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Long-Term Monitoring of the Atlantic Jet Through the Strait of Gibraltar with HF Radar Observations

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Long-Term Monitoring of the Atlantic Jet Through the Strait of Gibraltar with HF Radar Observations Journal of Marine Science and Engineering Article Long-Term Monitoring of the Atlantic Jet through the Strait of Gibraltar with HF Radar Observations Pablo Lorente 1,*, Silvia Piedracoba 2, Marcos G. Sotillo 1 and Enrique Álvarez-Fanjul 1 1 Puertos del Estado, 28042 Madrid, Spain; [email protected] (M.G.S.); [email protected] (E.Á.-F.) 2 Centro Tecnológico del Mar (CETMAR), 36208 Vigo, Spain; [email protected] * Correspondence: [email protected]; Tel.: +34-917-229-816 Received: 19 November 2018; Accepted: 20 December 2018; Published: 2 January 2019 Abstract: The present work focuses on the long-term coastal monitoring of the Atlantic surface inflow into the Mediterranean basin through the Strait of Gibraltar. Hourly current maps provided during 2016–2017 by a High Frequency radar (HFR) system were used to characterize the Atlantic Jet (AJ) since changes in its speed and direction modulate the upper-layer circulation of the Western Alboran Gyre (WAG). The AJ pattern was observed to follow a marked seasonal cycle. A stronger AJ flowed north-eastwards during autumn and winter, while a weaker AJ was directed more southwardly during the middle of the year, reaching a minimum of intensity during summertime. A strong relationship between AJ speeds and angles was evidenced: the AJ appeared to be frequently locked at an angle around 63◦, measured clockwise from the North. The AJ speed usually fluctuated between 50 cm·s−1 and 170 cm·s−1, with occasional drops below 50 cm·s−1 which were coincident with abrupt modifications in AJ orientation. Peaks of current speed clearly reached values up to 250 cm·s−1, regardless of the season. A number of persistent full reversal episodes of the surface inflow were analyzed in terms of triggering synoptic conditions and the related wind-driven circulation patterns. High sea level pressures and intense (above 10 m·s−1), permanent and spatially-uniform easterlies prevailed over the study domain during the AJ collapse events analyzed. By contrast, tides seemed to play a secondary role by partially speeding up or slowing down the westward currents, depending on the phase of the tide. A detailed characterization of this unusual phenomenon in the Strait of Gibraltar is relevant from diverse aspects, encompassing search and rescue operations, the management of accidental marine pollution episodes or efficient ship routing. Keywords: HF radar; monitoring; circulation; Atlantic Jet; flow reversal; Gibraltar; Alboran Sea 1. Introduction The Strait of Gibraltar (SoG hereinafter), the unique connection between the semi-enclosed Mediterranean basin and the open Atlantic Ocean, is characterized by a two-layer baroclinic exchange which is hydraulically controlled at Camarinal Sill (Figure1a). Whilst saltier Mediterranean water flows out at depth, an eastward surface jet of relatively fresh Atlantic water (AJ) flows into the Alboran Sea by surrounding the quasi-permanent Western Anticyclonic Gyre (WAG) and the more elusive Eastern Anticyclonic Gyre (EAG) in a wavelike path. As the WAG owes its existence to the input of new Atlantic waters provided by the AJ, both structures are widely considered to be coupled and usually referred to as the AJ-WAG system [1]. A significant number of analytical, field and modeling studies have previously attempted to disentangle the AJ-WAG system and properly explain the underlying physical processes [1–3]. J. Mar. Sci. Eng. 2019, 7, 3; doi:10.3390/jmse7010003 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, 3 2 of 16 J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 2 of 17 FigureFigure 1.1. (a) StudyStudy area:area: surface Atlantic Jet (AJ) flowingflowing through the StraitStrait ofof GibraltarGibraltar intointo thethe AlboranAlboran Sea,Sea, feedingfeeding the the Western Western Alboran Alboran Gyre Gyre (WAG); (WAG); isobath isobath depths depths are labeledare labeled every every 200 m.200 Red m. dotRed indicates dot indicates a topographic a topographic feature: feature: Camarinal Camarinal Sill (CS). Sill ( b(CS).) HFR (b) hourly HFR hourly data availability data availability for the periodfor the 2016–2017:period 2016–2017: solid black solid squares black squares represent represent radar sites, radar black sites, crosses black indicate crosses buoysindicate locations, buoys locations, blue dot denotesblue dot Tarifa denotes tide-gauge Tarifa tide-gauge (TG) situation. (TG) Blacksituation. line andBlack the line related and whitethe related square white indicate square the selectedindicate transectthe selected and its transect midpoint, and respectively; its midpoint, (c) HFR-derived respectively; mean (c) HFR-derived surface circulation