Alfredo Izquierdo

Simulation of the semidiurnal tides in the Strait of Gibraltar

The M2 and S2 surface tides in the Strait of Gibraltar are simulated using a two-dimensional, nonlinear, boundary-fitted coordinate model with a nominal resolution of ∼0.5 km. Good agreement is achieved with tide gauge and bottom pressure observations, as well as with current measurements made during the Gibraltar Experiment. The cotidal charts and the maps of tidal current ellipse parameters, which have been constructed on the basis of the model results, reproduce all of the known features of the spatial structure of the M2 and S2 tidal waves. These results also show that a ∼90° phase difference between tidal velocity and elevation is detected in much of the Strait of Gibraltar, thus suggesting a small mean tidal energy flux through the strait. The model results give evidence of the general direction for the M2 and S2 net tidal energy fluxes to the west. This finding is consistent with an observed southwestern tidal phase propagation and remains qualitatively unchanged when varying the straits geometry as well as boundary and astronomical forcings.

Regionally coupled atmosphere‐ocean‐sea ice‐marine biogeochemistry model ROM: 1. Description and validation

The general circulation models used to simulate global climate typically feature resolution too coarse to reproduce many smaller-scale processes, which are crucial to determining the regional responses to climate change. A novel approach to downscale climate change scenarios is presented which includes the interactions between the North Atlantic Ocean and the European shelves as well as their impact on the North Atlantic and European climate. The goal of this paper is to introduce the global ocean-regional atmosphere coupling concept and to show the potential benefits of this model system to simulate present-day climate. A global ocean-sea ice-marine biogeochemistry model (MPIOM/HAMOCC) with regionally high horizontal resolution is coupled to an atmospheric regional model (REMO) and global terrestrial hydrology model (HD) via the OASIS coupler. Moreover, results obtained with ROM using NCEP/NCAR reanalysis and ECHAM5/MPIOM CMIP3 historical simulations as boundary conditions are presented and discussed for the North Atlantic and North European region. The validation of all the model components, i.e., ocean, atmosphere, terrestrial hydrology, and ocean biogeochemistry is performed and discussed. The careful and detailed validation of ROM provides evidence that the proposed model system improves the simulation of many aspects of the regional climate, remarkably the ocean, even though some biases persist in other model components, thus leaving potential for future improvement. We conclude that ROM is a powerful tool to estimate possible impacts of climate change on the regional scale.

Vertical structure of the semidiurnal tidal currents at Camarinal Sill, the strait of Gibraltar

Abstract The dynamical mode decomposition (DMD) technique is applied to the data of currentmeter and CTD measurements taken during the 1985–1986 Gibraltar Experiment and the 1989 survey so as to clarify features of the vertical structure of the M2 and S2 tidal currents at the Camarinal Sill. It is shown that in conformity with the inference made on the basis of the empirical orthogonal function (EOF) decomposition technique, these currents are mainly due to the M2 and S2 barotropic modes. At the same time the first three baroclinic modes are responsible not only for the vertical variability of the tidal currents but also for the velocity and density amplitude variances at semidiurnal frequencies. Certain quantitative discrepancies between the values of barotropic tidal current characteristics as deduced from DMD and EOF decomposition techniques are revealed. In order to eliminate these, new currentmeter data are required with a finer vertical resolution than those which are available.

The South Atlantic Anticyclone as a key player for the representation of the tropical Atlantic climate in coupled climate models

The key role of the South Atlantic Anticyclone (SAA) on the seasonal cycle of the tropical Atlantic is investigated with a regionally coupled atmosphere–ocean model for two different coupled domains. Both domains include the equatorial Atlantic and a large portion of the northern tropical Atlantic, but one extends southward, and the other northwestward. The SAA is simulated as internal model variability in the former, and is prescribed as external forcing in the latter. In the first case, the model shows significant warm biases in sea surface temperature (SST) in the Angola-Benguela front zone. If the SAA is externally prescribed, these biases are substantially reduced. The biases are both of oceanic and atmospheric origin, and are influenced by ocean–atmosphere interactions in coupled runs. The strong SST austral summer biases are associated with a weaker SAA, which weakens the winds over the southeastern tropical Atlantic, deepens the thermocline and prevents the local coastal upwelling of colder water. The biases in the basins interior in this season could be related to the advection and eddy transport of the coastal warm anomalies. In winter, the deeper thermocline and atmospheric fluxes are probably the main biases sources. Biases in incoming solar radiation and thus cloudiness seem to be a secondary effect only observed in austral winter. We conclude that the external prescription of the SAA south of 20°S improves the simulation of the seasonal cycle over the tropical Atlantic, revealing the fundamental role of this anticyclone in shaping the climate over this region.

