Modeling circulation patterns induced by spatial cross-shore wind variability in a small-size coastal embayment

This contribution shows the importance of the cross-shore spatial wind variability in the water circulation in a small-sized micro-tidal bay. The hydrodynamic wind response at Alfacs Bay (Ebro River delta, NW Mediterranean Sea) is investigated with a numerical model (ROMS) supported by in situ observations. The wind variability observed in meteorological measurements is characterized with meteorological model (WRF) outputs. From the hydrodynamic simulations of the bay, the water circulation response is affected by the cross-shore wind variability, leading to water current structures not observed in the homogeneous-wind case. If the wind heterogeneity response is considered, the water exchange in the longitudinal direction increases significantly, reducing the water exchange time by around 20%. Wind resolutions half the size of the bay (in our case around 9 km) inhibit cross-shore wind variability, which significantly affects the resultant circulation pattern. The characteristic response is also investigated using idealized test cases. These results show how the wind curl contributes to the hydrodynamic response in shallow areas and promotes the exchange between the bay and the open sea. Negative wind curl is related to the formation of an anti-cyclonic gyre at the bay's mouth. Our results highlight the importance of considering appropriate wind resolution even in small-scale domains (such as bays or harbors) to characterize the hydrodynamics, with relevant implications in the water exchange time and the consequent water quality and ecological parameters.Peer ReviewedPostprint (author's final draft

Subtidal circulation in a microtidal Mediterranean bay

We examine the role of different forcings on the subtidal circulation in a microtidal bay with freshwater inputs in the NW Mediterranean Sea: Alfacs Bay. Observations of subtidal flow in summer 2013 and winter 2014 reveal a two-layered, vertically sheared circulation. During the summer, there is a significant positive correlation between surface currents and winds along the main axis of the bay, while a negative correlation is observed between wind and the bottom layers. During the winter, the cross-shore response is correlated with the most energetic winds, showing a two-layered vertical structure inside the bay and a nearly depth-independent water motion caused by high wind speeds at the bay mouth. The vertical structure of the velocities, as determined through empirical orthogonal function analysis, confirms that surface layers are affected by winds and bottom currents correlated negatively with winds as a response of the wind set-up. Seasonal mean circulation reveals gravitational exchange at the bay mouth during the summer. However, mean circulation is unclear in the inner bay and close to the drainage channels. Observed flow patterns are supported by modelling results that confirm the persistence of averaged current in the low-frequency dynamics. Re-circulation areas in the inner bay indicate the rich spatial variability in flow at low-frequency time scales.Peer ReviewedPostprint (published version

Modelling assessment of the tidal stream resource in the Ria of Ferrol (NW Spain) using a year-long simulation

The availability of tidal stream energy in the Ría of Ferrol (NW Spain) has been assessed using a long term hydrodynamic simulation (351 days). A priori, a strait in the central part of the estuary seems a promising site for tidal energy tapping, but the results show that barotropic currents rarely exceed 0.9 m/s during spring tides, with a maximum peak power density of 0.45 kW/m2 estimated at a spring tide mid-ebb. The maximum annual energy density is estimated at 415 kWh/m2, significantly lower than at other nearby estuaries. A comparison of the annual resource estimates shows that differences of up to 35% can be introduced depending on whether a simulation of one tidal cycle, one lunar month, or a full year is used for the calculations. This proves that usual tidal resource estimations, based on a single tidal cycle, can significantly misestimate the tidal energy potential of a site.Peer ReviewedPostprint (author's final draft

Characterization of bottom sediment resuspension events observed in a micro-tidal bay

In this study we investigate the variability in near-bottom turbidity in Alfacs Bay (in the northwestern Mediterranean Sea). The bay is characterized by a micro-tidal environment and seiching activity that may lead to flow velocities of more than 50 cm s−1. A set of current meters and optical sensors were mounted near the sea bottom to acquire synchronous hydrodynamic and optical information from the water column. The time-series observations showed an evident relation between seiche activity and sediment resuspension events. The observations of turbidity peaks are consistent with the node–antinode location of the fundamental and first resonance periods of the bay. The implementation of a coupled wave–current numerical model shows strong spatial variability of the potential resuspension locations. Strong wind events are also a mechanism responsible for the resuspension of fine sediment within the bay. This is confirmed by suspended sediment concentration maps derived from Sentinel-2 satellite imagery. We suggest that the sequence of resuspension events plays an important role in the suspended sediment concentration, meaning that the occurrence of sediment resuspension events may increase the suspended sediment in subsequent events. The suspended sediment events likely affect the ecological status of the bay and the sedimentary process over a long-term period.Peer ReviewedPostprint (published version

