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The Mediterranean Outflow in the Strait of Gibraltar and Its Connection

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The Mediterranean Outflow in the Strait of Gibraltar and Its Connection Ocean Sci., 13, 195–207, 2017 www.ocean-sci.net/13/195/2017/ doi:10.5194/os-13-195-2017 © Author(s) 2017. CC Attribution 3.0 License. The Mediterranean outflow in the Strait of Gibraltar and its connection with upstream conditions in the Alborán Sea Jesús García-Lafuente, Cristina Naranjo, Simone Sammartino, José C. Sánchez-Garrido, and Javier Delgado Physical Oceanography Group, Department of Applied Physics 2, University of Málaga, Málaga, Spain Correspondence to: Jesús García-Lafuente ([email protected]) Received: 23 November 2016 – Discussion started: 13 December 2016 Revised: 24 February 2017 – Accepted: 28 February 2017 – Published: 24 March 2017 Abstract. The present study addresses the hypothesis that the mediate and the Tyrrhenian Dense waters, both of interme- Western Alborán Gyre in the Alborán Sea (the westernmost diate nature. The Winter Intermediate Water is formed along Mediterranean basin adjacent to the Strait of Gibraltar) in- the continental shelf of the Liguro-Provençal sub-basin and fluences the composition of the outflow through the Strait Catalan Sea (Conan and Millot, 1995; Vargas-Yáñez et al., of Gibraltar. The process invoked is that strong and well- 2012), exhibits marked interannual fluctuations that include developed gyres help to evacuate the Western Mediterranean years of no formation (Pinot et al., 2002; Monserrat et al., Deep Water from the Alborán basin, thus increasing its pres- 2008), and is characterized by an absolute minimum of po- ence in the outflow, whereas weak gyres facilitate the outflow tential temperature. Its volume transport is much less than the of Levantine and other intermediate waters. To this aim, in Levantine Intermediate Water and it flows embedded inside situ observations collected at the Camarinal (the main) and this water mass at relatively shallow depths. The Tyrrhenian Espartel (the westernmost) sills of the strait have been anal- Dense Water is the result of mixing of old Western Mediter- ysed along with altimetry data, which were employed to ob- ranean Deep Water residing in the Tyrrhenian Sea and newly tain a proxy of the strength of the gyre. An encouraging cor- entered Levantine Intermediate Water flowing into the west- relation of the expected sign was observed between the time ern Mediterranean Sea through the Strait of Sicily (Rhein series of potential temperature at the Espartel Sill, which is et al., 1999; Millot et al., 2006). It is slightly denser than shown to keep information on the outflow composition, and the Levantine Intermediate Water but lighter than the West- the proxy of the Western Alborán Gyre, suggesting the cor- ern Mediterranean Deep Water and spreads between both of rectness of the hypothesis, although the weakness of the in- them. volved signals does not allow for drawing definitive conclu- In the Alborán Sea, which is the upstream basin for the sions. outflow, these waters approach the strait following different paths, with the Levantine and Western Intermediate waters flowing closer to the Spanish continental slope and the West- ern Mediterranean Water attached to the Moroccan coast 1 Introduction (Bryden and Stommel, 1982; Parrilla et al., 1986; Millot, 2009; Naranjo et al., 2012). This spatial differentiation is The Mediterranean outflow through the Strait of Gibraltar still observed before they flow over the Camarinal Sill (see can be considered to be formed by up to four water masses of Fig. 1) in the eastern half of the strait (García-Lafuente et different origins (see Naranjo et al., 2015, for a recent anal- al., 2000; Naranjo et al., 2012, 2015). The severe mixing and ysis of the outflow composition). Of all the four, the Levan- dissipation that takes place in the Tangier Basin downstream tine Intermediate Water, characterized by an absolute maxi- (oceanward) of the Camarinal Sill (Wesson and Gregg, 1994; mum of salinity, and the Western Mediterranean Deep Wa- García-Lafuente et al., 2009, 2011; Sánchez-Garrido et al., ter, which is the densest and often the coldest water, have 2011) blurs this spatial pattern and tends to form a rather been historically seen as the main contributors to the out- mixed outflow with little participation of entrained North At- flow. The two remaining water masses are the Winter Inter- Published by Copernicus Publications on behalf of the European Geosciences Union. 