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 the AJ AJ surface dynamic inflow during through a 2-year the entireperiod selected (2016–2017) transect by using (Figure novel1b) during,remote-sensed at least, high- 48 h whenresolution intense current easterlies estimations. episodes Secondly, were prevalent. to provid Thee triggering further insight met-ocean into factorsa singular (i.e., event: the atmospheric the quasi- andpermanent tidal forcing) full reversal were explored of the AJ along surface with inflow the physical through implications the entire in selected terms of transect sharp variations (Figure 1b) in theduring, sea surface at least, temperature 48 h when during intense summertime. easterlies ep Aisodes variety were of full prevalent. reversal episodesThe triggering was detected met-ocean and thefactors related (i.e., wind-induced the atmospheric circulation and tidal patterns forcing) were were interpreted. explored along with the physical implications in termsTo this of end,sharp a High-Frequencyvariations in the radar sea surface (HFR) systemtemperature was used, during since summertime. it provides quality-controlled A variety of full hourlyreversal maps episodes of the was surface detected currents and the in related this first-order wind-induced geostrategic circulation spot patterns (Figure were1b). interpreted. As shown, data availabilityTo this end, was a significantlyHigh-Frequency high radar (almost (HFR) 100%) system in the selectedwas used, transect since forit provides the study quality- period, decreasingcontrolled hourly in the easternmostmaps of the sectorssurface of currents the coverage in this domain.first-order The geostrategic accuracy of spot HFR (Figure observations, 1b). As whichshown, are data often availability affected was by intrinsic significantly uncertainties high (alm (radioost 100%) frequency in the interferences,selected transect antenna for the pattern study period, decreasing in the easternmost sectors of the coverage domain. The accuracy of HFR observations, which are often affected by intrinsic uncertainties (radio frequency interferences,

J. Mar. Sci. Eng. 2019, 7, 3 3 of 16 distortions, environmental noise, etc.), was previously assessed by comparing them against in situ data provided by a current meter [18]. Skill metrics improved when calibrated antenna patterns were used to process radial data. Recent research with this HFR network successfully investigated the water exchange between Algeciras Bay (Figure1a) and the SoG [ 19], the impact of the atmospheric pressure fluctuations on the mesoscale water dynamics of the SoG and the Alboran Sea [20], or the dominant modes of spatio-temporal variability of the surface circulation [21]. As a consequence, this HFR network can be considered to be an appropriate device to effectively monitor the AJ dynamics in near real-time, with current pulses often exceeding 200 cm·s−1 and time-averaged north-eastward speeds around 100 cm·s−1 in the narrowest section of the SoG (Figure1c). In the same line, this observing platform is also assumed to be capable of reliably portraying the AJ variability and the aforementioned episodic full reversals of the surface flow thanks to its significant spatio-temporal resolution, hence constituting an additional asset for wise decision-making of Algeciras Bay Harbor operators [22]. A detailed characterization of this unusual phenomenon in the SoG is relevant from diverse aspects, encompassing search and rescue operations (to adequately expand westwards the search area), the management of accidental marine pollution episodes (to establish alternative contingency plans), or safe ship routing (to maximize fuel efficiency). The paper is organized as follows. Section2 outlines the specific instrumentation used in this study. Section3 describes the main results obtained. Finally, a detailed discussion of the results, along with future plans, is presented in Section4.

2. Materials and Methods The study domain includes an array of three coastal buoys and a tide-gauge operated by Puertos del Estado (Figure1b), providing quality-controlled hourly-averaged observations of sea surface temperature (SST) and sea surface height (SSH), respectively. To ensure the continuity of the data record, occasional gaps detected in time series (not larger than 6 h) were linearly interpolated. Basic features of each in-situ instrument are described in Table1.

Table 1. Description of in situ instruments operated by Puertos del Estado in the Strait of Gibraltar.

Name Punta Carnero Tarifa Ceuta Tarifa Instrument Buoy Buoy Buoy Tide-gauge Type Directional Directional Directional Radar Manufacturer WatchKeeper Triaxys Triaxys Miros Year of deployment 2010 2009 1985 2009 Longitude 5.42◦ W 5.59◦ W 5.33◦ W 5.60◦ W Latitude 36.07◦ N 36.00◦ N 35.90◦ N 36.01◦ N Depth 40 m 33 m 21 m ——- Frequency sampling 60 min 60 min 60 min 1 min