Use of a hydrodynamic model for the management of water renovation in a coastal system

In this contribution we investigate the hydrodynamic response in Alfacs Bay (Ebro Delta, NW Mediterranean Sea) to different anthropogenic modifications in freshwater flows and inner bay–open sea connections. The fresh water coming from rice field irrigation contains nutrients and pesticides and therefore affects in multiple ways the productivity and water quality of the bay. The application of a nested oceanographic circulation modelling suite within the bay provides objective information to solve water quality problems that are becoming more acute due to temperature and phytoplankton concentration peaks during the summer period when seawater may exceed 28 °C, leading to high rates of mussel mortality and therefore a significant impact on the local economy. The effects of different management “solutions” (like a connection channel between the inner bay and open sea) are hydrodynamically modelled in order to diminish residence times (e-flushing time) and water temperatures. The modelling system, based on the Regional Ocean Modeling System (ROMS), consists of a set of nested domains using data from CMEMS-IBI for the initial and open boundary conditions (coarser domain). One full year (2014) of simulation is used to validate the results, showing low errors with sea surface temperature (SST) and good agreement with surface currents. Finally, a set of twin numerical experiments during the summer period (when the water temperature reaches 28 °C) is used to analyse the effects of proposed nature-based interventions. Although these actions modify water temperature in the water column, the decrease in SST is not enough to avoid high temperatures during some days and prevent eventual mussel mortality during summer in the shallowest regions. However, the proposed management actions reveal their effectiveness in diminishing water residence times along the entire bay, thus preventing the inner areas from having poor water renewal and the corresponding ecological problems.Peer ReviewedPostprint (published version

Ocean reanalyses

Ocean reanalyses are becoming increasingly available and useful, and may eventually attract a similar applications base as atmospheric reanalyses. Here we look at how they are being evaluated against both assimilated and independent data, and emphasise that circulation and transport estimates are critical. The Ocean Reanalysis Intercomparison project, ORA-IP, has been comparing many products for consistency on a regional and global basis, including ocean heat content, air-sea fluxes, and recently polar properties including sea ice. The Atlantic meridional overturning circulation as measured by the RAPID array at 26N, is now a challenging new target for simulation. This chapter shows that reanalyses may represent interior ocean basin circulations well (better than free-running models) but they still fail to consistently constrain boundary currents, where most meridional heat transport takes place. There is new work ongoing to try to physically interpret observation increments in reanalysis products, and to look at how to best develop long period reanalysis in earlier years when ocean observations were scarce. Finally, we look at new coupled ocean-atmosphere reanalysis that, by always maintaining a coupled ocean-atmospheric boundary layer, may lead to reduced assimilation increments and air-sea fluxes across domains

Hydrodynamic response of a shallow and microtidal estuary : Alfacs Bay. From high to low frequency processes

Using both observations and modelling tools, it is investigated the main hydrodynamic response mechanisms of a microtidal and shallow bay in the Mediterranean Sea: Alfacs Bay. Main objectives are considered using differenttim e-scales approximations: from high frequency processes (corresponding periods of 1 to 3h), through tidal and daily wind influence, to low frequency and average response (days and weeks ). Results revealed a clear circulation dominated by seiches on high frequency scales, demonstrating the effects on water column stratification. Moreover, spatial wind variability becomes a relevant mechanism influencing the hydrodynamic response under energetic wind events. The response at low frequency scales is determined by the combined effects of winds and gravitational circulation mainly due to the fres hwater inputs. Both Bay's geometry -shape and shallow depths- and stratification are found to be much relevant in all the hydrodynamics mechanisms analysed. The results allow the better understanding of main hydrodynamics patterns of an area with strong anthropogenic pressure and notably social and economic role in the region, and being applicable to similar domains. As such, these bodies of water are exposed to several environmental and management problems and need to be considered for pollution protection issues .A partir de l’ús combinat d’obs ervacions i eines de modelat numèric s'han investigat els principals mecanismes de resposta hidrodinàmica en una badia micromareal i poc profunda de la mar Mediterrània: la badia dels Alfacs. Els principals objectius van des de l’anàlisi de processos fís ics d’alta freqüència (períodes associats de 1 a 3h), passant per l’anàlis i de les marees i els vents, fins l’anàlisi del comportament hidrodinàmic de baixa freqüència. Els resultats mostren un clar domini de les seiches en les corrents d’alta freqüència, demostrant la influencia d’aquest fenomen sobre l’estratificació en la columna d’aigua. La variabilitat es pacial dels vents es devé un mecanisme rellevant alhora de considerar la resposta de la badia als esdeveniments més energètics. Finalment, la resposta en baixa freqüència es veu fortament condicionada per la interacció entre els vents i la circulació gravitacional induïda per les aportacions d'aigua dolça. Tant la geometria de la badia (forma i escassa profunditat) com l’estratificació tenen una importància cabdal en els mecanismes hidrodinàmics descrits. Els resultats obtinguts permeten conèixer millor la hidrodinàmica en una zona de forta pressió antropogènica i amb una rellevància social i econòmica important, sent les conclusions extrapolables a dominis similars. Així mateix, aquest tipus d’entorn es troba contínuament exposat a impactes ambientals i problemes de gestió, i el correcte coneixement de les seves característiques resulta indispensable per a les corresponents polítiques de protecció i conservaci

