. Campus de Excelencia Internacional Andalucía Tec

Dos décadas de fenómenos de mesoescala a uno y otro lado del estrecho de Gibraltar

Mesoscale circulation patterns in the adjacent basins of the Strait of Gibraltar were investigated by means of altimetry data. In the Gulf of Cádiz, the pattern is relatively stable with two gyres: a cyclonic gyre close to the southern Iberian coast and an anticyclonic one on the western side of the Strait of Gibraltar. Both structures are located in the right place to convey the surface circulation towards the Strait and feed the Atlantic inflow. In the Alboran Sea, our results confirm that the western anticyclonic gyre is the most stable feature observed, while the eastern cyclonic gyre is subject to great variability. The mesoscale structures fluctuate at seasonal and interannual frequencies, but they may also undergo great changes in a very short time scale. A simple correlation analysis suggests that changes in the upstream Gulf of Cádiz basin may be transmitted through the Strait of Gibraltar to the Alboran Sea with a time delay of around one week.Los patrones de circulación de mesoescala en las cuencas adyacentes al Estrecho de Gibraltar han sido investigados por medio de datos de altimetría. En el Golfo de Cádiz, el patrón se mantiene relativamente estable con dos giros, uno ciclónico cerca de la costa sur Ibérica y otro anticiclónico situado frente de la boca occidental del Estrecho de Gibraltar. Ambas estructuras se encuentran en el lugar adecuado para dirigir la circulación superficial hacia el Estrecho, alimentando el flujo de entrada Atlántico. En el mar de Alborán, nuestros resultados confirman que el giro anticiclónico occidental es la característica más estable, mientras el giro oriental aparece como una característica sujeta a una gran variabilidad. Las estructuras de mesoescala fluctúan en las frecuencias estacionales e interanuales, pero también pueden sufrir grandes cambios en escalas de tiempo cortas. Un análisis de correlación simple sugiere que los cambios en la corriente de la cuenca del Golfo de Cádiz se pueden transmitir a través del Estrecho de Gibraltar hasta el mar de Alborán, con un retraso de una semana

The influence of tides on the exchange through the Strait of Gibraltar and on properties of the Mediterranean Sea

A. Ishlinsky Institute for Problems in Mechanics, Rusi

Mediterranean outflow and its link with upstream conditions in Alboran Sea

The Western Alboran Gyre (WAG) at the eastern entrance of the Strait of Gibraltar can influence the Mediterranean outflow (MOW) by favoring or hampering the flow of Levantine and Western Mediterranean (LIW and WMDW) waters, the main constituents of the MOW. Observations collected at Camarinal sill in the Strait and AVISO data are used to investigate this issue.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Heat fluxes between the Guadalquivir river and the Gulf of Cádiz Continental Shelf

An 18-year time series of daily sea surface temperature of Gulf of Cadiz and an 18-month time series of temperature collected in the vicinity of the Guadalquivir estuary mouth have been analyzed to investigate the heat exchange between the estuary and the adjacent continental shelf. The first time identifies a continental shelf area where seasonal thermal oscillation signal (amplitudes and phase) changes abruptly. In order to explain this anomaly, the second data set allows a description of thermal fluctuations in a wide range of frequencies and an estimation of the upstream heat budget of the Guadalquivir estuary. Results show that high frequency thermal signal, diurnal and semidiurnal, and water flux signal through Guadalquivir mouth, mainly semidiurnal, apparently interact randomly to give a small exchange of thermal energy at high frequency. There is no trace, at the estuary's mouth, of daily heat exchanges with intertidal mudflats probably because it tends to cancel on daily time scales. Results also show that fluctuations of estimated air-sea fluxes force fluctuations of temperature in a quite homogeneous estuarine, with a delay of 20 days. The along-channel thermal energy gradient reaches magnitudes of 300-400 J m-4 near the mouth during the summer and winter and drives the estuary-shelf exchange of thermal energy at seasonal scale. Particularly, the thermal heat imported by the estuary from the shelf area during late fall-winter-early spring of 2008/2009 is balanced by the thermal heat that the estuary exports to the shelf area during late spring-summer of 2008. In summary, Guadalquivir river removes/imports excess of thermal energy towards/from the continental shelf seasonally, as a mechanism to accommodate excess of heat from one side respect to the other side.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Autoridad Portuaria de Sevilla (APS