Internal waves in the Strait of Gibraltar and their role in the vertical mixing processes within the Bay of Algeciras

Abstract

This article presents some of the results of an oceanographic survey carried out in the Bay of Algeciras (Strait of Gibraltar) as part of a research project intended to assess the environmental quality of the Bay. One of the most interesting findings was the step-like patterns presented by density profiles within the Bay, which were indicative of notable vertical mixing activity there. The analysis of the observations indicates that those mixing processes may be explained by the interaction between the local internal tide dynamics and the large amplitude internal waves entering the Bay, which seem to originate from the Camarinal Sill region.

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

“…Here, the calculated Rossby numbers are below 0.06 (compared with values of $0.3 present at the mouth of the Canyon) and net vertical currents are insignificant, in contrast with the net upwelling/downwelling processes observed in isolated canyons with no coastal blocking at the head [see, e.g., Hickey, 1997;Boyer et al, 2006]. However, the instantaneous vertical tidal currents reach values of 0.5 cm s À1 at some places within the northern-most part of the Algeciras Canyon, and the role of diffusive processes related to the internal waves field on the vertical transport may be significant, as established by Chioua et al [2013] from field measurements.…”

“…Currents measured by the vm‐ADCP (Figure , left) are plotted together with the corresponding results from the numerical experiment for the same tidal stages (Figure , right). The most striking feature seen in both observed and predicted data is the presence of the mentioned tidal counter‐current system between the upper Atlantic and the lower Mediterranean water layers, so the maximum water inflow in one layer is close in time to the maximum outflow in the other layer, the velocity decaying to zero close to the interface between the two layers, at a depth of about 50–100 m. The quantitative and qualitative discrepancies between observed and modeled results (i.e., the phase differences present in some of the panels and the higher observed currents) are related to the effect of constituents other than the M 2 during the measurements, since they were made close to the spring phase of the tide [see Chioua et al ., ].…”

“…However, the instantaneous vertical tidal currents reach values of 0.5 cm s −1 at some places within the northern‐most part of the Algeciras Canyon, and the role of diffusive processes related to the internal waves field on the vertical transport may be significant, as established by Chioua et al . [] from field measurements.…”

“…This argument is supported by A lvarez et al [2011] regarding the effects of incoming baroclinic internal wavefronts, generated in the Strait of Gibraltar over the Camarinal Sill, on the dynamics of Algeciras Bay. However, except for that study, the pioneer work by De Buen [1924], the preliminary information published in the Environmental Quality Plan for the Campo de Gibraltar Dossier [2004], and the recent work by Chioua et al [2013], there are notably few studies of this topic. In particular, no specific studies have been conducted to date on the detailed characteristics of the M 2 tidal circulation in Algeciras Bay, nor its connection with the larger system constituted by the Strait of Gibraltar.…”

Mass exchange at the Strait of Gibraltar in response to tidal and lower frequency forcing as simulated by a Mediterranean Sea model

“…These last authors showed that the eastward advection of the mixed waters west of the Camarinall Sill, the entrainment further east and the friction inside the mixed layer all contribute to the dynamics of the interface layer. The generated waves are found to reach the Bay of Algeciras where they are suggested to lead to notable vertical mixing (Chioua et al, 2013). The wave formation is influenced by the subinertial variability (Va´zquez et al, 2008).…”