I' .

Not to be cited without prior reference to the authors

International Council for theExploration of the Sea

CM 19921C:19Hydiogmphy Coinm.it~

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HYDROGRAPIDe OBSERVATIONS AT THE CONTINENTALSHELF EDGE NORTHWEST OF SCOTliAND

by

W R Turrell, E W Henderson, G Slesser,R Payne and R D Adams

SOAFD Marine LaboratoryPO Box 101; Victoria Road

Aberdeen AB9 BDBScotlnnd, UK

ABSTRACT

ResultS from an experiinent northwest of Scotlnnd are presentCd during which sixrecording current meter moorings were deployed nnd recovered; arid two hydiographicsurveys were performed (June and September 1989). Results from two moorings deployedon the coritinentnl slope confirmed the presence of a persistent shelf edge current.Computed volume transports in the slope current and on the shelf agreed weIl withpreVious estimates.The experiment took place during a pei-iod of noted high salinitiesin the northwest Atlnntic, and this nnomaly is discussed in the light of the observationsobtained within the slope current. _

INTRODUCTION

Acomponent ofthe oceanographic studies conducted overthe past 20 years by the ManneLaboratoI-y, Aberoeen, has been directed at the description and underStD.nding of theresidual circulation ofthe riortheni North Sea nnd adjacent continental shelfareas. Thesestudies have beeil in support of fisheries research and pollution monitonng.

Results from the major international JONSDAP (Joint North Sea Data AcquisitionProgramme) exercise conducted during the 1970s included evidence of a predominaritlywind driven cycloriic circulation in the northern Noi-th Seri. (Fig. 1), comprised partly ofthe Fair Isle and Dooley Currents (Dooley and Furnes, 19B1). These observations tookplaceduririg a winter period wheri the northern North Sea was vertically well-mixed.Theresults were extended to iriclude an examination of the circulrition wider stratifiedconditions dUrlng the Autumn Circulation Experiment (ACE) in 1987/1988. Observationsfrom this experiment suggested a significant non-wind driven componerit ofthe Fair IsleCurrent existed in ihe northern North Sea during the sllIIlrilcr months (Turrcll et aZ., .

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1990). Substantial evidence was also obtained ofa persistent inflow ofAtlantic water eastofShetland (Fig. 1) which resulted in high salinity water entenrig thc North Sea ofTsharefrom the Fair Isle Current (Turrell et al., 1992).

The origins of the Fror Is1e Current west of the Orkney/Shetland gap ,were alsoinvestigated duringACE. The current was eiamined elose to the West Orkney "C"(wo"e") mooring 10eation(Fig. 1), and foünd to be tightly constrained by the topographyat this point, and to be predominantly wind driven throughout the year (Turrell et eil.,1990). Comparison ofthe along-slope volume transport Within the current with wind data'obtained from a meteorologiCal station on OrkDey indicrited a directional response similarto that predictcd by a wind-diiven two-dimensional numeriCal model (Pingree andGriffiths, 1980). MaXimUm flow into the North Sea occurred for winds from thesouthwest, and outflow occurred for Winds froin the north and cast.

Volume flux budgets f~r the northem North Sea and cantinental shelf west of Scotlandduririg thc periods October 1987. axid January i988 .werc, attempted using transportsestimated from eurrent meter observations combined with salinity estimates ofthc variouswater inasses involved (Ttirrell et eil., 1992). Some of the transports derived for thc -stratified period October 1987 are indicated on Figurc 1. •

It was estimated thai 17 x 104 m3S·l of Scottish coastal water (SCnVS) - T-ii.6°C

S-34.80psu) entcred thc shelf area north of Scotland. This water was niost probablyderived from 8 x 104 m3

. S·l of Irish Sea/Clyde water(lS/C - T-12.8°C S-34.20 psu) flowingnoi-th along the west Scottish coast, :IDixed.with 9 i 104 m3

S·l of Atlaritic Water (AW -T-I0A°c S-35.30 psu) drawn onto thc shelf dunng the northward passage of the coastalcurrent. Of the 17 x 104 m3

S·l of coastal water entering the shelf area north of Scotlarid,it was estimated that 6 x 104 m3

S·l miXed with an inflow of 11 x104 ni3 S·l of Atlanticwatcr, probably in the complex area around the fslarid ofRona. Out ofthis arca, and asaresult of thc niixtui-e of thc twowater types, flowed 16 x 1ü4 m3 S·l of Fair Isle CuiTentwater as observed west ofOrkney (FIC(WO) - T-ll.2°C S-35.13 psu). This transport flowedtowards thc North Sea as the Fair Isle Current, followmg the 100 m coritour thrOughmooring ,WO"C" and entcring through the tidally mixed area between Orkney imdShetland.

The remaining 11 x 104 m3S·l ofcoastal water also entered the Orkney/Shetland passage

rather thanflowing into the North Sea through the narrow Pentland Fii-th. Thc mixingthilt occurred during the passage between Orkney and Shetland resulted iii an iricreasedtransport within the Fair Isle Crirrent, but oflower saIinity water. Tbe Fair Isle CurrentaS,obscrved inthe North Sea (FIC(NS» had characteristic temperatures of 10.8°C andsalinities of 35.05 psu and a mean transport of 20 x 104 m3 S·l•. This implied thät7 x 104 m3

S·l of coastal water flowed south inshore from the Fair Isle Current inta theScottish North Sen coastal zone.

Thc above transports were derived from consfder~tions of mMs and salt continuity, Withobserved transports restricted ,to those in the Fair Isle Current immediately west ofOrkney combinedwith observations in thc northem North Sea. No measUrements cDstedof circulation of transport in the topogfaphically, complex area around Roriä and .thenorthwest of Scotland. The present study was initiated to examine. the coritinental shelfand slope urea northwest ofScoUand where it was believed the Fair Isle Current derivedits source water. In particular thc experiment was designed to examine t4e relationship

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between the slopc current CHuthnunce, 1986) and the amenint ofwriter impinging ontO thcshelf, and how this aITectcd the composition und magnitude of the Fair Isle Current.

In. addition, the experiment coincid~d WiÜi the period of high salinities obserVed inAthintic Waters both west of Scotland (Ellett lmd Turrell, in press) arid in thc northemNorth Sea (Herith et al.; 1991). Hence the hydrographie rind current observations mayadd some ftirlher insight into this anomalous penod.

DATA!"', J

Mooring Dcp]oymcnts

,Seven cu~rentmeter moorings were dcpioyed on the continental sheir lind slope noi1.h~estof Scotland between June 1989 arid September 1989 (Fig., 1). or these an were

. successfully recovered except one; thc North Hebrides moodng (58°43.6'N 6°47.S'W). TWocurrent meters were deployed on eaeh mooring; a nerir-surface meter at a depth ofapproximately 40 m, änd a nerir-bcd instrument 10 m above the bottorn. The instrumenttypes were Aarideraa RCM4 and RCM7s. In addition an Aanderaa Water Level Recorder(WLR) was" deployed on the sea bed at tbe East Rona (ER) moonng. Records from thcNorlhwest Flannan (NWF) inooring cerised at the beginmng of August owing 10 removalof tbe inooring by fishing gear, and thci record from thc riear-bed instrument at the ERmoonng ended earlyowing to instrument mrilfunetion. All otlier recovered records provedvalid over the fuH deployment period.

