ICES Annnal Science Conference 1999 CM 1999/L:21 Nordic Seas Exchanges

The -Faroe inflow of Atlantic water to the Nordic Seas

Bogi Hansen. Faroese Fisheries Laboratory. Box 3051. FO-flO T6rshavn. Faroe Islands. bogihan@frsfo Svein @sterhus. Geophysical Insl1tute. University of Bergen. Regin Kristiansen. Faroese Fisheries Laboratory, Faroe Islands Karin Margretha H. Larsen. Faroese Fisheries Laboratory, Faroe Islands

Abstract

The flow of Atlantic water between Iceland and the Faroe Islands is one of three current branches flowmg from the Atlantic into the Nordic Seas across the -Scotland Ridge. In the Nordic WOCE and the VEINS projects. ADCP's have been moored on a section going north from the Faroes. crossing the inflow. Combining these measurements with decade-long CTD observations from research vessel cruIses along this sectIOn. we compute the fluxes of water (volume). heat. and salt. For the period June 1997 to June 1999, we found the average volume flux of Atlantic water to range between 3.0 and 3.7 Sv (10' m'·s·'), and this water was found to carry a heat flux rangmg from 103 to II? TW (1 TW : 10" W) and a salt flux ranging from 106 to 129 kilotonnes per second. When compared to the latest estimates for the other two branches, the Iceland - F aroes in flow carries about the same amount of Atlantic water as the Faroe - Shetland inflow, but somewhat less heat and salt. 2

Introduction

The flow ofwann, saline water from the across the Greenland-Scotland Ridge into the Nordic Seas and the is of major importance, both for the regional climate and for the global thennohaline circulation. TIrrough its heat transport, it keeps large areas north of the ridge much wanner, than they would otherwise have been, and it completes the loop fonned by sinking in the northern regions and the deep overflows back into the Atlantic across the ridge. Thus, the salt, carried by the Atlantic water inflow to the Nordic Seas, provides density to the upper layers as a precondition for the fonnation of intennediate and deep water. In spite of the obvious importance of the inflow, quantitative estimates of its fluxes of mass, heat and salt have been missing. Up to now, one of the most frequent sources for these numbers has remained that of Worthington (1970), even though he based his estimates on a few, fairly uncertain, current measurements and budgets that involved exchanges of heat and freshwater with input parameters that even today are not well known (Simonsen and Haugan, 1996). Worthington's estimate has also been questioned by later budget estimates (McCartney and Talley, 1984), and quantitative estimates of the fluxes, based on measurements are needed to settle the uncertainties. This was a main reason for the initiation of the two projects "Nordic WOCE" and "VEINS" that provided the observations reported in this paper. The flow of Atlantic water into the Nordic Seas (hereafter tenned "inflow'') occurs through three main branches (Hansen et aI., 1998) as shown in Figure 1. Long-term measurements mdicate that the branch west 3 1 of Iceland transports about I Sv (10' m ·s· ) of Atlantic water (Kristrnannsson, 1998) and therefore on the order of 10% of the total inflow (Hansen et al., 1999). Of the other two branches, the flow through the Faroe­ Shetland Channel is generally cited (exphcitly or imphcitly) as by far the dominant one, but there has been little evidence to support that view (Hansen, 1985). The observatIOns, initiated in the Nordic WOCE project, were therefore designed to measure fluxes through both of the gaps between Iceland and Scotland (Fig. I). In these gaps, hydrographic observations have been carried out on standard sections for almost a century in the Faroe-Shetland Channel and for a decade in the Iceland-Faroes Gap. To provide flux estimates, the Nordic WOCE project established a series of quasi-pennanent mooring sites to measure currents directly. Since 1997, the measurements have been continued within the framework of the VEINS project.

Atlantic Ocean

Figure 1. The lightly shaded area on the figure is the part of the Greenland-Scotland Ridge which is shallower than 500 m. Arrows indicate the three branches of Atlantic water inflow to the Nordic Seas. The Faroe Current is the branch studied in this work. The thick lines termed "N" and "S" indicate two CTO standard sections which were instrumented with AOCP's in the Nordic WaCE and VEINS projects. 3

