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 M 2 and S 2 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 M 2 and S 2 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. © 2000 Ifremer/CNRS/IRD/É ditions scientifiques et médicales Elsevier SAS barotropic and baroclinic modes / tidal current / dynamical mode decomposition / Strait of Gibraltar Résumé-Structure verticale des courants de marée semi-diurne sur le seuil du détroit de Gibraltar. La structure verticale des courants de marée M 2 et S 2 sur le seuil Camarinal a été étudiée en appliquant la technique de décomposition en mode dynamique (DMD) aux données de courantométrie et d'hydrologie de l'expérience Gibraltar 1985-1986 et à celles de la campagne 1989. Les résultats, en accord avec l'analyse par fonctions empiriques orthogonales (EOF), indiquent que les courants sont engendrés par les modes barotropes M 2 et S 2. Les trois premiers modes baroclines sont responsables de la variabilité verticale des courants de marée et des variances d'amplitude de la vitesse et de la densité aux fréquences semi-diurnes. Certains écarts quantitatifs apparaissent entre les valeurs des caractéristiques du courant de marée barotrope obtenues par les techniques de décomposition DMD et EOF. Pour éliminer ces écarts, il faudrait disposer de nouvelles données de courantométrie avec une meilleure résolution verticale. © 2000 Ifremer/CNRS/IRD/É ditions scientifiques et médicales Elsevier SAS modes barotrope et barocline / courant de marée / décomposition en mode dynamique / détroit de Gibraltar

Accuracy assessment of high frequency radar current measurements in the Strait of Gibraltar

“…For instance, the tidal transport associated with M2 and S2 constituents reach values as high as 4 Sv. 34,35,36 Even though the meteorological wind forcing effect is restricted to the superficial layer, it might also be a key factor in specific processes such as punctual inflow interruptions or the circulation pattern in the Bay of Algeciras 37,38 (Fig 1b). The HF radar system covers the easternmost area of the Strait of Gibraltar, including the mouth of the Bay of Algeciras and part of the western boundary of the Alboran Sea.…”

Baroclinic M2 tidal circulation in Algeciras Bay and its implications for the water exchange with the Strait of Gibraltar: Observational and 3-D model results

“…[], Bruno et al . [], Tsimplis [], and Izquierdo et al . [], using interpolation/extrapolation techniques and basic flow‐conservation laws.…”

“…The vertical profiles of the M 2 current ellipse parameters over the Camarinal Sill published by Candela [], Bruno et al . [], and Tsimplis [] were used to describe the main characteristics of the M 2 tidal currents in the Strait of Gibraltar (CS1–CS4 in Figure ). At the four stations located in nearshore areas of Algeciras Bay, the profiles of the ellipse parameters were obtained from the harmonic analysis of a 2 month experimental current series recorded during November to December 2006 by three ADCPs installed at a depth of ∼20 m (AS1, AS3, and AS4), and two current meters moored at depths of 20 and 180 m (AS2).…”

“…Observed (black) and predicted (gray) vertical profiles of the M 2 current ellipse parameters: (a) stations over the Camarinal Sill; CS1, CS4: Candela []; CS2: Bruno et al . []; CS3: Tsimplis [] and (b) stations in the nearshore areas of Algeciras Bay; AS1–AS4. Squares: semimajor axis; diamonds: ellipse phase (in Greenwich degrees); triangles: ellipse inclination (0° eastwards, positive anticlockwise).…”

“…This behavior is shown by field data from current profilers throughout the water column [ Bruno et al ., ; Tsimplis , ], and explains the good agreement between barotropic, depth‐averaged models of the Strait [e.g., Tejedor et al ., ] and three‐dimensional models [ Sannino et al ., ]. For the principal astronomical tidal constituents, the barotropic mode accounts for more than 90% of total variance of tidal currents over the Camarinal Sill [ Candela et al ., ; Bruno et al ., ; Sánchez‐Román et al ., ] and about 70% in the eastern side of the Strait [ García‐Lafuente et al ., ]. However, in particular areas of the Strait of Gibraltar, such as Algeciras Bay, this may be not the case.…”