TWo tidal coinponents (M2, S2) have been caIculated froIn thc reeoids recovered from theinstruinentS using a standard harmonie analysis programme. In addition low-freqüeneycurrents have been examined by applying a low-pass filter to the cUfrent meter data(Godin, 1967). Volume transports have been computed usingthe sections desCrlbed below.

Wind DaÜi

In addition to the CUITEmt meter rind ~aicir level reeorder drita; Wind data bas beenobtained from Stornoway in the north of the Hebddes.

• Hydrographie Survcys

TWo hydrographie surveys were perfornied, one during thc deployment of the currentmeter moonngs between 21 and 30 Jtine 1989, aIid one during the recovery of tbemoorings between 26 September and 6 Oetobcr 1989. MeasurementS were made usingan Applied Micrösystems STD12 CTD imd a Neil Brown "Smart" CTD. ,Water sampIeswere also collected, to check the CTD data, using KriudScn reversing water bottles.

RESULTS

Cürrcnt Meter Data

Table 1 provides a summary of thc mean residual currents reeofdrid over the fulldeployment penod at erieh ofthri cuITent meter loeations, while .Tables 2a aild2b presentthe results of the harmonie analyses öf the cill-rent meter records. Vec::tor plots of each

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,current meter recoro time-series rire presented in FigiIres 2a and b. A vector plot ~f thewmd reeord obtamed from Stornoway alorig With the low-pass mtered reeord of sea bedpressure obtained at the ER mooring nie presented in Figure 3.

TIdal ellipse parameters: in general these agree weIl with published chartS of thc area(Howarth; 1990). The amplitudes of thc semi-major rixis obtained at thc North Milleh(NM) mooring, however, ure greater than previously detennined (+ 10 ein S·l), whilethesenses ofrotation ofthe S2 componEmt at the slope moonngs are in the opposing directionto thnt indicated by thc charts.

Low 'frequency ,eurrents: The recOrds fram thc two slopc moormgs (NWF and NWH)revealed a stable (>99%) and persistent eurrerit flowirig towaros the northeast along thetopographY. Reduction of the flow, particularly at the near-surfaee instrument at NWH;occuired for orie major event during the deployIDent; between 3 ririd 10 September 1989.CurrentS ät the lower instrument at mociring NWF gener8.lly cxceeded those rit the near­surface instrument. A stable (>95% at 40 ni) northeristerly flow also existed west of thcHebrides (WH). .

At the North Minch mooring <NM) flow was generally directcd towardS the north out [rom ' •thc Mmch, althciugh periodS of reversal occurred. Thc rcsultS from the mooring cast ofROna revealed less stable currcnts ofsmaller amplitude, although again generally directedtowards thc northeast. Results from the WO"C" mcioring showed another stable (>90%)eurrent directcd towriros the eaSt, again alorig thc topography.

At each mooring the pnncipal direction (along which the residual eurrents demOIistratemaximum variance) derived from the near-bed instruments coincided With thc direetioncifthe Ioeal tcipography as determined usirig Admiralty charts. These directions have beenused in order to computc along-slope and across-slope current componcntS. Monthlymeans or"these time-series are presented in Tables 3a and b. . '

Lciw-freguency wind events: The pericid ofmooring deployment began with relatively weakand ,variable wirids (Fig. 3a) associated with a region of high atmospherie pressureestablished over the North Atlantic., Wind speeds began to increase from mid-Augustfollowrng the disappearance ofthc high pressurc area aild a resumption ofthe eaSterIyprog:fess of cyclonic wenther systems to thc north of Scotland. A blockirig situation againcommeneed duririg 25-26 August, äiid lasted until 2 September, with an associated •reduction in wind speedS. Tbc northwest European shelf was then subjected, tosouthwesterly wirids as the result ofalow pressure system developing over Ieelrind. Highpressure again becanie established by 8 September. Onee this blocking miti-cyclone hadbeen removcd, a particwarly intense low pressure area developed over the North Atlimticduring the 16-17 September, and travellcd towardS the northeast to the west of theBritish Isles., .This resUlted in strong southerly winds being iinposed over ScoUandbetween 18-20 September, which, it may be postulated, produced a storm surge resultingiIi thc even~ in sea-bed pressure recorded at thc East Rana (ER) mooring (Fig. 3b).

Alom!-slope currents: It is clerir from Tables 3a and,3b that significant along-slopecurrents existed at the two slope moorings NWF and NWH. Along-slope speedS at NWHare typically 20 cm S·l, With a mean speed of 13 cm S·l at thc near:-bcdinstrument. Tbereduced morithIy meriri speed in September may have been thc resUlt ofthc reversal eveiitnoted above. Along-slope. spee~ at thc near-bed instrument of thc NWF mooring

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excecded thosc at thc ncar-surface instrument. Mean along-slope speeds at the cither, moorings are gcncrally <10 cm S·l. .

.Across-slope currerits: Across-slopc currcnt speeds werc generally an order ofmagnltudcsmaller than the along-slopc speeds. Flow was directed onto thc shelf at thc NWFniooring throughout thc deploymcnt, whilc at the NWH mooring, ftii-ther offshore, thcacross-slopc componcnt of current väried, bcing directrid onto the shelf during July aridSeptembcr arid off thc shclf dunng August. .

. Voiunie transports: Monthly mean volume transports have bcen ciU.cuiated using thcalong-slope mean current speeds givEm in Table 3a, multiplied by the. appropriate cross­sectional area at each mooring site. Tbc sections einployed to calculate the transports areshown in Figurc ,4. Apart from thc slopc section, a standard section Width of 15 krii hasbcen employed. Thc approxi.iD.atc depth-mcan saIinities these sections eneompass are alsoshom in FigUre 4. The resulting mcan volume transportS are preserited in Table 4, andthc overall mean transports iridicated in Figure 5.

Hydrographie SurvcysThc survcys performec! consisted of sections perpcndicular to thc coast, approxiiriatelythrough thc mooring sites, and additional standard sectioris in the noi-them North Sea(the JONSIS arid EC sections, Turrell et Cll., 1990). FigUres 6ri and b show thc stationlocations in relation to thc mooring sitcs, and also contoured values of bottom salinity.