The waters surrounding the Fatoes are heavily fished, and expenence has taught, that traditional moorings extending far up into the water column have short survival. In the Nordic WOCE project it was therefore decided to rely on the newly-developed Btoadband ADCP (Acoustic Doppler Current Ptofiler) ptoduced by RD Instruments. After some initial ptoblems with these instruments, the choice was found to have been good. In the Iceland-Faroes Gap, the Atlantic water is generally confined to a region north of the Faroes by the Iceland-Fatoes Front (Fig. 2) and the three quasi­ pennanent mooring sites "NA", "NB", and "NC" are located on a north-southgoing standard section which ctosses the flow. Figure 2. The CTD standard section N is shown as a line with the standard stations indicated by black Here we report results of the ADCP measure­ rectangles, labeled N01 to N14 northwards. Main ments carried out in this area from October 1994 to ADCP mooring sites are indicated by circles. Dotted July 1999 at the three main mooring sites, plus a curve indicates the general location of the Iceland· shorter deployment at a site "ND" (Fig. 3) between Fames Front. Shaded areas are shallower than 500 m. NB and NC. We combine the ADCP measurements with results of CTO observations on the 14 standard stations along the sectlOn which have been regularly (about 4 times a year) occupied by the Faroese Research vessel R/V Magnus Heinason since the late eighties. The observations contain a lot of information on the considerable mesoscale aCllvity which occurs in this area, but that will be treated elsewhere. In this work we focus on average fluxes through the section. In Hansen et al. (1999), a preliminary flux estimate for the Atlantic water flow between Iceland and the Faroes was discussed in context with the other in- and outflows. Here we include data from additional observations and provide a more thotough treatroent ofthe results.

The data 62°3Q'N 63°00'N 63°30'N 64°00'N I I I I Both the hydrographic data and the current N03 NOS N07 N09 N11 N13 measurements were acquired on the same section, standard section N (Fig. 3), extending from 62°20'N .. 6°05'W to 64°30'N 6°05'W (Fig. 2). The section has depths reaching below 3000 m, but we focus on the .. uppermost 700 m which contain all the Atlantic water .. that has passed over the Iceland-Faroe Ridge. • ... CTn observations. The hydrographic data was acquired through 52 cruises along standard section N .. (Fig. 2) in the period 1987-1999. The standard section ... Faroe Plateau has 14 standard stations, labeled NOI to N14, with 10 ONe nautical miles equidistant spacing between stations. .. Lack of time or bad weather restncted the northward extension of the section on some occaSlOns, and a Figure 3, Standard section N extends northwards from few intermediate stations have been deleted in the the Fame Plateau with 14 standard stations labeled N01 quality assessment procedure as indicated in Table 1. to N14. ADCP mooring sites are shown by circles. with sound beams shown by shading (with exaggerated Several different instruments of type Neil Brown, width\. EG&G, and SeaBird have been used through the period. In later years salinity samples were obtained on each station and analyzed with an Autosal salinometer for calibration. 4

Table 1. CTD cruises along standard section N carried out with RN Magnus Heinason in the period July 1987 to June 1999. Each cruise is identified with a four digit number where the first two indicate the year. The date (yy/mmldd) for the occupation of station N01 is indicated. Stations successfully occupied on each cruise are indicated with a "+".