The HFR system employed in the present study consists of a three-site shore-based CODAR Seasonde network, installed within the framework of TRADE (Trans-regional Radars for Environmental applications) project and supported by European FEDER funding. The system covers the easternmost area of the SoG, including the mouth of the Algeciras Bay and part of the western Alboran Sea (Figure1b,c). The sites, currently owned and operated by Puertos del Estado, were deployed in two different stages. The first two sites started data collection on May 2011: Ceuta (39.90◦ N, 5.31◦ W) and Punta Carnero (36.08◦ N, 5.43◦ W). Afterwards, the third site Tarifa (36.00◦ N, 5.61◦ W) was installed in October 2012 in order to properly resolve the baseline between Ceuta and Punta Carnero and also to gain accuracy over the entire radar coverage. Hereafter the sites will be referred to by their four-letter site codes: CEUT, CARN, and TARI, respectively (Figure1b). Each site operates at a central frequency of 26.8 MHz with a 150 KHz bandwidth, providing hourly radial measurements with a cut-off filter of 250 cm·s−1, a threshold defined according to the historical values observed in the area. The maximum horizontal range and angular resolution are J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 4 of 17 J. Mar. Sci. Eng. 2019, 7, 3 4 of 16 km and 5°, respectively. The estimated current velocities are representative of the upper 0.5 m of the water column. Only calibrated antenna patterns are used to process radial data due to their ◦ 40positive km and impact 5 , respectively. in HFR Theaccuracy, estimated as currentprevious velocitiesly proved are representative by Reference of the[18]. upper Radial 0.5 mcurrent of the watermeasurements column. Onlyfrom calibratedthe three sites antenna are patternsgeometrically are used combined to process with radial an averaging data due to radius their positiveset to 3 impactkm, in inorder HFR to accuracy, estimate as hourly previously total proved current by ve Referencectors on [a18 Cartesian]. Radial current regular measurements grid of 1 × 1 from km thehorizontal three sites resolution. are geometrically combined with an averaging radius set to 3 km, in order to estimate hourlyA totalsource current of error vectors to be on considered a Cartesian in regular the computation grid of 1 × 1 kmof the horizontal total vectors resolution. is the so-called GeometricalA source Dilution of error of toPrecision be considered (GDOP). in The the GDOP computation is defined of theas a total dimensionless vectors is thecoefficient so-called of Geometricaluncertainty that Dilution characterizes of Precision how (GDOP). radar Thesystem GDOP geometry is defined may as impact a dimensionless on the measurements coefficient of uncertaintyaccuracy and that position characterizes determination how radar errors, system owing geometry to the may angle impact at which on the measurementsradial vectors accuracyintersect and[23]. positionMaps of determinationeast and north errors,GDOP owingfor an HFR to the sy anglestem generally at which follow radial vectorsa pattern intersect where their [23]. values Maps ofincrease east and with north the GDOPdistance for from an HFRthe radar system sites generally and along follow the abaselines pattern where(lines connecting their values two increase HFR withsites) thedue distance to poor from intersecting the radar beam sites andgeometry, along theas baselinesthe combining (lines connectingradial vectors two are HFR increasingly sites) due toparallel poor intersectingand the orthogonal beam geometry, component as thetends combining to zero. radial vectors are increasingly parallel and the orthogonalFigure component2a, b illustrates tends tothe zero. zonal and meridional components of the GDOP over the HFR domain,Figure estimated2a,b illustrates according the zonal to the and formulation meridional of components Reference of[23]. the GDOPThe values over theof GDOP HFR domain, in the estimatedbaselines accordingare low, less to the than formulation 1.5 for both of Reference componen [23ts.]. The valuesspatial ofdistribution GDOP in the of baselinesthe sites areallows low, lessresolving than 1.5the for central both region components. of the SoG, The spatialminimizing distribution the GDOP of the of sitesthe system allows in resolving this sensitive the central area. regionThe baseline of the SoG,between minimizing CEUT and the GDOPCARN ofis theresolved system using in this radial sensitive vectors area. from The baselineTARI, while between the CEUTbaseline and between CARN TARI is resolved and CEUT using is radial resolved vectors using from radial TARI, vectors while from the baselineCARN. On between the other TARI hand, and CEUTthe GDOP is resolved for the using meridional radial vectorscomponent from increases CARN. On in thethe other eastern hand, region the GDOPof the fordomain, the meridional reaching componentvalues of 2.5 increases in the boundary in the eastern (Figure region 2b). In of this the area, domain, only reaching radial vectors values from of 2.5 CARN in the and boundary CEUT (Figureare used2b). to compute In this area, the onlytotal velocities radial vectors and the from direct CARNion of and their CEUT radial are vectors used to is computealmost parallel. the total In velocitiesthis context, and the the transect direction here of their used radial to examine vectors the is almost AJ surface parallel. inflow In this was context, readily the selected transect as here the usedassociated to examine total GDOP the AJ surface(Figure inflow 2c) was was reduced readily selected(below 1.3) as the and associated the spatial total and GDOP temporal (Figure data2c) wascoverage reduced were (below optimal 1.3) during and the 2016–2017 spatial and (Figure temporal 1b). data A similar coverage approach were optimal was previously during 2016–2017 adopted (Figurein Reference1b). A similar[24] to approach evaluate was the previously water rene adoptedwal mechanism in Reference and [ 24 ]the to evaluate related theinshore-offshore water renewal mechanismexchanges in and the the Gulf related of Naples. inshore-offshore From an oceanogr exchangesaphic in the perspective, Gulf of Naples. the election From an of oceanographic such transect perspective,was also convenient the election to ofbetter such transectcharacterize was alsoboth convenient the intensity to better and characterizedirection of boththe AJ, the intensitysince its andmidpoint direction covers of the the AJ, area since where its midpoint the highest covers peak the of area current where speed the highest is usually peak ofdetected current and speed also is usuallywhere the detected inflow and orientation also where is not the inflowinfluenced orientation yet by the is not water influenced exchange yet between by the water Algeciras exchange Bay betweenand the Strait Algeciras of Gibraltar, Bay and as the shown Strait in of Figure Gibraltar, 1c. as shown in Figure1c.

Figure 2.2. GeometricGeometric Dilution Dilution of of Precision Precision (GDOP) (GDOP) for fo ther geometrythe geometry of Gibraltar of Gibraltar HFR system:HFR system: (a) Zonal (a) GDOP;Zonal GDOP; (b) Meridional (b) Meridional GDOP; GDOP; (c) Total (c) GDOP. Total GDOP.

Finally, the prevailing synoptic atmospheric conditions were inferred from three-hourly predictions ofof sea sea level level pressure pressure (SLP) (SLP) and and zonal zonal wind wind at 10 mat height10 m (U-10),height provided(U-10), provided by the European by the CenterEuropean of MediumCenter of Weather Medium Forecast Weather (ECMWF). Forecast (ECMWF).

3. Results The scatterscatter plotplot ofof HFR-derived HFR-derived eastward eastward current current speed speed against against direction direction at theat the midpoint midpoint of the of · −1 selectedthe selected transect transect revealed revealed that that currents currents stronger stronger than than 50 cm 50s cm·sﬂowed−1 flowed predominantly predominantly towards towards the ◦ North-East,the North-East, forming, forming, on average,on average, an anglean angle of 63 of measured63° measured clockwise clockwise from from the Norththe North (Figure (Figure3a).

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J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 5 of 17 · −1 The3a). mean The eastwardmean eastward current current speed speed was 109 was cm 109s cm·sfor− the1 for analyzed the analyzed 2-year 2-year period period and speedand speed peaks −1 −1 clearlypeaks reachedclearly reached values upvalues to 250 up cm to· s250. cm·s Velocities−1. Velocities below below 50 cm ·50s cm·swere−1 were registered registered along along the entire the ◦ rangeentire of range eastward of eastward directions direct (0–180ions). Westwards(0–180°). Westwards currents, albeit currents, minority albeit (1256 minority hourly (1256 observations hourly duringobservations 2016–2017, during 8% of2016–2017, the time), 8% were of the also time), observed were and also tended observed to predominantly and tended to formpredominantly an angle of ◦ −1 −1 274formwith an averagedangle of 274° speeds with about averaged 35 cm· sspeeds(Figure about3b). 35 Higher cm·s−1 velocities (Figure 3b). (>50 Higher cm ·s velocities) were scarcely (>50 ◦ ◦ observedcm·s−1) were for directions scarcely rangingobserved from for 240 directionsto 290 .ranging Since the from AJ flowed 240° predominantlyto 290°. Since north-eastwardsthe AJ flowed (15,487predominantly observations, north-eastwards 88% of the time),(15,487 a observations, scatter plot of88 zonal% of the versus time), meridional a scatter plot current of zonal speed versus was alsomeridional computed current to infer speed the was relationship also computed between to in bothfer the components relationship in between terms of both strength components of the flow in (Figureterms 3ofc). strength According of the to theflow results (Figure derived 3c). Accordin from theg bestto the linear results fit, derived they were from highly the best and positivelylinear fit, correlatedthey were (0.74). highly Moreover, and positively thezonal correlated velocity (0.74). component Moreover, emerged the zonal to velocity be, on average,component almost emerged three timesto be, stronger on average, than almost the meridional three times current stronger speed than (as the reflected meridional by a current slope of speed 0.37), (as thus reflected highlighting by a theslope relevance of 0.37), of thethus along-shore highlighting transport the relevance in the of water the exchangealong-shore between transport the in Atlantic the water Ocean exchange and the Mediterraneanbetween the Atlantic Basin, inOcean agreement and the with Mediterranean [4]. Basin, in agreement with [4].