Horizontal distribution cif bottom salinity: The bottom saÜnity obscrved iriJune 1989(Fig. 6a) revealed some general features of thc circulation; High salinity water (S>35.35)was present along the slope, through moorings NWH and NWF. Some intrusion anto theshelf occurred west of Rona coinciding with a change in direction of the isobaths towrirdsthe north. Intermediate saliIiity boUom water was constrained weIl inshore,both westof thc Hebrides arid.west of Orkriey witli water of S>35.11)ring over much of thc shclf.Low salinity coastal water (S<35.0) lay in a naiTow strip along the ScotÜsh coast.Companson of the large gradients at thc North MinClt (NM) mooring, marking thcoffshore boundary of thc low salinity coastal water, with thc less stecp gradients andgreater salinities wcst of thc Hcbridcs might iridicilte that; at this. time, most of thecoastal cuiTent flowed north through thc Minch. Thc passage ofthc Fair Isle Currerit iotathc riorthcril North Seawas traccd by thc 35.1 isohaline. An arca exhibiting a salinitymaXimUin (S>35.3) lay offshore from thc Fair Isle CuiTent in thc North Sea arid may havcbccn associatrid with water from the East Shetlaild Atlantic lriflow (TWTell, iri press).

Conditions iri September i989 wcrc vCIY simiiar. Again a strip of high salinity bottolnwater comcided with the strong residual cUITcnts abovc the slope, thcFair ~sle Currcntwas aSsociated with the 35.1 isohalinc, and thc northem Norlh Sea eXhibited a salinitymaximum offshore frOIIl the Fair Islc Clirrent, with bottom water ofreduced saliriity lyingoffshorc from this. Conditions had altered west of thc Hebridcs, with coastal water(8<34.9) obscrved at thc inshorc stations, und stecp gradiörits obscrved offshore from thiswater, probably marking thri boundriry of the northerly flowing coastllI current. Withinthc Minch a northerly directCd tonguc of higher srilinity (S>35.1) water was observed.This coincided with verj wriak and occasionrilly revcrsed flow in thc Minch (Fig. 2),stiggesting that most of thc coastal CUrTcrit dliririg this period may have flowed iiorth, tothe west ofthe Hebridcs; Extremely low saliriity water was found at the most easterlystation in the Minch (8<34.4).

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Shelfsections - June 1989: Vertical sections oftemperatw-e and sallillty through thc shelfcurrent meter moorings obtained dunng June 1989 are shown in Figurc 7. Thc.WcstHcbndcs (WH) mooring lay inshore froin the maxiinUni horizontal salÜlity gradicntS irian ärea of domed isotherms. A core ofhigh saIiDity water (S>35.4) was obscrVcd at thissection lying above thc 150 misobath at a depth of 50:-100 In.· The section tlITough thcNorth Minch (NM) moormg agwn demonstrated that the mooring laY,Within a zone ofsteep honzontal salinity gradients marking the efTshore boundary of low salinity watcr.

The section th:fough the East Rona (ER) mooring showed that thc mooiinir lay ofTshorefrom the maximum horizontal salinity gradients, in a region of homogeneeus salinity of8>35.3 psu..Thus most of thc. mixirig that occUrred between Atlaritic Watcr and coast3Iwater took placc inshorc from thc ER mooring. The temperaturc field at thc mooririg sitedemonstrated some indication of reduced vertical stiitification either as the result of,increased vertical mixing mound Rona, or as the result of thc adveCtion of mixed waterinto the area. Tbe section tlu-ough the West Orkney "C" meoring also showed that thccurrent there, located abeve thc 100 m contour, was associated with increased verticalhomogeneity with reduced vertical temperaturo gradientS.. Tbe area of thc sectionoccupied by water ofsalinitiesbctween 35.1psu and 35.2 psu had increased in theWestOrkney section compared to the East Rona section~ as had the mean temperature of thiswater. The area of maximum honzontal salinity iri the East Rolm Seetion was confinedto the inshorelOO m contour lying io thc south ofthedeeper channel separating the Ronaplateau from the mairilroid. This contour continues towurds thc West Orkney "C" site.

Shelf sections - September 1989: The shelfsections in September 1989 demonstrated thatalmost completc vertical mixing had occurrcd by thEln, with thc removal of the seasonalthermocline .(Fig. 8).. Saliriity at thc West Hebrides (WH) moeririg rcmmned atapproximately 35.2 psu although niean temperattires had risen to 12.5°C. Conditions inthe Minch remained similar, with low saIinity water agailist the Scottish coast~ aridcentral and bottom waters having greater saliniÜes.

The East Rana secHon again showed maximum horizontal salinity gradients werc lo~tedabove the inshorc 100 m contour, although these were associated with reduced salinitiesof between 34.9_psu and 35.1 psu. Tbe saliriities above the 100 mcontour at the WestOrkney "C" mooring were also less than iri June 1989, being between 34.8 psu and35.0 psu. .

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Slope sections: The secÜons ebtained tbrough the slope. moorings in June 1989 ami •September 1989. ure shown in Figure9. In June 1989 the seasonal thermoclinc laybetween 50 ni lind 80 m on the shelf, deepening and becoming less intense at the shelfedge. Thc near-bottom instrimient at thc North West Flannan (NWF) moering lay 8.t theinshore edge of a corc of high salinity water (S>35.4 psu). This watcr lay betwcen 100 mand 250 m depth, above the 300 m coritour, with the highest salinity obserVed within thecorc being 35.413 psu.

In September 1989, although the shelf stations exhibitcd vertical homogeneity, a deep. (100 m) thermoclinc persisted ofTshore from the 100 m contour. A cora of high salinity. water (S>35.4 psu) ,was again observed above the 300, m contoiIr, although it had

expanded in areal extent. The maximum salinity observed in this section was 35.431 psu.Greater horizontal salinity gradients were present on thc shelfadjaecnt 10 thc eoust ofthcHebrides where, as noted above, water of low salinity was found (8<35.0 psu).

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Teniperature-salinÜ,y eharaeteristies: Teniperature-Salinity diugfarD.s for jtinc 1989 aridSeptember 1989 are shown in Figure 10.,The menn temperaturcs arid aalinities at eaebmooring site are also indicated. The North Minch (NM) mooring was located within waterwitb salinities typical of Scottisb west coast water (SC(WS) - TUrrell et al., 1992). TheWcst Hcbrides (WH) inooring lay in water whose salinity bad increased from coastUJ.values Wter mixing with Atlnntic Water drawn onto the sbelf. The NWF arid NWHmoonrigs lay in core slope water eXhibiting inaiirilUm saliriities rind relatively warmerteinperatures compared to the higb salinity water found Oll. the shelf. WithiD tbe NorthSea a tJpiciU T-S eurvc was observed (TurieIl, in press); moVing out from tbc eoast of

\ OrkD.ey warm, low saIinity coastaI water was observed, followed by water associated witbthc Fair Isle Current; offshorc from which lay a salinity maXimum associated Witb theEast Sbetlrind Atlaritic Inflow. Offshorc from this maximum, temperatures and saliniiiesfell to those of Cooled Atlantic Water (CAW) found in tbe,deep basin ofthe northernNorth Sea. SaliD.ities at the eaSt ROna Inooririg were similrir to those obscrv-ed in the