Cruise Date NOI N02 N03 N04 N05 N06 N07 N08 N09 NI0 NIl N12 N13 N14

8720 87/07/05 + + + + + + + + + + + 8826 88/05/27 + + + + + + + + + + + + 8838 88/06/20 + + + + + 8936 89/05/23 + + + + + + + + + + + + + 8952 89/07/07 + + + + + + + + + + + + + 8984 89/11/10 + + + + + + + + + + + + + + 9038 90/05/11 + + + + + + + + + + + + +. + 9068 90/08/20 + + + + + + + + + + 9088 90/11/09 + + + + + + + + + + + + 9140 91/05/18 + + + + + + + + + + + + + 9160 91/08/22 + + + + + + + + + + + + + + 9188 91/11/22 + + + + + + + + + + + + + 9220 92/03/06 + + + + + + + + + + + + + 9240 92/05/29 + + + + + + + + + + + + + + 9252 92/07/13 + + + + + + + + + + + + 9272 92/10/29 + + + + + + + + + + + + + + 9312 93/02/05 + + + + + + + + + + + + + 9342 93/05/26 + + + + + + + + + + + + + + 9344 93/06/05 + + + + + 9356 93/08/16 + + + + + + + + + 9368 93/09/27 + + + + + + + + + + + + + + 9404 94/01/29 + + + + + + + + + + + + + + 9412 94/03/06 + + + + + 9416 94/03/22 + + + + + 9428 94/04/09 + + + + + 9432 94/04/23 + + + + + 9440 94/05/14 + + + + + + + + + + + + + + 9508 95/02/17 + + + + + + + + + + + 9524 95/04/11 + + + + + + + + + + + + + + 9556 95/09/01 + + + + + + + + + + + + + + 9568 95/11/12 + + + + + + + + + + + + + + 9607 96/02/16 + + + + + + 9608 96/02/17 + + + + + + + + + + + 9628 96/04/28 + + + + + 9644 96/06/16 + + + + + + + + + + + + + + 9660 96/08/30 + + + + + + + + + + + + + + 9688 96/11/10 + + ~ + + + + + + + + + + + 9708 97/02/16 + + + + + + + + + + + + + 9724 97/04/03 + + + + + 9736 97/05/20 + + + + + + + + + + + + + + 9748 97/06/13 + + + + + + + + + + + + + + 9764 97/08/29 + + + + + + + + + + + + + + 9792 97/11/09 + + + + + + + + + + + + + 9808 98/02/12 + + + + + + + + + + + + + + 9832 98/05/19 + + + + + + + + + + + + + + 9840 98/06/12 + + + + + + + + + + + + 9848 98/07/06 + + + + + + + + + + + + + + 9864 98/09/15 + + + + + + 9888 98/11/08 + + + + + + + + + + + + 9908 99/02/23 + + + + + + + + + + 9932 99/05/24 + + + + + + + + + + + + + + 9940 99/06/14 + + + + + + + + + + + 5

The early CTD observations were less frequently calibrated, but the slow changes in the deep water salinities allow adjustments that keep the salinity uncertainties below 0.01. In this work we use temperature and salinity observations averaged over 10m depth intervals in 70 layers, the uppermost one centered at 5 m depth and the deepest at 695 m.

ADCP observations. Current measurements were obtained through a number of deployments at the three Nordic WOCE standard mooring sites, NA, NB, and NC, and through an additional deployment at the site ND, between NB and NC (Fig. 3). Details of the deployments are listed in Table 2 where each record is identified with a six-character record name indicating the site and year and month (yynun) of deployment. At site NA, a 150 kHz RDI Broadband ADCP was placed on the bottom within a specially constructed trawl­ proof protection frame (0sterhus and Hansen, 1995 with later modifications). At the other sites, 75 kHz RDI Broadband ADCP's were moored in the top of single point moorings using ADCP buoys produced by Flotation Technology and acoustic releases from MORS. At the deep sites, NB, NC, and ND, acoustic echo from the surface could be used for all records except NB9807 to adjust the depth of the instrument. These observations show that drag·down of the instrument only rarely exceeded 5 m and could be ignored. Sound velocity variation along the beam path was also found to be sufficiently small to be Ignored.

Table 2. Details of ADCP deployments along standard section N in the period October 1994 to July 1999. The last two columns indicate the vertical extent of data for which the daily averaged velocities were 100% good.

Mooring position Depth to Int Observational period 100% good Record erval Latitude Longitude Bottom Instr minut Iyy/mm/dd-yy/mm/dd days Bins Top

NA9601 62°42.370'N 6°QS.310'W 302m 301m 15 96/01/25-96/05/25 123 2-20 97m NA9606 62°41.92B'N 6°04.665'W 292m 291m 20 96/06/17-97/05/21 340 2-18 105m NA9706 62°42.31S'N 6°0S.170'W 300m 299m 20 97/06/15-98/06/08 359 2-18 113m NA9807 62°42.178'N 6°0S.043'W 297m 296m 20 98/07/08-99/07/01 360 2-19 100m

NB9410 62 Q 5S.088'N 6°04.630'W 963m 6S4m 5 94/10/23-95/02/16 118 1-21 124m NB9706 62°S4.81B'N 6°04.957'W 925m 659m 20 97/06/14-98/06/12 365 1-17 223m NB9807 62°55.158 'N 6°04.B44'W 961m 708m 20 98/07/05-99/06/18 349 1-19 222m

NC9410 63 D 16.3S0'N 6°06.300'W 173Gm 616m 5 94/10/23-95/02/16 118 1-19 136m NC9606 63 D 16.082'N 6°Q6.509 ' W 1738m 640m 20 96/06/17-97/05/21 339 1-19 154m NC9706 63 D 16.42S'N 6 D 06.600 ' W 1731m 659m 20 97/06/14-98/06/12 364 1-18 198m NC9807 63 D 1S.944'N 6 D 06.299 ' W 1739m 655m 20 98/07/06-99/06/18 349 1-17 219m