Sea Level Variations in the Western Mediterranean Studied by a Numerical Tidal Model of the Strait of Gibraltar

Aspects of the sea level changes in the western Mediterranean Sea are investigated using a numerical tidal model of the Strait of Gibraltar. As a prerequisite, the performance of this model, that is, a two-dimensional, nonlinear, two-layer, boundary-fitted coordinate numerical model based on the hydrostatic approximation on an f plane, is assessed in the simulation of mean and tidal circulation of the Strait of Gibraltar. The model is forced by imposing mean interface and surface displacements as well as M2, S2, O1, and K1 tidal components along the Atlantic and Mediterranean model open boundaries. Model results are compared with observations and with results obtained from a tidal inverse model for the eastern entrance of the Strait of Gibraltar. In general, good agreement is found. A sensitivity study performed by varying different model parameters shows that the model behaves reasonably well in the simulation of the averaged circulation. The model is then used to investigate the climatological sensitivity of the simulated dynamics in the Strait of Gibraltar to changes in the density difference between Atlantic and Mediterranean waters. For this purpose, given a certain density difference between Atlantic and Mediterranean waters, the authors iteratively searched for that sea level drop between the Atlantic and the Mediterranean that fulfills the mass balance of the Mediterranean. It is found that an increase of the density difference leads to an increase of the exchange flow and to an increase of the sea level drop between the two basins. A trend in the sea level drop of O(1 cm yr21), such as the one observed between 1994 and 1997, is explained by the model as the result of a trend of O(10 24 yr21) in the relative density difference between the Mediterranean and Atlantic waters. The observed north‐south asymmetry in this trend is also captured by the model, and it is found to arise from changes in the along-strait velocity. Results suggest that the dynamics within the Strait of Gibraltar cannot be neglected when sea level changes in the western Mediterranean basin are investigated.

Weak wave–tide interaction formulation and its application to Cádiz bay

Abstract Using a single-point, one-equation ( k – l ) model for an oscillatory turbulent bottom boundary layer (BBL) above a hydrodynamically rough bottom and varying the external determining parameters over a wide range, we show that nonlinear wave/low-frequency current interaction effects are smaller, the greater are the ratio of near-bottom wave orbital velocity amplitude to friction-free, low-frequency current velocity amplitude and the ratio between frequencies of wave and low-frequency components of motion. Specifically, in shallow waters the bottom stress oscillations with wave and tidal frequencies are, with fair accuracy, weakly correlated, thereby suggesting that wave-tide interaction is substantially weak interaction. A new weak wave–tide interaction formulation is proposed. It involves a relationship for the drag coefficient in a wave-affected tidal flow and the surface Rossby number dependences for the scaled wave and tidal friction velocity amplitudes inferred from the resistance law for an oscillatory turbulent BBL over a hydrodynamically rough surface. This formulation is implemented within a 2D nonlinear, finite-difference, high-resolution, hydrodynamic model and the modified model is applied to quantify the wave-induced changes in the tidal dynamics and energetics of Cadiz Bay. The model results reveal one unexpected feature in the fields of maximum tidal velocity and mean tidal energy flux. Namely, wave–tide interaction responsible for enhancing the mean bottom stress throughout the bay tends to increase the maximum tidal velocities and the mean tidal energy fluxes at deeper depths and to reduce them at shallower depths. The reason for appearing this feature is an overall amplification of the mean tidal energy transport into the bay from Gulf of Cadiz. Based on the sensitivity study to varying wave parameters, the wave-induced seasonal variability in the M 2 tidal characteristics is found to be not pronounced in Cadiz Bay. This, however, does not rule out a clearly defined manifestation of such a variability in other shallow basins and/or in other tidal frequency bands. Special attention is given to identify the regions of potential suspended sediment transport and their wave-induced changes.

TIDES AND TIDAL ENERGETICS OF THE STRAIT OF GIBRALTAR : A MODELLING APPROACH

Abstract The main semidiurnal (M2 and S2) and diurnal (O1 and K1) tidal waves in the Strait of Gibraltar are simulated by employing a 2D high-resolution, non-linear, boundary-fitted coordinate model. Agreement between observational evidence and model results is good for the M2 and S2 tidal waves and satisfactory for the O1 and K1 tidal waves. The model reproduces all the known features of the spatial structure of these waves and predicts some new ones, namely, the general direction of the M2 and S2 mean tidal energy fluxes to the west, with a clear increase at the Camarinal Sill; the O1 amphidrome with anticlockwise rotation of cotidal lines in the Tarifa Narrows; and small-scale eddies in the M2, S2 and O1 mean tidal energy flux fields in the vicinity of the western and eastern boundaries of the Strait.