. nortbern North Sea salinity maximum, althoügh temperntures were greater., , . l , <

Conditions in September had altered,witb tbc near-bottom shelf water west of Oi-lciieygenerally warmcr resulting in a fully. developed T-S ctirve similar to thcit observcd in thenortberri Nortb Sea at that time. The water at the NWF moonng again demonstmtedextreme temperaturc lind salinitycharaetcristics eomparcd to water adjaeent to the coreslope watcr. Salinities rit the east Rona mooring were also similar to those obserVed intbc northern North Sea salinity maximum;

DISCUSSION

Thc cxistencc of a pcrsistEmt slope current bas for some time nowbeen weIl doctimented(eg Booth arid Ellett, 1983; Huthnanee, 1986). The dYnamits of this currcnt bavc beeninvestigatcd, lind it is most likely that the current is predominaritly driven by anortll-south pressure . gradient generated by meridional temperature differentes(Huthnarice, 1986). Hencc tbe stability nnd persistence of thc currentS obserVcd at thcNWF iind NWH slopc moorings dtiring this experiment. Dooley and Meinkc (1981)obtained similar results in the slope current northwest ofSnetlnnd. They found aconstnnt current of 20 cm S·l whieh was unrelated to the,Wind, altbough certain everitSiri the recoro did appear to coincide with the passage of stoms. Dooley et. al. (1976)suggcst that windS greater than 15 ins·1 pcrsisting over aperiod greater thiin thiee driyswere rcquired in order tö significaritly alter tbe slope current.

Thc loeation ärid continuityoftlle current has aIsobeen examined prcviousiy. Bootb andEllett (1983) obserircd astable current west of St Kilda abovc tbc 1,000 m contOur, Withmenn speedS iri the range 9-16 cm S·l. Inshore from this; above the 250 m contour, thecurrents were weak and vanable, being generaIly directed towrirds tbe southwest duriIigtbc summer, and reversing during tbc winter. The results from thc present study suggestthat no suCh inshore counter flow exists to thc northwest of the Hebiides during thesummer months. Edelsten et al. (1976) suggest that tne inshore counter flow may bedriven by the horizontal density gradients obscrved on thc Malin Shelf to the south of thepresent sttidy area.. Here cold dense bottom wüter persists on thc shelf through thesUnimer inonths, isolatcd from thc incursion of Atlantic Wnter by processes occumng 8tthc ahelf edge.

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Hydrographie observations made dUring June 1989 suggest that.wthough bottom watersare indeed cooler thari. those abovo the seasonal thennocline, they do not maintain wintertemperatures to the same extcnt as observcd furthcr south, suggcsting greater exchangein the narrowshelf region northwest of the Hebrides. The absence of an inshore reverseshelfflow confinns that a similar dynamical balarice as suggcsted by Edelsten et eil. (1976)does not cxist to the northwest of the Hebrides.

The 10eaÜon of the core of the sl~pe current in relation to the topography has also beenexamined at severalloeations along thci European shelfedge. Pingrce and Le Cann (1989)observed the slope cuircnt to thä southwest ofthe Uriited Kingdom; aroUnd Chapel Bankon the edgc of thc Ccltie shclf, and found thc corc of thc current thore lay above the100-300 m contours. Dooley arid Martin (1969) obscrvcd a corc of high salinity waterlying above the 200-300 in contotir, extcnding from thc west of Ireland to the northwestofShetlaIid; while Dooley and Creasc (1978) cstimatcd the core ofnortheastedy transportofNorth Atlantie water within the Faroe-Shetland Channellay 8nove the 500 m eontoUr.Dooley and Martin (1969) suggcst that water flowing along the shelf edgc in depths lessthan 300 ni may enter the northern North Sea around Tampen Bank and along thewestern. edgc of thc Norwegian Trench. Dooley et eil. (1976) found euirents typiCaIly of20 eni 5.1 above thc 200 m eontour northwest ofShetland, while above thc 1,000 m contour .•cUITcnt speeds were typically 40 cin S·l.

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The prescnt study. dCDlOnstrates thai for the area northwest of the Hebrides, while thefllament of maximum salinity lies above the 300 m cantour, along-slope current speedsiricrease offshore, being typically 10 ein S·l ribove thc 200 m contour and 20 em S·l aboveihe 500 in contour. The two-dimensiorial model of Pingree and Lc Cann (1989), drivensolely by an imposed north-south oceanie gradient ofdynamie hcight; indicates that whilcto thc west of thc Hebndes the maximum, alorig-slope eurrents lie between the200-1,000 m eontours, to the northwest of thc Hebrides the currents are corifined to anarea much closer to thc 200 m contour, presumably owing 1.0 the changes in thetopography iha~ oceur in thc region. Noi1.h ofthe area of confinement öfthe eurrent, thcmodel suggests that thc flow broadens significantly With an inflow ofslope water onto theshclf occurring to thc north and south of Rana.

In gencrai the estimate of 1.8 Sv (180 x 10· m3 S·l) for the slope transport occurrlng to thenorthwest of the Hebrides found in this sttidy (Table 4) agrees weIl with preViouscstimates in the same area (Table 5). Thus approximatcly 30% ofthc warm, saline watersthat cntcr the Norwcgian Sea through thc Faroe-Shetland Charinel are supplied by theslope cUrrent. The incrcase in transport along thc alopc towards the north is aecouritedfor by recruitmEmt of oceanie watCr (Pingree and Lc Carin, 1989).

The seasonality of the slope transport has been e~amined by Gouid et eil. (t985). Theydctermined thrit the northwcsterly flow through the Friroc-Shetland Channcl (between thc200 m and 1,000 m contours) was ai a minimum in about July (5 Sv) increasing to amaximum in Deeember (9 Sv). Thc monthly estimates shown in Table 4 also indicatC anincrcasing transport from. Junc (1.6 SV) until Septcmber 1989 (2.0 Sv).. Fromhydrographie observations, Ellett et al. (1986) found that the cuuent broadened in thewiriter, which is also confirmed by Figurc 9. .