ND9711 62°S7.S40'N GOQS.6QO ' W 1276m 670m 20 97/11/12-98/06/12 214 1-17 234m In the early verSIOns of the trawl-protective frame used at NA, the steel construction affected the ADCP compass considerably. This was circumvented by short-term calibration deployments with a traditional (Aanderaa) current meter mooring at NA while the ADCP was operating. During one of the deployments at NB (NB9807), one of the four ADCP sound transducers was malfunctionmg. This record was analyzed with only three beams therefore, but simulations on other records at NB indicate that this has not SIgnificant effects for the averaged types of data used in this work. The deployments at site NA were set up with a 10 m bin length (vertical averaging layer) while the other deployments had 25 m bins. Sound speed variations are sufficiently small that the bin lengths do not need adjustment. To ensure long· term records, only one ping was used in each ensemble. This produced fairly noisy records and many observations had to be deleted during the data editing process. This process included an automatic flagging procedure which detected outliers and large error velocities (Gordon, 1996). In addition, all observations were manually scanned in a graphical editing package, specially developed using MA TLAB routines. The resulting data records were fairly gappy, especially for the bins most distant from the ins!runIent. In this work, high frequency variations are, however, irrelevant and we use only daily averaged current values. 6

These were produced by averaging the east and north velocity components over an interval as close as possible to 24hours and 50minutes to reduce both diurnal and semi-diurnal tides. Before averaging, flagged values were interpolated between the two neighbouring bins if they were non-flagged and single-ensemble gaps were interpolated between neighbouring ensembles. In the most distant bins, the error rate may, however, depend upon diurnal migrations of organisms and special care had to be taken to prevent tides to bias the average current in case of large contiguous data gaps. For the deepest bin, the average of all non-flagged values was used if the total non-conttguous error gap for that day did not exceed 2 hours. Otherwise, the bin was flagged for that day. For higher bins, the simple average was again used if the total error gap did not exceed I hour. If the total error gap exceededl8h50m (good data less than 6 hours) the bin was flagged for that day. If the total error gap was more than 2 hours, but less than 18h50m, a special interpolation routine was employed which for each bin uses observations from the two bins below to correct the estimate based upon the simple average. This interpolation routine assumes the same tidal signal in neighbouring bins and has been tested with simulations. With this procedure, daily averaged current velocity components were produced for all the series in Table 2 and the second to last row in the table indicates the bins for each series with complete daily averaged current values. For the records at site NA, the first bin generally had a high error rate, probably due to the protective frame, but otherwise, complete data coverage extended from the deepest bin up to the depth shown in the last column of Table 2. Above that level, the data series have missing days, but even up to 100 m depth, all the daily averaged series for the records in Table 2 have more than 70% coverage of the observational periods. For transport calculations, we only need the east-component, perpendicular to the section, but the velocity series have to be extended all the way to the surface. This was done in a two-step procedure. First, all the non-flagged daily averaged velocity data for each site was analyzed to determine the typical change from one bin depth to the next. The eastward velocities of the uppermost bins were generally found to be fairly well correlated and a set of regression coefficients was determined for each site which was used to fill in gaps in the uppermost bins from deeper bins for the same day. 'This extended all series to 90 m depth or higher, and above that we assume constant velocities. To simplify processing, the resulting records were then linearly mterpolated between bins to give observations at the same fixed depth levels as the cm data with 10m intervals. With gaps between successful deployments, the ADCP data extend from October 1994 to July 1999. Figure 4 summarizes the availability of both cm and ADCP data in this period.

NO"' ~u ~ ~ ~ ~ ~ " " O>0lO> , 'j i , , , I I I, , ~~-...:"t ,I ,( ... , .·,1 .... ·1 .. 'II II I . I ' 1_,,,. .. .. NO r .. i,l - - - - I II I , , _NB ---- .. - ---- .. , , . .. : • : I I r 1 II I II , ~ ••• I • .. , .' :1 NA r I 1111 I I III 1 I I II

1994 1995 1996 1997 1998 1999

Figure 4. Overview of AOCP data (thick horizontal bars labeled with record name as in Table 2) and CTO cruises along standard section N (thin vertical lines labeled with cruise number as in Table 1) from October 1994 to July 1999. All CTO occupations extended north of site NC except for the three cruises indicated by shorter lines. 7