Weak wave/tide interaction in suspended sediment-stratified flow: a case study

Abstract The formulation of weak wind-wave/low-frequency current interaction is extended to the case of suspended sediment-stratified flow. The influence of suspended sediment stratification on flow dynamics is described in terms of a sediment stratification parameter defined as von Karmans constant times a depth-independent function of the relative friction velocity and the relative settling velocity of suspended particles that is specified by a solution for the problem on the vertical structure of the suspended sediment-stratified near-bottom logarithmic layer. This ‘extended’ formulation is inserted in a two-dimensional non-linear, finite-difference, high-resolution hydrodynamic model and the modified model is applied to clarify the roles of wind-wave/tide interaction and suspended sediment stratification—individually and in combination—in the formation of the M4 and M6 overtides in Cadiz Bay. It is shown that the predictions for the M4 and M6 overtides have much in common and much in contrast with the M2 tide. For the M2 tide the influence of suspended sediment stratification shows up most vividly in the spatial variability of the tidal characteristics, but is not evident in changes in the M4 and M6 overtides. On the other hand, the influence of wave-induced changes on the M2 tidal amplitude and phase is only of minor importance, but for the M4 and M6 overtides these changes are quite significant. When taken together, the effects of the two factors under investigation are very nearly balanced. This, however, does not mean that the conventional assumption of ignoring these factors is valid in shallow-water dynamics. Simply, that their resulting effect vanishes.

Weak wind-wave/tide interaction over a moveable bottom: results of numerical experiments in Cádiz Bay

Abstract The formulation of weak wind-wave/low-frequency current interaction is extended to the moveable rough bottom case using the bottom roughness predictors of Nielsen (Coastal Eng. 7 (1983) 233) and Tolman (J. Phys. Oceanogr. 24 (1994) 994). This “extended” formulation is then implemented in a 2D non-linear, high-resolution hydrodynamic model and the modified model is applied to study the changes in the tidal dynamics of Cadiz Bay due to wind-wave/tide interaction and bottom mobility. It is shown that an agreement between the observed and predicted tidal elevation amplitudes and phases at the tide-gauge and bottom-pressure measurement locations within the bay tend to be improved if both of these factors are accounted for. Distinctions between the solutions derived when employing Nielsens and Tolmans bottom roughness predictors are considerable though not so much as might be expected. The sensitivity of the solution to the mean sediment grain size turns out to be either moderate or low depending on which of the above-mentioned bottom roughness predictors is adopted and much less than the sensitivity to the tidal reference bottom roughness length. Accordingly, if the wave and tidal reference bottom roughness lengths are set equal to each other, the changes in the fields of tidal characteristics become unreasonable, thereby eliminating the possibility of prescribing a single reference bottom roughness length.

Study of the influence of physical, chemical and biological conditions that influence the deterioration and protection of Underwater Cultural Heritage

Two wrecks related to the Battle of Trafalgar (1805) were studied. Following the guidelines of the UNESCO-2001 Convention for the Protection of the Underwater Cultural Heritage, a holistic and interdisciplinary approach based on the development of four of the thirty-six Rules of this international agreement was applied. A non-destructive survey technique was developed to obtain information from the scattered cannons and anchors without altering their condition (Rule 4). The work performed provided information about the origin of both wrecks, the Fougueux and the Bucentaure, two ships of the line of the French Navy, and allowed to characterize the state of conservation at each site without jeopardizing their future conservation in the marine environment. In addition, measurements of the main physical, chemical and biological variables allowed correlating the conservation status at each site with the marine environmental conditions (Rule 15). Thus, in Fougueux shipwreck large iron objects are corroding at a higher rate (between 0.180 and 0.246mmpy) due to high sediment remobilization and transport induced by waves at this site, causing damage by direct mechanical effect on metallic material and by removing the layer of corrosion products developed on the artefacts. Meanwhile artillery on Bucentaure site, covered with thick layers of biological concretion, is well preserved, with lower corrosion rates (0.073 to 0.126mmpy), and archaeological information is guaranteed. Finally, the effectiveness of the cathodic protection as a temporary measure for in situ conservation (Rule 1) was evaluated on a cannon. The use of a sacrificial anode after 9months reduced the average corrosion rate (from 0.103 to 0.064mmpy) and the percent of corrosion rate in 37.9%. These results are very useful for developing a decision making system of the Site Management Program, based on predictive models of artefacts permanence and risk factors in the marine environment (Rule 25).