Tr~sport of oceanie water onto the shelf to thc northwest of Shetland was examined byDooley et al. (1976). They found typieal current speeds dirccted onto the ahelf at thc200m contour of 0 to 2 cm S·l. Pingrcc arid Lc Crinn (1989) found that within thc slope

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current to the southwest of the Celtie shelf, transport directed örito the shelfocCurred inthe surfacc waters, whilc off-shclf flow occurrcd in the near-bottom water. Results fromNWF mooring (Tablc 3) indieatc that tyPical ori-shelf current speeds in the near-surlaccwaters were 2 ein S·l with smaller an-shelf speedS near thc sea bed (0.5 em S·l)•. ASsumingtypieal speeds direeted onto thc shelfof 1 em S·l approximately 0.8 Sv of oceanic water willcross anto the shelf betwcen secticins northwest of the Hebrides and noith of Shethmd.This agrees weIl with the estiIDated 0.6 Sv of Atlantic Water whieh enters the northernNorth Sea from the riorth (Turrell, 1992). Hence much of the sourec water to thcnortherly inflow tO the North Sea may be derived from thc shelf area northwest ofScotland. Tbe sniall transport towards the northeast at the eaSt Rana (ER) mooring(Fig. 5),ofwater With thc same charaCteristics ofthat creaiing thc salinity maXimUm in

,thc northem Ncii1.h Sea is eonsistent with this hypothcsis.

Thc observations of a stable eurrent at thc West Orkney "C" mooiing flo'Wing towards thceast may be compared tci previous records obtained from the same Ioeation (Table 6). Thceoiisistericy ofresults obtained from this very productive mooring sitc loeated directly onthc 100 ni eoritour, iri eonjunctiori With previous hydrographie studies (eg TuzTell et aZ.,1990), confIrnls thc presencc ofthe Fair Isle Current in this area observed before it entersthc North sea. The directional response of the current to wind stress has been examiriedby Turrell et al. (1990) who found a direct relation similar tö thrit deteriniried by Pingreearid Griffiths (1980) using atwo-diinensional wind-driven model. Turrell (in press) wönton to cxamine thc long-term ehanges in the wind-driven componerit of this current byemploying a 20 year reeord of wind data from Kirkwall on Orkriey.The same data setwaS employed to deterniine thc seasonalityofthe wind-driven Fair Isle Current, and theresults may be seen in Figure 11; The monthly transports ealculated from the reconISobtained at mooririg site WO"C" are also shown.. The monthly-mean transports derivedduririg the preserit study agree weIl with the previously determined seasonal eycle oftransport within thc Fair Islc Current.

Thc observations of high saliriities in the slope hydrographie seeiions eoincided With aperiod öfnotably high salinities in the North Atlantic water iIripingmg ontö the riorthwestEuropean shelf (Ellett and Turrell, in press). The core of high salinity watcr was weIlmonitored by thc NWF mooring during the deployment period, and henee an estimate oftransport,Within this eore may be obtained. In JuxlC 1989 thc core was approximately10 km wide and 150 m deep, while in September 1989, this had expanded toapproximately 20 ki:n widc although the vertical extent had reduced to' 100 in. Aeomparison of. thc area of thccore (approximately 2 x 106 m2

) tö that of the total areaemployed in, thc Calculation of slcipe transport (13.8 x 106 m2

- Table 4) suggestS thatapproximately 15% ofthe total slope transport consistcd ofwater haVing salinities greaterthan 35.4 psu. Thus some 0.27 Sv of high' saliriity water flowed alongthe shelf edgetowards the northwest. Heath et al. (1991) suggest that some of this water entered tbcnoi1.hern North Sea along the western edge ofthe Norwegian Trench; and that salimtiesgreater than 35.45 psu existed Within this water. 'Vater eXlllbitirig ,suCh extremesalinities was not observedwithin the slope eurrcnt northwest of the Hebrides in June1989 or lätc September 1989.

, '~., , ~ .ACKNOWLEDGEMENTS

The authorswould likc to acknowledge the help ,or the, officers and crew of theFRV Scotia. 'Virid data was supplied by the Meteorolcigieal Office, Brackriell.

9

REFERENCES

Booth, D.A. lind Ellett, D.J. i983. The Scottish continent81 slope current. ContimmtalShelf Research, 2(213), 127·146.

CONSLEX. 1984. The UR slope experiment lOS and 5MBA current meter data. lOSInternal Report, 217; 239pp.

Dooley, H.D. and Martin, J.H.A. 1969. Currcnts at the continentaI slope ofthe northernNcirth Sea. ICES CM 1969/C:4. '

Dooley, H.D., Martin, J.H.A. and Payrie, R. 1976. Flow across the continentaI slope offilOrthern Scotland. Deep Sea Research, 23, 875-880.

Dooley, H.D. and Crease, J~ 1978. ObserVed and geostropWc currentS sciuth lind east ofFaroe during overflow '73. ICES CM 1978/C:53.

Dooley, H.D. and Mcinke, J. 1981. Circulatiori and water niasses in the FaroeseChannels during Overflow73. Deutsche Hydro. Zeit., 34, 41·54. •

Edelsten, D.J., Ellett, D.J~ and Edwards, A. 1976. Preliminary results from current. measurements at thc Scottish continental shelf-edge. ICES CM 1976/C:12.

. .

Ellett, D.J., Edwards, A~ and Bowers, R. 1986. Thc hydrography of the ROckallChannel-an ovemew. Proc. Roy. Soc. Edin., 88ß, 61-83.

Ellett, D J and Turrell, W.R. Iri press. Increased salinity levels in the NE Atlantic.ICES CM 19921C:20.

Godin, G. 1967. The analysis of current observations. Int. Hydrogr. Rev., 44, 149.

Gould, W.J., Loynes, J. and Backhaus, J. 1985. Seasonality in slope current transportsNW of Shetland. ICES CM 1985:C:7.

Heath, M., Henderscin, E.W., Slesser, G. and Woodward, E.M.S. 1991. High salinit.y inthc North Sea. Nature, 352, 116.

. .

Howarth, M.J. 1990. Atlas of tidal elevations and currcrits around the British Isles.Department of Energy Offshore Technology Rcport, HMSO. .

Huthnance, J. 1986. The Rockall slope current and shelf-edge processes. Proc. Roy. Soc.Edin., 88ß, 83-102.

McCartney, M.S. and Talley, L.D. 1984. Warm-to-cold water conversion in the northcrnNorth Atlantic Ocean. Journal ofPhysical Oceanography, 14, 922-935.

Pingree, R.D. and Griffiths, D.K. 1980. Currents driven by steady uniform wind stresson the shelf seas around the British Isles. Oceanologica Acta, 3, No. 2, 227-236.

Pingree, R.D. and Cann, B. Le. 1989.' Celtic and Armorican slope arid shelf residualcurrents. Prog. Oceanog., 23, 303-338.

10

•

•

•

Turrell, W.R 1992. New hypotheses concerning the circulation of the northern NorthSea and its relation to North Sea fish stock. recruitment. lCES Journal ofMarineScience, 49, 107-123.

Turrell, W.R In press. The East Shetland Atlantic Inflow. lCES Marine ScienceSymposia,195.

Turrell, W.R, Henderson, E.W. and Slesser, G. 1990. Residual transport within the FairIsle Current observed during the Autumn Circulation Experiment (ACE).Continental Shelf Research, 10, No. 6, 521-543.

Turrell, W.R, Henderson, E.W., Slesser, G., Payne, R and Adams, RD. 1992. Seasonalchanges in the circulation of the northern North Sea. Continental ShelfResearch,12,213, 257-286.

