Begoña Tejedor

A note on sea-breeze-induced seasonal variability in the K1 tidal constants in Cádiz Bay, Spain

Abstract The response of Cadiz Bay to sea-breeze wind stress and tidal boundary forcing—individually and in combination—is studied using a 2D depth-averaged, non-linear, high-resolution hydrodynamic model. Linear superposition of the solution for the K 1 and S 1 constituents, like the solution obtained with an allowance for both the input functions together, is shown to give rise to a modulation of the K1 tidal dynamics. It is precisely this modulation which is responsible for the observed seasonal variations in the K 1 tidal constants in Cadiz Bay.

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.

Coastal Altimetry Products in the Strait of Gibraltar

This paper analyzes the availability and accuracy of coastal altimetry sea level products in the Strait of Gibraltar. All possible repeats of two sections of the Envisat and AltiKa ground-tracks were used in the eastern and western portions of the strait. For Envisat, along-track sea level anomalies (SLAs) at 18-Hz posting rate were computed using ranges from two sources, namely, the official Sensor Geophysical Data Records (SGDRs) and the outputs of a coastal waveform retracker, the Adaptive Leading Edge Subwaveform (ALES) retracker; in addition, SLAs at 1 Hz were obtained from the Centre for Topographic studies of the Ocean and Hydrosphere (CTOH). For AltiKa, along-track SLA at 40 Hz was also computed both from SGDR and ALES ranges. The sea state bias correction was recomputed for the ALES-retracked Envisat SLA. The quality of these altimeter products was validated using two tide gauges located on the southern coast of Spain. For Envisat, the availability of data close to the coast depends crucially on the strategy followed for data screening. Most of the rejected data were due to the radar instrument operating in a low-precision nonocean mode. We observed an improvement of about 20% in the accuracy of the Envisat SLAs from ALES compared to the standard (SGDR) and the reprocessed CTOH data sets. AltiKa shows higher accuracy, with no significant differences between SGDR and ALES. The use of products from both missions allows longer times series, leading to a better understanding of the hydrodynamic processes in the study area.

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.

Reversal in Longshore Sediment Transport without Variations in Wave Power Direction

Abstract The mouth of the Guadiaro River (SW Spain), oriented in a NE-SW direction, is often closed, resulting in environmental, social, and economic problems. An analysis of the factors influencing this phenomenon was performed using the Coastal Modeling System, a numerical model based on the mild slope equation (waves, currents, and sediment transport), and a coastal evolution study was performed comparing aerial photographs to the monitoring campaigns carried out in 2005 and 2006 (topographic surveys, current meters, tidal gauges, and water quality parameters). Global transport at the Spanish south Mediterranean coast occurs in a southwestern direction. However, the spit that closes the Guadiaro River grows in the opposite direction. The points and conditions where transport direction changes occur along the coastline were identified. The results, obtained from 200 different propagations, were consistent with field data, showing that northeastern littoral transport during easterly waves was sufficient to block the river mouth. Furthermore, significant differences in sediment transport patterns were found despite the existence of a fixed wave power direction. Through this research we also learned that use of wave propagation numerical models by nonexperts is prone to error, as small variations in input parameters can induce dramatically different results that may be contradictory in certain bathymetric configurations.

Estimation of the seasonal sea level variations in the Gulf of Cadiz (SW Iberian Peninsula) from in-situ measurements, satellite altimetry and numerical models

Time series (1997-2008) of near-shore altimetry data and in-situ tide gauge records have been analyzed to investigate the seasonal variability of sea level along the Gulf of Cadiz. A high level of agreement was obtained between altimeter and in-situ observations, indicating that altimeter data can be a valuable tool to study the sea level seasonal cycle near the coast. Harmonic analysis showed that more than 95% of the average seasonal cycle is explained by the annual and, to a lesser extent, semiannual components. The average seasonal cycle of sea level anomalies is very similar at the four coastal stations, with minimum values during winter and maximum during autumn. Atmospheric pressure accounts for 20-38% of the sea level variability, its effect diminishing toward the Strait of Gibraltar. The steric contribution is notable at the westernmost stations (32-37%) and it also decreases eastward (9-17%). River discharge explains about 15% of the sea level variability, indicating that its effects should be taken into account. The contribution of direct atmospheric forcing for a section of the sea level time series (1997-2001) has also been explored using the output of a barotropic oceanographic model (HIPOCAS project) forced with wind and atmospheric pressure, revealing that the contribution of wind is generally small (6-12%) at seasonal time scales. Small but significant correlations are found between the time series of winter-autumn mean sea level and the winter-autumn North Atlantic Oscillation (NAO) indices. Analysis show that the effect of NAO is mainly reflected on atmospheric pressure, wind and river runoff.

From ENVISAT RA-2 to CRYOSAT SIRAL: validation of altimeter products near the coast (the ALCOVA Project)

Satellite altimetry has proven to be a useful tool to study oceanic processes in the deep ocean; however, its use is still limited in shallow waters near the coast where two main issues still need a more detailed analysis. On one side, the local characteristics of each coastal region imply that certain corrections applied to the altimetry measurements need to be reanalysed. On the other side, the radar signal retracking algorithms need to be improved because the waveforms do not follow the Browns model, which is designed for deep waters. The ESA mission Envisat was launched in March 2002 with a dual-frequency radar altimetry (RA-2). The satellite was operative until the end of the mission in May 2012. The ESA mission Cryosat-2 was launched in April 2010 being still in operation. The radar instrument on-board Cryosat-2 improves the capabilities of previous pulse-limited altimeters, such as Envisat RA-2. The Spanish-funded ALCOVA project aims at analyzing and improving the altimetry measurements obtained from these two altimetry missions. Regarding the RA-2 data a new prototype retracker -ALES- has been developed under the frame of the ESA-DUE eSurge project. Two pilot regions are proposed, namely, the Gulf of Cadiz and the Strait of Gibraltar in the Southwestern Iberian Peninsula. Cryosat-2 data (in SAR mode), the newly corrected RA-2 data (based on ALES) and the standard RA- 2 product (based on Browns model) are being validated with available in-situ data (sea level height) to ensure their correct performance in the selected coastal areas.

Spatial structure of the sea level seasonal cycle within the Gulf of Cadiz

Sea level anomaly maps from altimeter (AVISO) were retrieved for the Gulf of Cadiz (GoC) for the period 1997-2008, along with maps of Dynamic Atmospheric Correction (DAC), atmospheric pressure at sea level and satellite Sea Surface Temperature (SST). Data were averaged in time to obtain maps of monthly mean time series in order to analyze the seasonal variability of sea level and its main forcing agents along the GoC. Moreover, a very high resolution climatology for the region was combined with the SST maps to explore the steric contribution with enough spatial resolution near the coast. The AVISO sea level anomaly monthly maps were initially de-corrected using the DAC product and then corrected using the inverted barometer method. Atmospheric pressure explained more than 55% of the sea level variance offshore and between 35-45% within the continental shelf. The amplitude of the pressureadjusted sea level semiannual signal was considerably reduced, confirming its meteorological origin. The steric contribution on the pressure-adjusted sea level was addressed by considering local, open ocean, basin-wide and continental shelf steric effects. The open ocean contribution explained the highest percentage of variance all over the basin with the exception of the western shelf, where the best results were obtained with the local contribution. After correcting for the best steric contribution, the amplitude of the remaining offshore annual signal was negligible (0.5-1.0 ± 1 cm). As for the continental shelves, 2- 3 cm (± 0.5-1 cm) of the annual signal remained unexplained, probably due to local effects related with the shelves dynamics.