FigureFigure 3. 3.2-year 2-year scatter scatter plot plot of of hourly hourly Atlantic Atlantic Jet Jet (AJ) (A currentJ) current direction direction (angle (ang measuredle measured clockwise clockwise from thefrom North) the versusNorth) speedversus estimations speed estimations at the midpoint at the midpoint of the selected of the transect:selected transect: (a) AJ ﬂowing (a) AJ eastwards flowing (positiveeastwards zonal (positive speed, zonal U > 0); speed, (b) AJ U ﬂowing > 0); (b westwards) AJ flowing (U

ScatterScatter plots plots were were also also computed computed on on a quarterly a quarterl basisy basis to infer to infer potential potential differences differences for each for seasoneach (Figureseason4 ).(Figure During 4). winter During (defined winter (defined as January-February-March as January-February-March –JFM-), –JFM-), the AJ reachedthe AJ reached its highest its quarterly-averagedhighest quarterly-averaged speed (123 speed cm·s −(1231) with cm·s a− related1) with meana related orientation mean orientation of 63◦ (Figure of 63°4a). (Figure Most of 4a). the directionalMost of the estimations directional were estimations comprised were in the comprise first quadrantd in the (0–90 first◦ ),quadrant whereas only(0–90°), isolated whereas values only lay inisolated the range values of 90–180 lay in◦ the(i.e., range flowing of 90–180° towards (i.e., the flowing South-East). towards Likewise, the South-East). weaker westwardLikewise, weaker currents werewestward residually currents observed were (7% residually of the wintertime), observed (7% mainly of the flowing wintertime), to the NW mainly with aflowing mean angleto the around NW 283with◦ (Figure a mean4b). angle around 283° (Figure 4b). DuringDuring spring spring (defined (defined asas April-May-JuneApril-May-June –AMJ-), both both AJ AJ mean mean speed speed and and angle angle were were lower, lower, 113113 cm cm·s·s−1−1 andand 6060°,◦, respectivelyrespectively (Figure 44c).c). Again, the AJ flowedflowed mostly towards towards the the North-East, North-East, withwith scarce scarce observations observations ofof surfacesurface currentscurrents flowingflowing to other quadrants quadrants (4% (4% of of the the time, time, Figure Figure 4d).4d). InIn summer summer (July-August-September (July-August-September -JAS-)-JAS-) somesome noticeablenoticeable differences were were observed: observed: the the eastward eastward −1 AJAJ speed speed (angle) (angle) quarterly-averaged quarterly-averaged value value reachedreached aa minimumminimum (maximum)(maximum) ofof 9999 cmcm·s·s− 1(68°),(68◦), with with thethe AJ AJ directed directed more more southwardly. southwardly. HigherHigher scatterscatter was detected along along the the directional directional axis, axis, with with surface currents often flowing towards the South-East (Figure 4e), the South-West and the North- surface currents often flowing towards the South-East (Figure4e), the South-West and the North-West West (Figure 4f). By the end of the year (October-November-December -OND-), the AJ mean speed (Figure4f). By the end of the year (October-November-December -OND-), the AJ mean speed partially partially recovered (102 cm·s−1) and the currents were again directed predominantly to the North- recovered (102 cm·s−1) and the currents were again directed predominantly to the North-East with an East with an averaged angle of 63° (Figure 4g), thereby completing the annual cycle. Likewise, flow averaged angle of 63◦ (Figure4g), thereby completing the annual cycle. Likewise, flow reversals were

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J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 6 of 17 sporadicallyreversals were observed sporadically (9% of observed the autumn (9% time), of the with autumn a prevailing time), with direction a prevailing towards direction the west, towards forming ◦ anthe angle west, of forming 277 (Figure an angle4h). of 277° (Figure 4h).

FigureFigure 4. 4.Seasonal Seasonal scatter scatter plots plots of hourly of hourly Atlantic Atlantic Jet (AJ) Jet current (AJ) current direction direction (angle measured (angle measured clockwise fromclockwise the North) from the versus North) speed versus estimations speed estimations at the midpoint at the midpoint of the selected of the selected transect, transect, with AJ with ﬂowing AJ eastwardsflowing eastwards (left) and (left) westwards and westwards (right): ((right):a,b) Winter (a,b) Winter (JFM); (JFM); (c,d) Spring (c,d) Spring (AMJ); (AMJ); (e,f) Summer(e,f) Summer (JAS); (g(JAS);,h) Autumn (g,h) Autumn (OND). (OND). Quarterly Quarterly mean and mean standard and standard deviation deviation values ofvalues both AJof both speed AJ and speed direction and aredirection provided. are Analysisprovided. performed Analysis pe forrformed a 2-year for period a 2-year (2016–2017). period (2016–2017).