Worthington, L.V. 1976. On the North Atlantic circulation. The John HopkinsOceanography Studies, 6, 110pp.

11

•TABLE 1

Summary statistics oflow-frequency currents observed durlng the depIoyment perlod. Subscrlpts indicate Ioeation ofmeter in water coIumn(1 =near-surface, 2 =near-bed). sn refers to the standard deviation of the current

Current Record Period of Position Sounding Meter ü SDu \t SD. SF% Prinedirn Meanmeter no valid data (m) depth (em S·I) (em S'I) (em S'I) (em S·I) (:1:180·) dirn

(m)

WHI 3121 250689· 58·10.32'N 101 36 6.6 3.8 8.3 4.8 96.0 035· 039·

WH2 3122 260989 7"52.54'W 91 1.0 2.7 3.5 3.5 81.9 036" 016"

NWF! 3111 250689· 58·33.BB'N 197 30 6.8 3.2 4.7 2.4 97.8 056" 055"

NWF! 3112 11 08 89 8"2.21'W 187 8.4 3.0 9.8 3.7 99.7 038" 040·

NWH! 3101 250689· 58"40.71'N 509 60 17.8 5.3 11.9 4.3 99.5 053" 056·

NWH! 3102 260989 8"16.51'W 480 10.6 3.5 7.6 2.6 99.5 055" 055"

NM! 3131 270689· 58"30.57'N 114 29 1.9 3.5 7.0 6.6 78.2 350" 015·

NM! 3132 250989 5"31.08'W 104 ·0.8 2.1 2.6 6.0 47.0 356" 343·

ER! 3141 270689· 59·9.65'N 125 40 2.9 5.7 5.0 4.8 73.3 051" 031·280989 5"16.31'W

E~ 3142 270689· 115 0.3 2.5 4.5 7.3 65.8 016" 004·240889

WO"C"I 3151 280689· 59·9.21'N 101 36 8.5 4.6 0.4 2.3 95.9 084" 088·

WO"C"! 3152 180989 3"55.59'W 91 5.8 2.8 -0.1 2.3 92.9 066" 091"

•

•

-- ------- ---------------

TABLE 2'

Summary of harmonie analyses of the current meter records. Rotational current ellipseis counterclockwise if amplitude of minor axis is positive, clockwise if negatiye

a) M2 componentS

Tidalellipse East component North component

Current Record Semi· Semi· . Orientation Amp Phase Arnp Phasemeter no major minor (em S·I) (cm S·I)

(cm S'I) (ern S·I)

WHI 3121 14.4 1.5 1980 1950'.

4.6 13.8 2140

Wl~ 3122 12.8 4.5 2040 6.7 1580 11.9 2060

NWF1 3111 15.4 -0.8 1760 1.4 420 15.3 2580

..mVF. 3112 12.1 3.1 1700 3.7 1280 12.0 2510

NWH1 3101 7.4 4.3 1950 4.6 1560 7.3 2300.

NWI~ 3102 12.8 0.1 1850 1.0 2310 12.8 2330

NMl 3131 25.2 3.8 1930 6.8 2070 24.5 2420

mr. 3132 19.9 13.2 760 19.5 1730 13.7 2520

ER l 3141 27.9 0.7 2360 23.0 2350 15.8 2380

ER. 3142 14.5 12.5 1270 13.9 2070 13.2 3050

WORC"1 3151 19.5 5.1 0780 19.1 2420 6.5 2950

WO"C". 3152 18.7 7.6 1030 18.3 2360 8.4 3500

b) S2 components. .'>

Tidal ellipse East component North component

Current Rerord Semi· Semi· Orientation Amp Phase Amp Phasemeter no major minor (ern S·I) (ern S·I)

(cm S·I) (ern S'I)

WHI 3121 4.5 1.2 1900 1.5 2080 4.4 2670

WH. 3122 4.3 1.8 1880 1.9 1880 4.2 263°

NWF1 3111 3.8 0.5 1780 0.6 1920 3.8 298°

NWF. 3112 4.1 0.5 1700 0.9 148° 4.1 2950

NWH1 3101 2.7 1.2 2140 1.8 214° 2.3 264 0

NW~ 3102 3.6 0.5 1780 0.5 1790 3.6 287°'

NM! 3131 9.3 2.0 1970 3.3 252° 8.9 2910

mr. 3132 6.8 6.0 2020. 6.1 2170 6.7 . 3020

ER l 3141 9.8 1.7 234 0 8.0 2710 6.0 2910

ER. 3142 6.7 5.1 2320 6.1 2500 5.8 3250

WQRC"1 3151 8.0 2.6 0790 7.9 2690 3.0 3300

WORC". .3152 7.5 3.7 1070 7.3 2560 4.1 90

•

•

TABLE 3

Summary of monthly mean a) along-slope; and b) across-slope speeds. Negative across­slope values indicate flow directed onto the shelf (to the right). Menns derived from thefull records are also presented (OVerall). June means have been derived from only thelast few days of that month (sec periods of valid data - Table 1) and hence may not berepresentativc

a) Along-slope speeds

Current Record Sounding Meter Monthly mean along-slope cUrrent (em S·l)

meter no (m) depthJune July August September Overall(m)

WHI 3121 101 36 5.6 9.9 13.5 8.9 10.6

~ 3122 91 1.2 1.8 3.8 5.5 3.5

NWF1 3111 197 30 5.7 7.2 10.7 - 7.8

NWF. 3112 187 13.7 12.4 14.2 - 12.9

NWHI 3101 509 60 21.6 23.0 23.8 16.4 21.4

~ 3102 480 10.3 13.0 13.6 12.8 13.0,

NM. 3131 114 29 12.5 3.9 10.3 5.3 6.8

NM. 3132 104 8.7 0.7 6.3 -0.5 2.7

ER. 3141 125 40 7.6 4.7 5.7 6.2 5.6

ER. 3142 115 2.6 2.1 7.6 - 4.4

WO·O"1 3151 101 36 2.7 6.4 10.7 6.3 7.9

WO·O"•.. 3152 91 1.8 4.8 5.9 5.5 5.3

b) . Across slopc speeds....