Results

CTD observations. Average sections of temperature, salinity, and density are shown in Figure 5 based on 35 cruises in Table 1 whIch had complete coverage out to standard station Nl 0 with fewer cruises beyond that. Northwards from the Faroe shelf. a warm, saline area identifies the wedge of Atlantic water. Its boundary towards the colder, less salme waters reaches the bottom of the Faroe Plateau at depths of 300-500 m and slopes upwards to reach the surface m the Iceland-Faroe Front between standard stations NOS and N09. Figure 5 also shows the baroclinic velocity structure from geostrophic calculations using the average temperature and salinity fields. The baroclinic velocity is largest in the area of the sloping boundary between N04 and N07 with VelOCIty differences exceeding 20 cm s'\ between the upper and deeper waters.

N01 NJ3 N05 N07 f'.,()9 N11 N13 N11 N13

>34.90

<34.90

~3490~

N09 N11

c d

Figure 5. Average distributions of temperature (a). salinity (b). cr, (c) based on CTD observations by RN Magnus Heinason in the period 1987 - 1999 (35 cruises for stations NOI to Nl0, reducing to 24 cruises for station NI4). Geostrophic velocity shear (d) based on the average temperature and salinity distributions in (a ) and (b) and assuming zero velocity at the deepest level. Positive velocities out of the paper. Area with negative shear is hatched.

When single cruises are considered, they often look like the average distributions in Figure 5, although usually with sharper boundaries. Mesoscale activity, however, frequently deforms the distribution and more irregular sections are found. The water masses below the surface layer on the section are illustrated by Figure 6 where the main water masses as described by Read and Pollard (1992) and Hansen e/ al. (1998) are indicated. 8

10 ,~ I 9 f- MNAW a L , I 7 r--

6 ~ ~ E ~623m N6 ~, 5 .~ "§ ~ •c. 4 '- E ! ~ ~.~ 3 NA

"',-" I 2 , ~ ". 223m I MEIW i 10 crrVsec o 223m

·1 Figure 7. Progressive vector diagrams from daily averaged 34.60 3470 3480 3490 3500 35.10' 3520 3530 observations at sites NA, NB, and NC for the period June Salinity 1997 to June 1998. The traces have been scaled with the Figure 6. TS diagram for water below 150 m depth duration of each series so that they indicate residual based on 10 m averages from all cruises in Table 1. velocity. The depth of each trace is indicated.

ADCP observations. The general flow through section N is illustrated by the progressive vector diagrams in Figure 7 with observations from upper and deeper layers. At all three sites the water at about 200 m depth (and higher) has a residual flow somewhat south of east. In the deeper layer, below slll-level of the Iceland­ Faroe Ridge, the flow IS much weaker and mainly follows the topography towards the south and east. The depth dependence of the residual eastward flow is shown m Figure 8 for all the records.

Eastw ard velocrty (cm'sec) Eastward velocity (crrlsec) Eastward velcx::rty (cmsec) Eastw. velocity (em sec 10 20 0 10 20 30 0 10 20 0 10 of 01 I I i ~ I NA NB NO ~NC .s 100e 100- -" I I t 200c jJ / ! ! 2nd- • \J _ ./ 300L 'I .s 300- / • NA9601 -" ~ 'NA9606 ND9711 'NA9706 ! 40d- • NA9807 I 5QQ- ,•/' i • NB9410 .. NC9410 t • NB9706 "NC9606 60t t • NB980? +- NC9706 ·NC9S07

Figure 8. Avenage profiles of the eastward velocities for all the records in Table 2.

At NB, the flow IS seen to have a minimum at about 500 m depth, but all sites show eastward flow at all depths except sometimes for the deepest water at NCo Figure 8 indicates interannual variability in the flow field, but on shorter llme-scales, much larger variations are seen in Figure 9. 9

u • NC ~ 20 "- 10 ~ u 0 !\ "'•> -10 ~ -20 ill• u• ~ 40 "-,. 30 ~ u -" 20 •> 10 ~ 0 ~ u• ~ 40 "- 30 ~ u -" 20 •> 10 ~ 0 ill• 1997 1998 1999

Figure 9. Timeseries of the east velocity component at 205 m depth from mid-June 1997 to the end of June 1999 at the three main ADCP mooring sites. The figure shows 7 day running mean values.