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The long-term coastal monitoring of AJ speed and direction at the midpoint of the selected transectThe is long-term presented coastal in Figure monitoring 5. A strong of AJ speedrelationship and direction between at theremotely midpoint observed of the selected AJ speeds transect (red line)is presented and angles in Figure (blue 5line). A strongwas revealed relationship: with betweencurrent velocities remotely above observed 50 cm·s AJ speeds−1, the AJ (red appeared line) and to − beangles locked (blue at line) an angle was revealed: around with63°. currentIt is also velocities noticeable above that 50 AJ cm ·hourlys 1, the speed AJ appeared usually to fluctuated be locked ◦ −1 betweenat an angle 50 around cm·s−163 and. It is170 also cm·s noticeable−1, with thatpeaks AJ hourlyof 250 speed cm·s− usually1 in the fluctuated form of betweeneastward 50 pulses. cm·s −1 −1 Occasionaland 170 cm ·dropss , with below peaks 50 ofcm·s 250−1 cmwere·s coincidentin the form with of eastwardabrupt changes pulses. in Occasional AJ orientation. drops belowWhile − some50 cm ·ofs those1 were sharp coincident variations with abruptin directions changes were in AJ of orientation.short duration While (i.e., some a few of thosehours) sharp in response variations to thein directions joint action were of local of short wind duration forcing (i.e., (moderate a few hours) and/or in time-variable response to theeasterlies) joint action and tidal of local currents wind duringforcing the (moderate outflow and/or phase [25], time-variable others were easterlies) persistent and enough tidal currents (lasting duringmore than the 48 outflow h) to give phase rise [25 to], fullothers surface were inflow persistent reversals. enough (lasting more than 48 h) to give rise to full surface inflow reversals.

Figure 5. Annual evolution of the AJ orientation (b (bluelue line) and current speed (red line) for (a) 2016 and (b) 2017, as derived from hourly HFR estimations at the midpoint of the selected transect in the Strait of Gibraltar (Figure1 1b).b).

Complete collapsecollapse of of the the AJ andAJ and quasi-permanent quasi-permanent inversion inversion of the surfaceof the inflowsurface during inflow prevalent during prevalentintense easterlies intense iseasterlies a singular is phenomenona singular phenom that deservedenon that detailed deserved exploration. detailed Underexploration. this temporal Under premise,this temporal monthly premise, Hovmöller monthly diagrams Hovmöller were computed diagrams for were HFR-derived computed zonal for currents HFR-derived at the selected zonal currentstransect toat easilythe selected detect andtransect categorize to easily six 2-daydetect full and reversal categorize episodes, six 2-day represented full reversal by black episodes, boxes representedin Figure6. Inby Decemberblack boxes 2016 in Figure (Figure 6.6 b)In December and February 2016 2017 (Figure (Figure 6b) 6andc), briefFebruary inversions 2017 (Figure (related 6c), briefto less inversions intense easterlies) (related to preceded less intense the fulleasterlies) reversal preceded of the surface the full flow. reversal The eventof the detected surface flow. in March The event2017 consisted detected ofin an March abrupt 2017 interruption consisted of of the an eastward abrupt inflowinterruption and complete of the reversaleastward of inflow the surface and completestream (Figure reversal6d). of By the contrast, surface instream August (Figure 2016 (Figure6d). By 6contrast,a), August in August 2017 (Figure 2016 6(Figuree) and December6a), August 2017 (Figure(Figure6 f),6e) the and classical December AJ inflow 2017 (Figure into the 6f), Mediterranean the classical Sea AJ (redinflow color) into was the only Mediterranean observed in theSea (redsouthern color) part was of only the transect,observed whereas in the southern a weaker part coastal of the counter transect, current whereas (CCC) a weaker was detected coastal flowingcounter westwardscurrent (CCC) and was bordering detected the flowing northern westwards Spanish shoreline.and bordering the northern Spanish shoreline. The prevailingprevailing atmosphericatmospheric synoptic synoptic conditions conditions were were inferred inferred from from ECMWF ECMWF predictions predictions of SLP of SLPand U-10,and U-10, as shown as shown in Figure in Figure7. A significant 7. A significant latitudinal latitudinal gradient gradient of SLP was of SLP observed was observed in four episodes in four episodesthat took that place, took respectively, place, respectively, in August in 2016, Augu Februaryst 2016, 2017,February March 2017, 2017, March and December2017, and 2017,December with 2017,high pressureswith high over pressures the Gulf over of Biscay the Gulf and of isobars Biscay closely and isobars spaced inclosely the SoG spaced (Figure in 7thea,c,d,f), SoG leading(Figure −1 7a,c,dto extremely and f), intenseleading easterliesto extremely (above intense 10 measterlies·s ), channeled (above 10 through m·s−1), channeled the Strait duethrough to its the specific Strait due to its specific geometric configuration (Figure 7g,i,j and l). In December 2016, high pressures

J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 8 of 17 J. Mar. Sci. Eng. 2019, 7, 3 8 of 16 J.dominated Mar. Sci. Eng. the2018 , entire6, x FOR Western PEER REVIEW Mediterranean (Figure 7b), equally yielding strong along-shore 8 of 17 westward winds along both sides of the SoG (Figure 7h). In August 2017, the typical summer dominatedgeometricweather type conﬁgurationthe was entire observed Western (Figure with Mediterranean7 g,i,j,l).Azores InHigh December pressures(Figure 2016, 7b), governing highequally pressures theyielding Mid-Atlantic dominated strong Areaalong-shore the (Figure entire westwardWestern7e) with Mediterraneanmoderate winds along but persistent (Figureboth sides7b), easterly equallyof the winds yieldingSoG (Fblowingigure strong 7h). through along-shore In August the entire westward 2017, Western the winds typical Mediterranean along summer both weathersides(Figure of 7k). thetype SoG was (Figure observed7h). with In August Azores 2017, High the pressures typical summergoverning weather the Mid-Atlantic type was observed Area (Figure with 7e)Azores with Highmoderate pressures but persistent governing easterly the Mid-Atlantic winds blowing Area through (Figure 7thee) withentire moderate Western Mediterranean but persistent (Figureeasterly 7k). winds blowing through the entire Western Mediterranean (Figure7k).

Figure 6. Monthly Hovmöller diagrams of High-Frequency Radar (HFR)-derived zonal current Figurespeed at 6. Monthlythe selected Hovmöller transect diagrams in the Strait of High-Frequency of Gibraltar (Figure Radar (HFR)-derived 1b). Red (blue) zonal colors current represent speed Figure 6. Monthly Hovmöller diagrams of High-Frequency Radar (HFR)-derived zonal current ateastward the selected (westward) transect surface in the Straitflow. of2-day Gibraltar episodes (Figure of permanent1b). Red (blue) flow colorsreversal represent are marked eastward with speed at the selected transect in the Strait of Gibraltar (Figure 1b). Red (blue) colors represent (westward)black boxes. surface ﬂow. 2-day episodes of permanent ﬂow reversal are marked with black boxes. eastward (westward) surface flow. 2-day episodes of permanent flow reversal are marked with black boxes.