Current Record SoundingMetCr Monthly mean across-siope current (cni. S·l)

meter no (m) depthJune July August September Overall(m)

WH. 3121 101 36 -0.2 <0.1 -0.5 -1.1 -0.5

WIIt 3122 91 1.5 1.0 1.5 1.4 1.3

NWF1 3111 197 30 -1.1 ·2.3 -3.8 - -2.5

NWF. 3112 187 -0.5 -0.5 -1.0 - -0.6

NWHI 3101 509 60 1.1 -0.5 0.4 -1.1 -0.3

NWIIt 3102 480 1.5 -0.1 0.5 <0.1 0.2

NM. 3131 114 29 -2.5 -2.7 -1.5 -3.1 -2.4

NM. 3132 104 -1.1 0.4 1.2 0.4 0.6

ER. 3141 125 40 2.1 -0.1 -1.4 -3.3 -1.4

ER. 3142 115 1.2 1.3 0.5 - 1.0

WO·O"1 3151 101 36 -0.1 -2.9 -3.9 -2.7 -3.2

WO·O"t 3152 91 -2.4 -2.1 -2.3 -3.3 -2.5

•

•

TABLE 4

Volume transports calculated using the along-slope monthly mean current speedspresented in Table 3a. Units are 104 m3

S·l. Flow through the continental slope section(slope total) has been computed using the sum of that through the NWF and NWHsubsections. These are also given in fuH., along with the areas of each section. Thebracketed figures under the North Minch section refer to the transports found ü the fullwidth of the Minch is used in the transport calculations

Current Record Area June July August September Overallmeter no (x106 m2)

NWF1 3111 0.90 5.2 6.5 9.6 - 7.0

NWF2 3112 3.29 45.0 40.8 46.6 - 42.5

NWH1 3101 0.90 19.4 20.7 21.4 14.8 19.2

NWH2 3102 8.72 89.6 113.6 118.3 111.5 113.3

Seetion

Slope total 13.81 159.2 181.6 195.9 - 182.0

West Hebridces 1.50 5.1 8.8 13.0 10.8 10.6

North Minch1.35 14.6 3.3 11.5 3.7 6.7

(7.56 75.3 13.4 57.7 11.3 30.7)

EastRona ·1.50 7.7 5.1 10.0 - 7.5

West Orkney "C" 1.50 3.9 9.2 13.2 9.9 10.7

•TABLE 5

Summary of previous estimates of volume transports within the slope current. Units are x 106 m3S·l (Sv)

Author Source Region Depth range Value Comment

Worthington (1976) Budget FS Channel Entire Inflow 9.0

4.7McCarthy and Talley (1984) Budget FS Channel. Entire Inflow

8.5 High Norwegian Sea heat flux

Shelf - 600 m 4.1 NA and MNA and AIDooley and Meinke (1981) Fitted Geostrophy FS Channel

Shelf - 400 m 2.0 NA

5.0 Summer (July)Gould et al. (1985) Direct FS Channel 200-1,000 m

9.0' Winter. (December)

5.0 Non-wind driven (11 = 1007)

Pingree and Le Cann (1989) Model FS Channel Shelf - 1,000 m2.0 SWwind

Pingree and Le Cann (1989)- Direct W Hebrides 200-1,000 m 2.0 58°N

SW Ireland 200-1,000 m 1.0 52°N.Celtic Sea 200-1,000 m 0.6 47°N

Pingree and Le Cann (1989) Model W Hebrides Shelf - 1,000 m 4.0 Non-wind driven (11 = 1007)

Booth and·Ellett (1983) Direct WHebrides 500 m (±5 km) 0.5

CONSLEX (1984) Direct W Hebrides· Shelf - 1,000 m 3.0 59°N

This study. Direct NW Hebrides 150-800 m 1.8

•TABLE6

Summary statistics of low-frequency eurrents observed during eight deployments at the West Orkney "C" site covering in all over12,400 observation hours. SD refers to the standard deviation of the current

a) Records from near-surface instruments

Reeord Position Period of No obs Sounding Meter Ü SDu v SD. SF% Prine Meanno valid data hours (rn) depth (ern S·l) (ern S·l) (ern S·l) (cm S·l) dirn dirn

(rn)

1271 59°9.4'N 070874- 452 113 33 13.4 5.0 0.3 3.1 97.1 073° 089°3°56.6W 260874

2471 59°9.4'N 010885- 1920 113 41 12.8 5.4 -3.1 3.9 95.4 106° 103°3°56.5W 201085

2661 59°9.3"'N 070886- 1410 99 30 12.0 5.3 -2.4 3.1 95.5 093° 079°3°56.8W 051086

2881 59°9.9'N No data4°5.8W

2991 59°9.2'N 210688- 2126 100 27 11.0 6.4 1.1 3.3 93.2 092° 084°3°56.2W 180988

3151 59°9.2'N 260689- 2058 101 36 8.5 4.6 0.4 2.3 95.9 084° 088°3°55.6W 200989

3231 59°9.3'N 250490- 3801 100 32 7.4 3.9 -0.4 2.3 93.3 101° 093°3°56.3W 300990

4231 59°9.2'N 260991- 647 97 28 5.6 3.1 0.4 2.2 91.4 088° 086°3°56.2W 23 1091

TABLE 6 (continued)

b) From near-bed instrnments

• l

Reeord Position Period of No obs Sounding Meter ü SDu v SD.. SF% Prine Meanno valid data hours (m) depth (ern S·l) (ern S·l) (ern S·l) (ern S·l) dirn dirn

(rn)

1272 59°9.4'N 11 08 74- 367 113 88 6.3 2.3 0.5 1.9 96.3 059° 086°3°56.6'W 260874

2472 59°9.4'N 010885- 1920 113 93 7.6 4.8 0.8 2.5 91.9 094° 084°3°56.5'W 201085

2662 59°9.3··N No data3°56.8'W

2882 59°9.9'N 190987- 1725 107· 94 6.4 6.3 -3.9 3.5 83.9 111° 121°4°5.8W 301187

2992 59°9.2'N 210688- 2301 100 90 7.4 4.6 0.2 2.9 90.3 092° 089°3°56.2W 250988

3152 59°9.2'N 260689- 2058 101 91 5.8 2.8 -0.1 2.3 92.9 066° 091°3°55.6'W 200989

3232 59°9.3'N 250490- 3801 100 90 5.6 3.1 0.4 2.2 91.4 105° 086°3°56.3'W 300990

4232 59°9.2'N 260991- 647 . 97 86 6.3 5.6 -0.5 4.4 79.2 101° 0940

3°56.2'W 231091

. . . ,

FIGURE LEGENDS

•

Figure 1

Figure 2

FigUra 3

Figure 4

Figure 5

Figura 7

Figure 8

FigUra 9

Figure 10

Loeation .of current nietrir moorings in relation to main features oi thctopography. Thc major current systems are also shom. Numbers Withincircles nie estimatcd volume fluxes x 10· in3

S·l from Oetober 1987 (TWTeUet al., 1992). The mooring Ioeations are Northwest Hebndes '. (NWH), .Northwest Flannan (NWF), West Hebrides (WH), North MinCh (NM), EastRana (ER) and West Orkriey "C" (WO':C"). The small filled square indieatesthe Ioeation of Stornoway, from which willd data has been obtained.

Current vectors derived from tbe low-pass riitered records of a) the near­surface instrumentS; arid b) near-bed instruments.