Modeling circulation patterns induced by spatial cross-shore wind variability in a small-size coastal embayment

This contribution shows the importance of the cross-shore spatial wind variability in the water circulation in a small-sized micro-tidal bay. The hydrodynamic wind response at Alfacs Bay (Ebro River delta, NW Mediterranean Sea) is investigated with a numerical model (ROMS) supported by in situ observations. The wind variability observed in meteorological measurements is characterized with meteorological model (WRF) outputs. From the hydrodynamic simulations of the bay, the water circulation response is affected by the cross-shore wind variability, leading to water current structures not observed in the homogeneous-wind case. If the wind heterogeneity response is considered, the water exchange in the longitudinal direction increases significantly, reducing the water exchange time by around 20%. Wind resolutions half the size of the bay (in our case around 9 km) inhibit cross-shore wind variability, which significantly affects the resultant circulation pattern. The characteristic response is also investigated using idealized test cases. These results show how the wind curl contributes to the hydrodynamic response in shallow areas and promotes the exchange between the bay and the open sea. Negative wind curl is related to the formation of an anti-cyclonic gyre at the bay's mouth. Our results highlight the importance of considering appropriate wind resolution even in small-scale domains (such as bays or harbors) to characterize the hydrodynamics, with relevant implications in the water exchange time and the consequent water quality and ecological parameters.Peer Reviewe

Hydrodynamic response in a microtidal and shallow bay under energetic wind and seiche episodes

In this contribution we investigate the hydrodynamic response in a micro-tidal and shallow semi-enclosed domain. We chose a set of observations which include currents, hydrography and meteorological data obtained in Alfacs Bay (NW Mediterranean Sea). Short-term response to energetic wind events was found in the hydrography and water velocity observations, sometimes inverting the estuarine circulation or developing one-layered flow. In comparison to previous investigations in Alfacs Bay, we observed that water current variability, and also maximum velocities, were directly related to the development of surface standing waves (i.e. seiches). Mixing mechanisms versus buoyancy sources are studied through potential energy anomaly equation, proving the leading freshwater contribution to stratification, enhanced by heat fluxes in summer. On the other hand, mixing is directly related to winds, mainly in winter and early spring when both buoyancy forces are lower. We also study turbulent bottom mixing by seiches through observations, dimensionless relations and numerical modeling. Seiche induced mixing is suggested as an eventual mechanism that may break the stratification within the Bay under special circumstances.Peer Reviewe

Tidal transformation and resonance in a short, microtidal Mediterranean estuary (Alfacs Bay in Ebre delta)

Tidal and subtidal waves are analyzed with sea-level data and numerical modeling in a short and micro-tidal embayment, Alfacs Bay (NW Mediterranean Sea). Data analysis exhibits tidal wave amplification and seiching (characteristic period of 3.5h) along the bay. Numerical results show an eight-fold increase in quarter-wave resonant wave amplitudes from the mouth to the head of the bay. This amplification follows the classical description of a standing wave. Moreover, resonant wave velocities measured and computed at the bay mouth (node location) are about one order of magnitude higher than tidal currents. Analysis of astronomic tidal propagation in the bay reveals similar behavior for diurnal and semidiurnal constituents. Tidal waves amplify along the bay by 3% for diurnal and 10% for semidiurnal constituents. Numerical simulations conducted with different domains indicate that geometric effects dominate over frictional influences in causing the wave behavior. This behavior is consistent with the existence of a quasi-steady standing wave within the bay.Peer Reviewe