Figure 7.7. Synoptic patterns related to the six Atlantic JetJet (AJ) inflowinflow reversal eventsevents analyzedanalyzed duringduring thethe periodperiod 2016–2017:2016–2017: 2-day averaged maps of sea level pressure (SLP: a–f) and zonal wind at 1010 mm Figureheight (U-10:7. Synoptic g–l), patterns asas providedprovided related byby the theto the EuropeanEuropean six Atlantic CenterCenter Jet ofof (AJ) MediumMedium inflow Weather Weatherreversal ForecastForecastevents analyzed (ECMWF). (ECMWF). during the period 2016–2017: 2-day averaged maps of sea level pressure (SLP: a–f) and zonal wind at 10 m height2-day (U-10: averagedaveraged g–l), maps mapsas provided ofof modeledmodeled by the European windwind field field Center at at 10 10 of m m Medium height height are Weather are presented presented Forecast in in Figure(ECMWF). Figure8 for 8 for each each of theof the six six events events analyzed. analyzed. As shown,As shown, intense intense easterlies easterlies prevailed prevailed over over the entire the entire study study domain, domain, with −1 uniformwith2-day uniform velocities averaged velocities usually maps usually of above modeled above 10 m wind· s10 .m·s field Only−1. Onlyat the 10 fifth mthe height episode,fifth episode,are correspondingpresented corresponding in Figure to August 8to for August each 2017 of(Figure2017 the (Figure six8e), events exhibited 8e), analyzed.exhibited lower windlowerAs shown, speed wind intense (inspeed accordance (ineasterlies accordance with prevailed Figure with7 k)overFigure and the more7k) entire and spatially-variable morestudy spatially-domain, −1 withwindvariable uniform conditions. wind velocitiesconditions. usually above 10 m·s . Only the fifth episode, corresponding to August 2017 (Figure 8e), exhibited lower wind speed (in accordance with Figure 7k) and more spatially- variable wind conditions.

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Figure 8.8. 2-day2-day averaged averaged maps maps of windof wind at 10 at m 10 height m height (provided (provided by the Europeanby the European Center of Center Medium of WeatherMedium ForecastWeather (ECMWF)) Forecast (ECMWF)) related to the related six Atlantic to the Jet six (AJ) Atlantic inﬂow reversalJet (AJ) eventsinflow analyzedreversal duringevents theanalyzed period during 2016–2017. the period 2016–2017.

Short-lived reversals of the surfacesurface inflowinflow have been previously reported to occur almost every tidal cycle in Camarinal Sill (western end of SoG: Figure1 1a)a) mainlymainly duedue toto thethe contributioncontribution ofof thethe semidiurnal tidal component M [25–28]. Since the mean inflow of Atlantic water is modulated by semidiurnal tidal component M2 [25–28]. Since the mean inflow of Atlantic water is modulated by barotropic tidal currents, hourly-averaged SSH obse observationsrvations provided by Tarifa tide-gauge were used to elucidate if the six 2-day inflowinflow reversal events in the eastern end of the Strait could have been mostly influencedinfluenced by spring-neap tidal cycle fluctuationsfluctuations (Figure9 9).). AlthoughAlthough thethe fortnightlyfortnightly variability waswas clearly clearly observable observable in ain monthly a monthly time ti seriesme series of SSH, of no SSH, cause-effect no cause-effect relationship relationship could be visuallycould be inferred visually from inferred the inspection from the inspection of zonal velocities of zonal at velocities the selected at the transect selected (Figure transect6). Apparently, (Figure 6). evidenceApparently, of preferenceevidence of for preference a specific tidalfor a cycle specific was tidal not observed cycle was as not the sixobserved flow reversal as the episodessix flow tookreversal place episodes during took different place tidalduring conditions, different tidal ranging conditions, from neap ranging tides from (Figure neap9a,c) tides to spring(Figure tides9a, c) (Figureto spring9d,f). tides (Figure 9d, f). To supportsupport the the previous previous statement, statement, hourly hourly maps ma ofps HFR-derived of HFR-derived surface surface circulation circulation were depicted were fordepicted eight differentfor eight test-casesdifferent (Figuretest-cases 10 ),(Figure defined 10), according defined toaccording the prevailing to the met-oceanprevailing conditionsmet-ocean (Tableconditions2). The (Table first two 2). analyzedThe first events two analyzed took place ev inents February took 2017place under in February strong westerlies 2017 under (Figure strong 10a) duringwesterlies spring (Figure tides 10a) (Figure during 10c), spring when thetides tide (Figure was at 10c), flood when (event the 1) andtide later was atat ebb flood (event (event 2). In1) bothand cases,later at the ebb wind-driven (event 2). In flow both was cases, directed the wind-d eastwardsriven (Figure flow was10g,i), directed although eastwards more vigorously (Figure 10g, in the i), −1 casealthough of event more 1. By vigorously contrast, events in the 3 andcase 4 of occurred event under1. By extremelycontrast, intenseevents easterlies3 and 4 (aboveoccurred 15 munder·s ) duringextremely neap intense tides. easterlies Regardless (above of the 15 tide m·s height,−1) during the surface neap tides. circulation Regardle patternsss of werethe tide similar height, in both the casessurface (Figure circulation 10k,m), patterns with a were manifest similar reversal in both of thecases main (Figure flow towards10k, m), thewith west, a manifest as reflected reversal in the of monthlythe main Hovmöllerflow towards diagram the west, (Figure as reflected10e). In Decemberin the monthly 2017, eventsHovmöller 5 and diagram 6 were registered(Figure 10e). again In underDecember strong 2017, easterlies events (Figure5 and 6 10 wereb), but registered this time agai duringn under spring strong tides easterlies (Figure 10 (Figured). According 10b), but to this the hourlytime during HFR-derived spring tides maps, (Figure the westward 10d). Accordin inversiong to of the the hourly flow was HFR-derived evidenced independentlymaps, the westward of the tidalinversion phase of (Figure the flow 10 wash,j). evidenced Finally, events independently 7 and 8 took of placethe tidal during phase neap (Figure tides, 10h, when j). Finally, persistent events and very7 and intense 8 took westerlies place during were neap blowing. tides, The when associated persistent surface and circulation very intense maps westerlies exhibited anwere acceleration blowing. −1 ofThe the associated Atlantic inflowsurface into circulation the Mediterranean maps exhibited (Figure an 10 accelerationl,n), reaching of velocities the Atlantic above inflow 160 cm into·s theat theMediterranean selected transect (Figure (Figure 10l, n),10f). reaching velocities above 160 cm·s−1 at the selected transect (Figure 10f).