. ,

Vector plot ofthe Iow-pass filtered a) Wind record obtained from Stornoway(six hourly vaIues plotted); and b) the sea bad pressme ailomaly (dbar)obtamed froin the Water Level Recorder deployed at thc ER moöring.

Cross-sections employed to compute along-slope volume transports.Approximate depth mean salinities are shown at· the extreme of eachsection, cis observed during the deployment survey in June 1989.

Mean volume transports perpendicular to the sacÜons indiCri.tad at each ofthe mooring sites calculated over thc fuH deployment penod. Directions Ureparallel to the priricipal direetion of the bottom current meter at eachmoonng. It has beeri assumed that these directioris are equivalent to the10Cal along-slope diiectioris and may be compared to the courSe topogiaphy

.indicated on the chart. Monthly vanability of these fluxes are given inTable 4.

Contoured values of bottom water salinity observcd dunng a) June 1989;arid b) September 1989. Hydrographic station loeations are indieated (smalIcircles), as are the loeations of the current meter mooririgs (large cireles).

Hydrographic sections thi-Ough the shelf current meter moonngs~ Juiie1989. Heavy solid cireles indicate recording current meter positions.

Hydrographie, sectioris through the shelf current, meter moonngs,September 1989. Heavy solid circles indieate recording current meterpositions. .

Hydrographie seetions through thc slope current meter Inoorings, June andSeptember 1989. Heavy solid circles indicate recording current meterpositions. .

Temperature-salinity diagrams derived from the hydrographic seetions ofFigure 6. Small solid circles indicate values derived from the stations westof Orkriey. Lrirger open circles indieate vaIues denved from stations in thenorthern North Sea. Observations for June 1989 above 50 m have beenexeludedto avoid temperature-salinity relatioriships above the seasonalthermoeIine. .

•

Figure 11 Monthly mean volume transports through the West Orkney "e" mooringderived from eight separate deployments at the same site. Mean statisticsof these deployments are presented in Table 5. The cross-section employedis that shown in Figure 4. The broken line indieates the transportsealculated using an empirieal diagnostie model describing the wind driventransport through the seetion (Turrell et aZ., 1990).

60N

59N

5aN

8W

NORTHWESTSHELFEDGE:MOORINGIDEPLOYMENJ ..~~

~~~

CJ\)

~~o

q,v

St~ Kllda

•6W·" 4W· 2W, OE

EASTSHETLANDATLANTICINFLOWr

I]

lQC.,ro

Il~'I!J\\ll

N

1a cm/s

•

I1IIII i I J i i 1II j I J i li I (I i i i 11 i IIII111111 i i i I1I i 11 i i i i i i i IIII i i i i i li 1I1 i i 1I1III i II1III1111111111111111 i2J 21 3 • U " 23 21 2 7 12 t7 23 71 , , 11 " 21 1Iö.', '777777 •••••• """ 11., .. .. .. ., ., .. ., ., ., .. ., .. .. ., ., ., .. .. ., ..

3101 NWH

3111 NWF

3121 WH

3131 NM

3141 ER

I I

3151 WO C

I 10 cm 5- 1

lJlOc.,(1)

NQ

....

N

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•

I I I I I I i I i iI I I I I I I I I I i I I I I i i i i I i i I I I I I I I I I I I I I I J i J J i i I i I i I i i I i i I I I i I I i i i I I I I I I I I I I I I I I I I I I j I I I I i I I I I I IZl 21 l • 0 11 Zl 21 2 7 12 17 21 71 1 , 11 " 21 3 1, , 7 7 7 7 7 7 • • • • • • I I , , , , 11

., 8l 8l B! B! B! B! B! 8l B! B! B! B! B! 8l B! 8l B! 8l 8l B!

3102 NWH

3112 NWF

3122WH

3132 NM

3142 ER

, ,3152 WOC

I 10 cm 5-1

-n-.lnC.., .(J)

Ner

Figure 3

-Icm s15

• STORNOWAY

5

0

-5

-10 i N

-1525 30 5 10 15 20 25 30 5 10 15 20 25 30 5 10 15 20 25

.JULY 19B9 AUGUST 19B9 SEPTEMBER 19B9

dbar.25

eO EAST RONA.15

.10

.05

.00

- .05

-.1025 15 20 25 30 5 10 15 20 25 30 5 10 15 20 25

.JULY 19B9 AUGUST 1989 SEPTEMBER 1989

I----+---!----+----( 50 m

NORTH MINCH....lIII(r--------- (63 km)---------..

~ ~ ~ i~ ~ 15 km ~ ~- -r - -- --- --- __---------r---- -- ---,I II I

: eNM1 :,(40 m) 1(40 m)" '" ",'"

, 50 m ",,,,'", '", '" '", '", ~m,,,,,,,,

15km

•WEST HEBRIDES

1-- -.1 100 m

1-----------1 50 m

~

9km ~~

~ 9km 9km 9km

SLOPETOTAL

120 m

780 m

160 m

EASTRONA

15km

WEST ORKNEY 'C'

15km

e WO'C'1

U),..~

I-----------t 50 m 1------------4 50 m

L.- --.I 100 m

e WO'C'2

L_-------- 95 m-105 m

8W 6W 4W 2W OE

OE"Tl-.lnC.,(1)

58N

60N

59N

,-,r ,,-' ,

I ", ­, ', I. ,, .'

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60N

58N

59N

rFigure 6

8W 6W 4W 2W o

58

3C:·?

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1989JUNE

35'35,. '

)

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4W

SEPTEMBER

0

2W

1989

0

Q

35·35r

35.35 tI I

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WEST HEBRIDES

1"1o 10Km

JUNE 1989

40.

60

80

0:00.

40.

60

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1989

HEBRIDES

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rioKm T

35-2

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SEPTEMBER

•Figure 9

T

JUNE 1989

r-,o 10~

10 10-5

SJUNE 1989

1989

S

SEPTEMBER

T

SEPTEMBER 1989

lTjo 10Km

11 11· 5m IL II 12

100:;- ~ 1

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Figure 10

35.2 35.3 35.4 35.57

34.6 34.7 34.8

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734.6 34.7 35.8 34.9 35.0 35.1 35.2 35.3 35.4 35.5

SALINITY

5 -

15 f-

WEST ORKNEY 'C'Mean Transport ( x 1OE4 cubic m / s )20 ,.....- --=-:::---__e---------,

·86 - 85

,8588e86e 74e88 ,--.--------.

~... ,'"'...... "I- e89, ,,'

I- ". e 85 ""~'91.. e88,"

I- ",

'.... e 90 ,/ e8710 - .... ,.. "'.89

.... e 89 90 ,/ e87''''', • 90 ."",

..... e90 ,.. . . .. , , ,_' ,, e 90

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e89

o L....--__.1.-'_--L.'__...L...._-1__....:'L....--__.1.-'_--L.'__...L...._-1__---L-__...1-'_---I

1 2 3 4 5 6 7

Month8 9 10 11 12