Discussion

Geostrophy. The barochnic velocity variation Eastw ard velocity (cnVsec) seen in Figure 5d is qualitatively in reasonable 5 10 15 20 25 agreement with the measured velocity profiles 0 in Figure 8. A more detailed comparison is made for site NB in Figure 10 which shows 100 GeOSlr°1 some similarity, but it appears that the geostrophic profile is more linear and gives a 5 200 ADCP smaller velocity difference (shear) between the .<: upper and deep water. The geostrophically 'E. ~ estimated shear between the 100 m and the 300 500 m levels was found to be 86% of that measured by the ADCP. The two profiles in Figure 10 are averaged 400 in different ways and over different periods, so a complete agreement could not be expected. This is not the case in Figure 11 which Figure 10. Averaged eastward velocity profiles at NB compares geostrophic velocities from specific determined from geostrophy and from ADCP measurements. cruises to simultaneous velocities measured by The geostrophic profile is based on 35 cruises in the 1987-99 ADCP. period. The ADCP profile is the average of all records at the site (Oct. 1994 to June 1999) after gap-filling. 10 - u 4~, • • +-/ ~ + .!:!- 1'.

Figure 11. Eastward velocity difference between two depth levels (shear) measured by ADCP (daily average) at site NB (a) and NC (b) compared to geostrophic shear between the same levels calculated from CTD observations at surrounding stations on the same day. .

Although the number of independent points in Figure Iia is too small for statistical estimates, the result is fairly good especially for differences between the 100 m and the 500 m depth levels (black rectangles in Figure II). Again, there seems to be a tendency for the geostrophic shear to be smaller than the daily averaged ADCP velocity. A regression analysis indicated that geostrophic shear between 100 m and 500 m was 83% of the shear measured by the ADCP between the same layers. It thus appears that the geostrophic velocity profile underestimates the 100 m to 500 m shear at NB by some 15%. This does not necessarily imply a geostrophic imbalance, however, because the geostrophic estimate is an average for the 10 nautical miles between standard stations N04 and NOS while the ADCP profile at NB covers a much narrower area. If NB is located in the high-velocity core region (as was mtended) then the geostrophic shear should be smaller than the shear at NB. On the northern side we may check this by comparing the ADCP measurements at NB with those at ND which was situated midway between NB and station NOS. For the period when both sites were occupied (Nov. 1997 to June 1998), the 100 m to 500 m shear at ND was about 84% of the shear at NB. If there is a similar decrease from NB towards shallower water, then the geostrophic shear is consistent with the measured shear. It is also seen from Figure 5 that the lower boundary of Atlantic water shallows about 150-200 m from N04 to NOS. The average velocity profile between N04 and NOS may therefore be expected to be more linear than the ADCP profile at NB. Geostrophy was also tested at site NC by comparing daily averaged ADCP shear to geostrophic shear computed between stations N06 and N07 (Fig. II b). At NC the correspondence was better than at NB and a regression analysis indicated that geostrophic shear between 100 m and 500 m was 95% of the shear measured by the ADCP between the same layers.

The latitudinal velocity variation. For the period June 1997 to June 1999, the ADCP measurements are sufficiently detailed to merit the estimation of an average velocity distribution along the section. This requires interpolation of velocity profiles between the ADCP sites and extrapolation if a complete section is required. Eastward velocity u(y,z,t) at latitude y and depth z at time t is then represented as: 11

U(y,z,t) = a(y,z)· UA (Z,t) + b(y,z)· UB (Z,t) + C(y, Z)· UcCz,t) (1) where UA(Z,t) is the measured ADCP velocity at depth Z of site NA for time t etc. In the preliminary calculations reported by Hansen et al. (1999), the assumption was made that the weight factors a, b, and c were independent of depth and varied linearly with latitude y, as represented by the lines denoted as "Linear" on Figure 12. This is one of the simplest assumptions imaginable and we find no reason to question it in the region between NA and NB. In parts of the region between NB and NC, the linear relation may, however, give too large velocities. At 100 m depth (Fig. 12a), a linear variation was consistent with the shorter-term measurement at ND (after adjusting for different periods), but not with geostrophy. In Figure 12a the top of the shaded area is the velocity at 100 m depth determmed by adding the average geostrophic shear (Fig. 5d) to the measured velocity at 500 m depth. Between stations N04 and N05 the correspondence is fairly good, but between N05 and N06, the geostrophic velocity was considerably smaller than indicated by a linear variation between ADCP sites.