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Figure 9. 9. MonthlyMonthly time time series series of ofhourly-averaged hourly-averaged sea seasurface surface height height (SSH) (SSH) provided provided by Tarifa by Tarifa tide- gaugetide-gauge (Figure (Figure 1b). 12-dayb). 2-day episodes episodes of permanent of permanent inflow inﬂow reversal reversal are marked are marked with with black black boxes. boxes.

The 2-day2-day averagedaveraged HFR-derived HFR-derived circulation circulation patterns patterns associated associated with with the six the events six hereevents studied here studiedare depicted are depicted in Figure in 11 Figure. Some 11. common Some common peculiarities peculiarities were exposed, were exposed, such us thesuch westward us the westward outflow outflowthrough through the narrowest the narrowest section ofsection the SoG of orthe the SoG prevalent or the prevalent anticyclonic anticyclonic inflow into inflow the Algeciras into the AlgecirasBay (with Bay a direct (with impact a direct on largerimpact residence on larger times residence and reduced times and water reduced renewal). water Three renewal). case studies Three caserevealed studies a predominant revealed a circulation predominant towards circulation the West/South-West, towards the West/South-West, together with a marked together acceleration with a −1 markedof the flow acceleration in the vicinity of the of flow Algeciras in the Bay, vicinity reaching of Algeciras speeds above Bay, 50reaching cm·s speeds(Figure above11a,c,e). 50 Oncm·s the−1 (Figurecontrary, 11a, two c, episodese). On the illustrated contrary, how two the episodes circulation illustrated in the easternmosthow the circulation region of in the the study easternmost domain regionfollowed of athe clockwise study domain rotation, followed likely feedinga clockwise the WAG,rotation, which likely was feeding out of the the WAG, picture which (Figure was 11 d,f).out −1 ofIn the firstpicture event, (Figure corresponding 11d, f). In tothe March first event, 2017, thecorresponding anticyclonic to gyre March was accelerated2017, the anticyclonic up to 80 cm gyre·s was(Figure accelerated 11d). The up second to 80 case cm·s (December−1 (Figure 2017)11d). wasThe substantiallysecond case less(December energetic 2017) and exhibitedwas substantially besides a lesscounter-clockwise energetic and recirculationexhibited besides structure a counter-clockw in the entranceise to therecirculation strait. (Figure structure 11f). The in the event entrance identified to thein December strait. (Figure 2016 represented 11f). The event an overall identified cyclonic in recirculation December cell2016 with represented a marked intensificationan overall cyclonic of the recirculationflow reversal cell nearby with the a marked Algeciras intensification Bay when westward of the flow currents reversal were nearby channeled the Algeciras through theBay narrow when westwardSoG (Figure currents 11b). Finally, were channeled two small-scale through counter-clockwise the narrow SoG eddies (Figure were 11b). observed Finally, detached two small-scale from the counter-clockwiseshore and enclosed eddies in the generalwere observed westward detached flow (Figure from 11 c,e).the shore and enclosed in the general westwardAmong flow the (Figure physical 11c, implications e). of the surface inflow reversal, abrupt increases in the SST field wereAmong revealed, the especially physical during implications summertime of the when surface warmer inflow Mediterranean reversal, abrupt waters increases outflowed in the into SST the fieldmild were Atlantic revealed, Ocean (Figureespecially 12). during During summertim August 2016,e when the aforementioned warmer Mediterranean CCC showed waters up theoutflowed 8th and intolasted the 10 mild days Atlantic (Figure Ocean12a). During (Figure this 12). period During (delimited August 2016, by green the aforementioned dotted lines in Figure CCC showed12a–d), theup ◦ ◦ ◦ theSST 8th field and experienced lasted 10 adays net increase (Figure of12a). 6 C, During from 18 thisC period to 24 C, (delimited according by to thegreen in situdotted estimations lines in ◦ Figureprovided 12a–d), by Tarifa the andSST Punta-Carnerofield experienced buoys a net (Figure increase 12b,c). of 6 Soon°C, from afterwards, 18 °C to a 24 sharp °C, decreaseaccording to to 17 theC inwas situ recorded estimations in few provided hours (steeper by Tarifa in the and case Punta- of TarifaCarnero buoy), buoys as a (Figure consequence 12b, c). of theSoon reestablishment afterwards, a −1 sharpof the intensedecrease (above to 17 130 °C cmwas·s recorded) AJ flowing in few eastwards hours (Figure(steeper 12 ina). the In thecase southern of Tarifa region, buoy), the as SST a consequence of the reestablishment of the intense (above 130 cm·s−1) AJ flowing eastwards (Figure 12a). In the southern region, the SST remained rather constant (~16.5 °C) and obviously unaffected

J. Mar. Sci. Eng. 2019, 7, 3 11 of 16

J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 11 of 17 remained rather constant (~16.5 ◦C) and obviously unaffected by the northern CCC (Figure 12d). by the northern CCC (Figure 12d). Notwithstanding, as easterly winds progressively dominated the Notwithstanding, as easterly winds progressively dominated the study area and persisted enough, the study area and persisted enough, the CCC broadened and the complete inflow reversal transported CCC broadened and the complete inﬂow reversal transported warmer Mediterranean waters to the warmer Mediterranean waters to the west through the entire transect, as reflected by the west through the entire transect, as reﬂected by the pronounced SST maximum (~22.5 ◦C) recorded in pronounced SST maximum (~22.5 °C) recorded in Ceuta buoy but surprisingly delayed by several Ceutahours. buoy A secondary but surprisingly peak of delayed SST was by monitored several hours. only Ainsecondary the northern peak shoreline of SSTwas by the monitored end of the only inmonth the northern (delimited shoreline by purple by thedotted end lines of the in month Figure (delimited9b, c). The bytemperature purple dotted increased lines infrom Figure 18 to9b,c). 22 ◦ The°C, temperature approximately, increased whereas from in 18the to southern 22 C, approximately, area of the SoG whereas no relevant in the southern changesarea in SST of the were SoG nonoticed relevant (Figure changes 12d). in SST were noticed (Figure 12d).