mean pattern surface for 2016–2017.circulation pattern for 2016–2017. The position, intensity and direction of the AJ fluctuate in a broad range of temporal scales, driving the upper-layerThe position, circulation intensity of theand Alboran direction Sea of with the subsequent AJ fluctuate physical in a broad and biological range of implications temporal scales, [4–6]. Fordriving instance, the upper-layer the presence circulation of an intense of AJthe close Alboran to the Sea northern with subsequent shore of the physical Alboran and Sea reinforcesbiological theimplications coastal upwelling [4–6]. For and instance, therefore the increases presence both of an the intense nearshore AJ close chlorophyll to the concentrationnorthern shore and of thethe spawningAlboran Sea of fishreinforces in this the region coastal [5,7 ].upwelling By contrast, and meteorologically-induced therefore increases both the inflow nearshore interruptions chlorophyll can triggerconcentration the weakening and the andspawning even theof fish decoupling in this region of the [5,7]. AJ-WAG By contrast, system, themeteorologically-induced subsequent eastward migrationinflow interruptions of the WAG can and trigger the genesis the weakening of a new gyreand even that coexiststhe decoupling with the of other the AJ-WAG two, giving system, rise to the a three-anticyclonic-gyresubsequent eastward migration situation [of8]. the WAG and the genesis of a new gyre that coexists with the otherWithin two, giving this context, rise to thea three-anticyclonic-gyre AJ pattern has been traditionally situation [8]. described to oscillate between two main circulationWithin modes this context, at seasonal the scaleAJ pattern [9,10]: (i)has a strongerbeen traditionally AJ flows northeastwards described to oscillate during the between first half two of themain year; circulation (ii) a weaker modes AJ flowsat seasonal more southwardlyscale [9,10]: towards(i) a stronger the end AJ of flows the year. northeastwards Sea level pressure during (SLP) the variationsfirst half of over the theyear; Western (ii) a weaker Mediterranean AJ flows basin more and southwardly local zonal towards wind (U) the fluctuations end of the inyear. the Sea Alboran level Seapressure have been(SLP) usually variations considered over the to be Western the main Mediterranean factors controlling basin and and modulating local zonal the AJwind [11 ,12(U)]. Atfluctuations higher time inscales, the Alboran diurnal Sea and have semidiurnal been usually variations considered of the meanto be the flow main through factors the controlling SoG are mainly and duemodulating to interaction the AJ of tidal[11,12]. currents At high wither thetime intricate scales, topographydiurnal and [ 13semidiurnal]. In particular, variations local wind of the has mean been largelyflow through invoked the as SoG the primaryare mainly driving due to agent interaction to explain of bothtidal thecurrents intensification with the ofintricate the surface topography inflow during[13]. In prevalentparticular, westerlies local wind and has also been extreme largely AJ in collapsevoked as events the primary recorded driving when agent intense to easterliesexplain both are predominantthe intensification [14]. Theof the zonal surface wind inflow intensity during has beenprevalent reported westerlies to follow and an al annualso extreme cycle AJ with collapse more westerlyevents recorded (easterly) when winds intense during easterlies winter (summer) are predominant months [15 [14].]. The The seasonal zonal variabilitywind intensity and occasional has been interruptionsreported to follow of the an Atlantic annual inflow cycle due with to more meteorological westerly (easterly) forcing have winds been during previously winter investigated (summer) withmonths in situ [15]. data The from seasonal fixed moorings variability [10 and,11]. occasional More recently, interruptions a considerable of the number Atlantic of satellite inflow trackeddue to driftersmeteorological were released forcing on have both been sides previously of the SoG investigated within the framework with in situ of MEDESS-4MSdata from fixed project, moorings thus providing[10,11]. More a complete recently, Lagrangian a considerable view ofnumber the Atlantic of satellite waters tracked inflow intodrifters the Alboranwere released Sea [16 ,on17 ].both sidesThe of mainthe SoG goal ofwithin this workthe framework is twofold: firstly,of MEDE to buildSS-4MS up uponproject, previous thus worksproviding and characterizea complete theLagrangian AJ dynamic view during of the Atlantic a 2-year waters period inflow (2016–2017) into the by Alboran using novel Sea [16,17]. remote-sensed high-resolution currentThe estimations. main goal Secondly, of this towork provide is furthertwofold: insight firstly, into to a singularbuild up event: upon the previous quasi-permanent works and full reversalcharacterize of the
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