~3 ~5 N06 ~7 ~30r------~------r------+------~ ~ 301------j------t------t------j ~ ~ ~ ~ .~ u ~ > ~ 10 ~ ~ b

Figure 12. Eastward velocity at 100 m and 500 m depth (a) and at 300 m (b). Circles indicate average velocities measured by AOCP's in the June 1997 - June 1999 pertod. The values for NO have been scaled by the relative velocities of the same depth at NB for the NO observational pertod in relation to the whole period. The shaded area indicates the geostrophic shear between 500 m and 100 m and 300 m respectively. The lines indicate velocity between AOCP moortng sites, assuming linear variation (continuous lines) Or a vartation which is more consistent with geostrophy (broken lines).

At 300 m depth (Fig. 12b), a linear variation was again in fairly good correspondence with geostrophy between stations N04 and N05 (although rather too high), but not between N05 and N06. At this depth, the scaled velocity measured at ND was also smaller than indicated by a linear variation between NB and NC. These arguments indicate that the velocity may decrease faster than the linear assumption north of ND at 100 m depth and north ofNB at 300 m depth. In Figure 12 we have suggested a variation (shown by broken lines) which is more consistent with geostrophy, although it seems difficult to reconcile geostrophy between N06 and N07 with the ADCP measurements at NC, perhaps due to the high variability in this region (Fig. 8) and different averaging periods.

Total volume Flux. The flux of water through the seclion or a part of it is the areal integral of the eastward velocity. In practice, the integral is supplanted by a sum, and we divided the section down to 700 minto boxes, 10m deep and extending 5 nautical miles to either side of each standard station so that one column of boxes (up to 70) was associated with each station. Each box had an area of 1.852·10' m' unless it hit the bottom and the water flux through it was found as the product of this area WIth the eastward velocity at this location. The total volume flux through the section, V,(I) then is:

Vr(t) = L L A.,j . (ak,j ,uA,j(t)+bk,j ,uB,j(t)+c.,j .uc,/t) (2) • j where AkJ is the area of the box at depth number j for standard station number k, and uAJI) is the daily averaged ADCP velocity at site NA for day 1 and averaged over the depth interval of boxj etc. Each box is assigned one set of weight factors: akJ, bkj, and ckJ, where indexes k and j replace latitude y and depth Z 12

respectively. Determination of the total volume flux then reduces to determining the weight factors for each box which involves interpolation. We have tried both interpolation schemes indicated in Figure 12. The "geostrophic" interpolation scheme has depth-varying weight factors for each station. Table 3 lists the weight factors at depths 100 m, 300 m, and 500 m, used for this scheme, consistent with Figure 12. The factors were assumed to extend unchanged from 100 m to the surface and from 500 m downwards. Between these levels, they were assumed to vary linearly with depth. For the boxes associated with N04 which are below the 300 m depth of the ADCP at NA, the measurement for that depth at NB was used directly. The weight factors for 500 m depth in Table 3 were determined by linear interpolation between the mooring sites, and in the "linear" interpolation scheme, these factors were used at all depths.

Table 3. Weight factors for the velocities measured at sites NA, NB, and NC to be used in equation (2) for the boxes associated with each standard station for the depths 100 m, 300 m, and 500 m, using "geostrophic" interpolation.

Site Depth N02 N03 N04 NOS N06 N07 N08 N09

NA 100 m 0.64 0.94 0.38 NB 100 m 0.62 0.73 0.24 NC 100 m 0.50 1. 00 0.50 0.00

NA 300 m 0.64 0.94 0.38 NB 300 m 0.62 0.69 0.21 NC 300 m 0.50 1. 00 0.50 0.00

NA 500 m 0.38 NB 500 m 0.62 0.76 0.29 NC 500 m 0.24 0.71 1.00 0.50 0.00

In order to exclude Faroese shelf water, the 6,------, section was taken to start mid-way between NOI and N02 that is with the box surrounding 5 "Linear" N02. This required extrapolation of the ~~ velocity profiles towards the south. Measure­ "Geostrophic" ments with traditional Aanderaa current meters indicate typical residual velocities on the order of6 em s') in the vicinity of NO 1 (Hansen and Larsen, 1999), and we have used this value to interpolate linearly between NO I and NA. In ",;' , 1 "'; ;',' the northern end of the section, we have used the average geostrophlC velocity field (Fig. 5d) to extend the section to include N08. To obtain an averaged total volume flux, we inserted into (2) the ADCP profiles Figure 13. Accumulated total volume flux from station N02 northwards, averaged for the period June 1997 - June 1999, averaged over the June 1997 to June 1999 using two different interpolation schemes between ADCP period and extended to the surface as described measurement sites. in the data section. The average total volume flux was calculated using both interpolation schemes and estimated to 4.5 - 4.8 Sv (1 Sv = 10· mJ·s·)) (Fig. 13). The difference between the two schemes is seen to be small, as is the contribution from the boxes outside of the region covered by the ADCP measurements (N02 and N08).