FigureFigure 10. 10.( a(,ab,b)) Monthly Monthly time time seriesseries ofof EuropeanEuropean Center of of Medium Medium Weather Weather Forecast Forecast (ECMWF) (ECMWF) 3-h 3-h zonalzonal wind wind at at 10 10 m m height height (U-10),(U-10), spatially-averagedspatially-averaged over the the Strait Strait of of Gibraltar, Gibraltar, for for February February and and DecemberDecember 2017, 2017, respectively. respectively. Red Red dots dots denote denote specific spec selectedific selected events, events, defined defined in Table in2 ;(Tablec,d) 2; Monthly (c,d) timeMonthly series time of hourly series sea of surface hourly height.sea surface (e,f) Monthly height. Hovmöller(e,f) Monthly diagrams Hovmöller of High-Frequency diagrams of High- Radar (HFR)-derivedFrequency Radar zonal (HFR)-derived current speed zonal at the current selected speed transect at inthe the selected Strait of transect Gibraltar. in 2-daythe Strait episodes of ofGibraltar. permanent 2-day flow episodes reversal of are permanent marked with flow black reversal boxes; are (g –markedn) Hourly with maps black of boxes; HFR-derived (g–n) Hourly surface circulation,maps of HFR-derived corresponding surface to the circulation, eight selected corresponding events, labeled to the in eight the upper selected panels. events, labeled in the upper panels.

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J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 12 of 17 Table 2. Description of eight events categorized according to different met-ocean conditions. Table 2. Description of eight events categorized according to different met-ocean conditions. Westerlies (U-10 > 0) Easterlies (U-10 < 0) Westerlies (U-10 > 0) Easterlies (U-10 < 0) TidalTidal Cycle Cycle SSH SSH (m) (m) Event Event (date, (date, hour) hour) SSH SSH (m) (m) Event Event (date,(date, hour)hour) SpringSpring tide tide 0.77 0.77 1 (12 1 (12 Feb, Feb, 3 pm) 3 pm) 0.84 0.84 5 5 (4(4 Dec,Dec, 22 pm)pm) SpringSpring tide tide −0.38−0.38 2 (12 2 (12 Feb, Feb, 8 pm) 8 pm) −−0.500.50 6 6 (4(4 Dec,Dec, 99 pm)pm) NeapNeap tide tide 0.52 0.52 7 (11 7 (11 Dec, Dec, 8 am) 8 am) 0.30 0.30 3 3 (20(20 Feb,Feb, 99 am)am) NeapNeap tide tide −0.20−0.20 8 (11 8 (11 Dec, Dec, 1 am) 1 am) −−0.070.07 4 4 (20(20 Feb,Feb, 33 pm)pm)

Figure 11. 2-day2-day averagedaveraged maps maps of of High-Frequency High-Frequency Radar Radar (HFR)-derived (HFR)-derived surface surface circulation circulation for each for eachof the of six the Atlantic six Atlantic Jet (AJ) Jet inﬂow(AJ) inflow reversal reve eventsrsal events analyzed analyzed during du thering period the period 2016–2017. 2016–2017.

Equally, a similar situation was encountered from from 11th 11th to to 25th 25th of of August August 2017 2017 (Figure (Figure 12e),12e), ◦ with a progressiveprogressive warmingwarming of of 7.5 7.5 C°C at at the the upper uppe oceanr ocean layer layer of theof northernthe northern shoreline shoreline (Figure (Figure 12g). 12g).The westward The westward CCC wasCCC revealed was revealed from thefrom 11th the towards 11th towards the end the of end the month,of the month, and conﬁned and confined at higher at higherlatitudes latitudes except except for the for already the already analyzed analyzed 2-day 2- eventday event of 14th–15th, of 14th–15th, when when a full a ﬂowfull flow reversal reversal was ◦ wasdocumented documented (Figure (Figure 12e) and12e) anand abrupt an abrupt increase increase of 4 C of was 4 °C observed was observed in Ceuta in buoyCeuta (Figure buoy (Figure12h). 12h).

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Figure 12.12.( a(–ad–d) Monthly) Monthly Hovmöller Hovmöller diagram diagram of High-Frequency of High-Frequency Radar Radar (HFR)-derived (HFR)-derived zonal current zonal currentspeed at speed the selected at the transectselected in transect the Strait in ofthe Gibraltar Strait of for Gibraltar August for 2016 August along with2016 monthly along with timeseries monthly of timeseriesin situ sea surfaceof in situ temperature sea surface (SST) temperature recorded by(SST) three recorded coastal buoysby three deployed coastal inbuoys the study deployed area: in Tarifa, the studyPunta area: Carnero Tarifa, and Punta Ceuta. Carnero (e–h) Idem, and Ceuta. for August (e–h) 2017.Idem, Twofor August full Atlantic 2017. JetTwo (AJ) full reversal Atlantic episodes Jet (AJ) reversalmarked withepisodes black marked boxes for with the 13th–14th black boxes August for 2016the (left)13th–14th and 14th–15th August 2016 August (left) 2017 and (right). 14th–15th Green Augustand purple 2017 dotted (right). lines Green denote and the purp beginningle dotted and lines end denote of SST the increase beginning as a result and end of the ofpartial SST increase inversion as aof result the ﬂow of inthe the partial northern inversion coastal areaof the of theflow Strait, in the and northern the subsequent coastal intrusion area of of the warmer Strait, and and saltier the subsequentMediterranean intrusion waters of into warmer the Strait. and saltier Mediterranean waters into the Strait.

4. Discussion 4. Discussion Motivated by the critical relevance of AJ over the upper-layer circulation in the Western Alboran Motivated by the critical relevance of AJ over the upper-layer circulation in the Western Sea, the ability of the HFR system to properly portray the AJ intensity and direction at the midpoint of Alboran Sea, the ability of the HFR system to properly portray the AJ intensity and direction at the the selected transect (Figure1b) was evaluated. A strong relationship between remotely observed AJ midpoint of the selected transect (Figure 1b) was evaluated. A strong relationship between remotely speeds and angles was evidenced: with current velocities above 50 cm·s−1, the AJ appeared to be locked observed AJ speeds and angles was evidenced: with current velocities above 50 cm·s−1, the AJ at an angle around 63◦ (Figures3a and5). This finding is in close agreement with previous results appeared to be locked at an angle around 63° (Figures 3a and 5). This finding is in close agreement reported in Reference [14], where an averaged angle of 60◦ was obtained by using satellite-derived with previous results reported in Reference [14], where an averaged angle of 60° was obtained by observations and hydrodynamic model simulations. The role of the underlying topography (Figure1a) using satellite-derived observations and hydrodynamic model simulations. The role of the on the prevailing current direction is also noteworthy in steeper regions like the SoG since it acts as underlying topography (Figure 1a) on the prevailing current direction is also noteworthy in steeper physical constraint, enclosing the directional variability of the flow. regions like the SoG since it acts as physical constraint, enclosing the directional variability of the The AJ speed usually fluctuated between 50 cm·s−1 and 170 cm·s−1 (Figure3a), with periodic flow. tidal-induced pulses reaching peaks above 200 cm·s−1 as a result of increases in current speed as they The AJ speed usually fluctuated between 50 cm·s−1 and 170 cm·s−1 (Figure 3a), with periodic encountered the constrictions of the local bathymetry [28]. Occasional drops below 50 cm·s−1 were tidal-induced pulses reaching peaks above 200 cm·s−1 as a result of increases in current speed as coincident with abrupt changes in AJ orientation (Figure5). A significant direct relationship between they encountered the constrictions of the local bathymetry [28]. Occasional drops below 50 cm·s−1 the zonal and meridional velocity components was showcased, with the former almost three times were coincident with abrupt changes in AJ orientation (Figure 5). A significant direct relationship stronger than the latter (Figure3c). Such dependence was confirmed by a high and positive correlation between the zonal and meridional velocity components was showcased, with the former almost three times stronger than the latter (Figure 3c). Such dependence was confirmed by a high and