196 J. García-Lafuente et al.: The Mediterranean outflow and upstream conditions lantic Central Water in which the Mediterranean waters are identification, and the time series they collect (described in barely distinguishable (García-Lafuente et al., 2011; Naranjo more detail in next section) are well suited to investigate et al., 2015). the spatial structure of the outflow through the Camarinal The bottom topography of the Camarinal Sill section de- Sill. Both aspects are of great interest to our study. How- picts two troughs (Fig. 1). According to the spatial differ- ever, taking into account that the timescale of the WAG vari- entiation in the Alborán Sea and east of the Camarinal Sill, ability is of the order of weeks to months (Vargas-Yáñez the Levantine and Western Intermediate waters (and Tyrrhe- et al., 2002; Sánchez-Garrido et al., 2013), these series are nian Dense Water partially) will flow preferably through the too short to draw unarguable conclusions about our hypoth- northern channel of the Camarinal Sill (CSN hereinafter; see esis. This drawback does not apply to the long series at ESP, Fig. 1), and the Western Mediterranean Deep Water (and which shows up other disadvantages: since the characteris- Tyrrhenian Dense Water partially as well) will go across the tics of the Mediterranean waters flowing through CSN and southern channel (CSS hereinafter). On the other hand, bear- CSS are not very different and the changing fractions that ing in mind the different paths they follow in the Alborán the WAG fluctuations can originate on these flows are also Sea, the Western Mediterranean Deep Water flow would ben- small, the footprint left by these fluctuations in the series col- efit from a well-developed Western Alborán Gyre (WAG), lected at ESP are expectedly weak. This remark is of con- a recurrent mesoscale feature of this sub-basin (Parrilla et cern because other processes taking place in the Mediter- al., 1986; Viudez et al., 1998), whereas the Levantine and ranean Sea produce signals that, when exported through the also the Western Intermediate waters would be favoured by strait, can mask the weaker ones associated with the WAG a weak or absent WAG. While the first process has been fluctuations. One of them is, for instance, the formation of dealt with by different authors (Bryden and Stommel, 1982; larger-than-average volumes of denser-than-average Western Naranjo et al., 2012), the influence of the WAG on the Lev- Mediterranean Deep Water in winter, as it happened in the antine/Western Intermediate waters flow seems to not have years 2005 and 2006, which can uplift old deep water near been addressed yet. the strait and facilitate its drainage rather independently of The hypothesis of the present study is that if the WAG has the WAG features, causing a cold signature in the temper- an effect on the evacuation of these water masses, one ex- ature series at ESP (García-Lafuente et al., 2007). The in- pected consequence at the Camarinal Sill section would be terannual variability of the Mediterranean water properties the variation of the outflow size across each channel, the may also interfere with the mechanism we propose. Last but presence of a well-developed WAG increasing the size of not least, the entrained North Atlantic Central Water that has the flow across CSS with regard to CSN, and vice versa. been incorporated to the outflow at this site, despite being a Two mooring lines equipped with autonomous conductivity– very small fraction (García-Lafuente et al., 2011), also con- temperature (CT) and upward-looking acoustic Doppler cur- tributes to blurring possible signals (see Appendix A). rent profilers (ADCPs) were deployed in these channels Despite the aforementioned sources of “unwanted noise” (Fig. 1) in order to get information about the characteristics and observational limitations, our analysis provides reason- and properties of the outflow at the Camarinal Sill section. able evidence to support the feasibility of the hypothesis, Since the flow through each channel advects water of dif- even if it cannot be validated indisputably. The main goal of ferent characteristics that mix downstream of the Camarinal this paper is, therefore, to present and discuss this evidence Sill, changing fractions of these flows can leave a footprint bearing in mind these limitations, and has been organized as in the hydrological characteristics of the resulting mixed wa- follows: the next section describes the experimental data and ter that courses towards the Atlantic Ocean and contribute to the data processing and presents some features visible from the low-frequency signals reported in the Mediterranean out- the processed data. Of particular interest is the corrobora- flow in the Gulf of Cádiz (Fusco et al., 2008; Bellanco et al., tion of the aforementioned north–south spatial distribution of 2016). This outflow is being monitored by a long-term mon- Levantine Intermediate/Western Mediterranean Deep waters itoring station deployed few tens of kilometres westwards through CSN/CSS channels. Section 3, which is divided into of the Camarinal Sill at the Espartel Sill in the year 2004 two subsections, presents the results of the data analysis. The (ESP hereinafter, Fig. 1; see Sammartino et al., 2015, for a first subsection analyses the 3-month field experiment carried complete description of this station) and the collected long out in the Camarinal Sill, while the second one makes use of time series has been analysed to investigate our hypothesis the multi-year time series collected at the Espartel Sill to test as well.
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