Interannual variability of the Mediterranean outflow observed in Espartel sill, western Strait of Gibraltar

Abstract

[1] Four-year time series of observations in Espartel sill at the western part of the Strait of Gibraltar have been analyzed in order to investigate the variability of the Mediterranean outflow. It is assumed that the observed variability comes from the changing properties of the dense waters that are located at the maximum depth from where they can be uplifted in the upstream basin (Alborán Sea, inside the Mediterranean Sea) and evacuated through the strait. From this perspective, the following three mechanisms are investigated: (1) the replenishment of the deep basin by newly formed Western Mediterranean Deep Water that, depending on its density, can either uplift old resident waters or lay above them leaving in any case a cold signature in the temperature series; (2) the presence/absence of the energetic anticyclonic gyres in the Alborán Sea, particularly the western one, which can transfer momentum to the underlying Mediterranean vein and provide it with additional energy to ascend over the sills of the strait; and (3) the meteorologically enhanced flows that follow the rapid changes of atmospheric pressure over the western Mediterranean basin, which would be able to aspire deeper waters residing in the upstream basin. The three mechanisms act on different timescales, from annual in case (1) to monthly in case (2) to weekly in case (3) although these two latter are modulated annually by the seasonal prevalence of the western Alborán gyre in summer and of the strong meteorologically driven fluctuations in winter. The mechanisms overlap at annual timescales making it difficult to separate out the different contributions.

Physical forcing and physical/biochemical variability of the Mediterranean Sea: a review of unresolved issues and directions for future research

“…The Mediterranean ecosystem structure(s) is(are) clearly influenced by the overall physical/chemical processes and exchanges active in the basin that favored the immigration of many species of Atlantic origin that dominate the Mediterranean communities and the invasion of alien species from either Gibraltar, Suez, and anthropogenic transport (Galil et al, 2007;Galil, 2000). The short time needed by several new non-indigenous species to colonize vast areas of the Mediterranean Sea hints at the presence of favorable conditions that may contribute to NIS success related to resources, natural enemies and the physical environment (Shea and Chesson, 2002).…”

“…13). The replenishment of the deep basin is by newly formed Western Mediterranean Deep Water that, depending on its density, can either uplift old resident waters or lay above them, leaving in any case a cold signature in the temperature series (Garcia-Lafuente et al, 2009). This mechanism then determines interannual variability of thermohaline properties of the outflowing Mediterranean waters, possibly changing their impact on the global conveyor belt.…”

The Mediterranean outflow in the Strait of Gibraltar and its connection with upstream conditions in the Alborán Sea

“…Such variability masks the characteristics that the different outflowing water masses would have if tides were absent. A procedure to obtain a realistic estimation of such characteristics is to select the coldest or saltiest sample registered during each semi-diurnal tidal cycle, as in García-Lafuente et al (2009). With regard to the Camarinal Sill section and under the hypothesis that Levantine Intermediate Water flows preferably through CSN, the saltiest option would be the right choice to select the representative long term there, whereas the coldest option is more adequate to select samples in CSS, since Western Mediterranean Deep Water has a preference to flow attached to the Moroccan coast.…”

“…1) in the eastern half of the strait (García-Lafuente et al, 2000;Naranjo et al, 2012Naranjo et al, , 2015. The severe mixing and dissipation that takes place in the Tangier Basin downstream (oceanward) of the Camarinal Sill (Wesson and Gregg, 1994;García-Lafuente et al, 2009Sánchez-Garrido et al, 2011) blurs this spatial pattern and tends to form a rather mixed outflow with little participation of entrained North At-lantic Central Water in which the Mediterranean waters are barely distinguishable Naranjo et al, 2015).…”

Two decades of mesoscale phenomena on either side of the Strait of Gibraltar

“…This fact discourages their use in the area of the GoC, SG and AS although there are interesting examples of their application to oceanographic problems. Viúdez et al (1998a) characterize annual and semi-annual variability of residual sea level in the Alboran Sea from TOPEX/Poseidon and ERS1 altimeter data; Flexas et al (2006) use sea level anomalies (SLA), SST and CTD data to describe an eastward migration of the WAG; García-Lafuente et al (2009) and Naranjo et al (2012) used SLA to explore the possibility of deep water suction in the AS by means of vertical transfer of momentum from the WAG to the underlying Mediterranean water that would acquire the requested energy to ascend over the sills of the SG.…”