J. Mar. Sci. Eng. 2019, 7, 3 14 of 16 coefficient of 0.74, thereby explaining the recurrent propagation direction of the surface stream when it was directed towards the North-East. The AJ pattern was observed to follow a marked seasonal cycle (Figure4), in high accordance with the sequence documented in the literature [9,14]. A stronger AJ flowed towards the NE during winter, whereas a steady weakening in mean AJ speed was noticed along springtime. A minimum was reached by summer when a weaker AJ was directed more southwardly. During autumn, the AJ intensity partially recovered and the currents flowed again predominantly to the NE. Notwithstanding, the main discrepancy with earlier works lies in the higher AJ intensity here reported: whereas HFR-derived mean speed was about 110 cm·s−1 (reaching peaks of 200–250 cm·s−1 regardless of the season), previous modeling works substantially underestimated monthly-averaged AJ speeds, always reporting values below 50 cm·s−1 [14]. Six full reversal episodes of the surface inflow through the SoG, persistent during at least 48 h, were detected by means of Hövmoller diagrams of HFR-derived zonal velocity component (Figure6) and subsequently analyzed in terms of prevailing synoptic conditions (Figures7 and8) and wind-driven circulation patterns (Figure 11). High pressures and intense (above 10 m·s−1), permanent, and spatially-uniform easterlies prevailed over the entire study domain, inducing a westward outflow through the SoG as revealed by the 2-day averaged HFR circulation maps. Local wind forcing at this scale seemed to play a primary role in explaining such AJ collapse and the related inflow reversals. It has been previously well-documented how the interface fluctuates at the northeastern entrance of the Strait in response to local wind forcing [25]. Under strong westerlies, the interface depth rises (reducing the upper layer thickness) and the eastward HFR-derived current speed increases, being fully representative of the entire upper layer flow. Conversely, under prevailing intense easterlies, the interface becomes deeper (increasing the upper layer thickness) and HFR estimations are thereby illustrative of only the shallowest part of such a layer. Future efforts should address the synergistic integration of HFR estimations and outputs from a 3D high-resolution coastal model to provide further insight into the entire water column and comprehensively characterize the circulation in the SoG and the water exchanges at different depth levels. According to both Table2 and Figure 10, tides appeared to play a secondary role in the full flow inversion as the six episodes were observed during strong easterlies, regardless of the tidal phase. The westward inflow reversal was moderately accelerated (decelerated) during the tidal outflow (inflow). On the contrary, the Atlantic inflow into the Mediterranean was favored and intensified under westerlies: the tidal flow only partially sped up or slowed down the eastern flowing current, depending on the phase of the tide. These results are in accordance with previous modeling studies [26] where the contribution of the semidiurnal tidal component to the transport was proved to be relevant over the Camarinal Sill, (incrementing the mean transport by about 30%, both for the inflow and the outflow), whereas it was almost negligible at the eastern end of the Strait. Equally, Reference [27] described the structure of the barotropic tide (sea level oscillation) and showed that the amplitude of the prevailing semidiurnal constituents diminished more than 50% from the western to the eastern end of the Strait and had little cross-strait structure. One of the direct physical consequences of persistent inflow inversion was the sudden and abrupt rise of SST (above 5◦) in the SoG during summertime when warmer Mediterranean waters were injected into the mild Atlantic Ocean (Figure 12). Some of those SST increases were confined to the northern shoreline of the Strait due to the presence of the CCC. Such coastal flow inversion has been previously described and is the subject of further analysis in Reference [25], where the CCC was demonstrated to exist regardless of the wind direction, being substantially intensified and broadened during predominant strong easterlies. Ongoing efforts are devoted to conduct a multi-model inter-comparison exercise, ranging from global ocean models to regional and high-resolution coastal models in order to elucidate: (i) the accuracy of each system to characterize the AJ dynamic; (ii) their ability to adequately reproduce the singular and sporadic small-scale coastal ocean process here described, the quasi-permanent J. Mar. Sci. Eng. 2019, 7, 3 15 of 16 full reversal of the AJ surface flow; and (iii) quantify the added-value of dynamical downscaling approaches in coastal areas. As an essential part of the coastal ocean observing system [29], HFR only provide information of surface currents. To better study three-dimensional coastal ocean circulation variability, it would be useful to adopt a multi-platform approach by combining HFR estimations with other observational data sets, such as in situ observation from moored ADCPs and satellite remote sensing products [30,31]. Finally, it is worth mentioning that the present three-site HFR network is planned to be extended, with the imminent integration of a new site (denominated Camarinal and managed by the University of Cádiz, Cádiz, Spain) to better monitor the coastal ocean dynamics in the westernmost section of the SoG. Within this framework, the top priority issue is not only the maintenance of continued financial support to preserve the infrastructure core service already implemented, but also the pursuit of funding to extend the network for an overall coverage.

Author Contributions: conceptualization, P.L. and S.P.; methodology, P.L.; software, P.L.; investigation, P.L.; writing—original draft preparation, P.L. and S.P.; writing—review and editing, M.G.S.; supervision, E.Á.-F. Funding: This research received no external funding. Acknowledgments: The authors gratefully acknowledge Qualitas Remos Company (Madrid, Spain) (partner of CODAR) for their useful technical suggestions. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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