Atlantic water fluxes. The total volume flux, presented in Figure 13, includes water from northern areas which has not crossed the Iceland-Faroe Ridge recently. In order to separate the component which derives 13 directly from the Atlantic, we need a water mass analysis. This is complicated by the number and variability of water masses in the region. In Figure 6, five different water masses were indicated, which all OCCur on the section in addition to the low-salinity water usually found in the surface layer north of the Iceland-Faroe Front. Usually, this low-salinity layer does not reach the southernmost part of the section, and neither does the NNAW (Norwegian North Atlantic Water). The Deep Water (NSDW) is sheltered from contact with Atlantic water by the Norwegian Sea Arctic Intermediate Water (NSAlW). Most of the volume flux passes through the southern part of the section (Fig. 13), and under these conditions, the situation is simplified to involve only three water masses: Modified North Atlantic Water (MNA W), Modified East Icelandic Water (MEIW), and NSAIW. If typical temperatures and salinities of these are known, a simple three point mixing model allows the determination of the relative content, a. of MNAW water from the temperature and salinity at each point When the temperature and salinity of each box on the section is known, the relative content of MNAW, akj may be calculated for each box. The flux of MNAW through the section VA(t) is then given by:

The characteristics ofNSAlW (T = O.soC, S = 34.9) are fairly well defined both 1TI the literature (Hansen et ai., 1998) and in Figure 6. MNAW varies more, both geographically and in time (Hansen et al., 1998). We may assume, that it is the northern and deeper part of the MNAW crossing the ridge which comes most closely in contact with the other watermasses and takes most part in the mixing. This component has ternperatures and salinities in the lower ranges, and we therefore assume the MNAW to have temperatures 7°C - goC and salinities 3S.IS - 3S.20. Most variable and uncertain are the characteristics of MEIW. This water mass is to a large extent formed in the region (Read and Pollard, 1992), but we need the water characteristics before it meets the MNAW. We have assumed the source temperature of MEIW to vary between 1°C and 2°C and the salinity between 34.7 and 34.8. With these assumptions we have calculated the averaged flux of Atlantic water through the 4 section and found it to range between 3.0 and 3.7 Sv. We used the June 1997 - June 1999 averaged _3 ~.-- ADCP profiles with Table 3. The range in flux ~ estimates does not represent temporal variations, x ~ / but rather uncertainties in the interpolalion '" 2 ~ scheme and in water mass characteristics. Figure .2 o 14 exemplifies how the accumulated flux > 1 / increases as we extend the section northwards. . . The largest contribution is seen to come under N04andNOS. a ---/ NJ2 NJ3 NJ4 I'll5 NJ6 NJ7 NJ8 NJ9 In addition to volume, we have calculated average fluxes of heat and salt through the Figure 14. Accumulated flux of Atlantic water through the section. Heat flux (relative to O°C) is found by section with "geostrophic" interpolation, MNAW with T = multiplying each term in (2) or (3) by tempera­ 7°C, S= 35.15 and the sources for MEIW having T = 1"C, S ture, heat capacity, and density. Salt flux is = 34.7. similarly found by multiplication by salinity and density. Some of the heat and salt carried through the section derives from other sources than the Atlantic, however, and care has to be taken when we want only the fluxes that have come directly across the Iceland­ Faroe Ridge. If therefore in (3), the relative content ofMNAW in a box akJ was found to be less than I, the assumed source characteristics of MNAW was used for temperature and salinity rather than actual values. 14

Using various combinations of interpolation schemes and water mass characteristics within the stated ranges, we found heat fluxes ranging from 103 to 117 TW (I TW = 10 12 W) and we found salt fluxes ranging from 106 to 129 kilotonnes per second. Again, the ranges express uncertainties rather than temporal variation. In a preliminary estimate, we reported somewhat higher fluxes, especially for volume (Hansen e/ al., 1999) which made the Iceland-Faroes Gap the most important contributor of Atlantic water to the Nordic Seas in terms of volume (though not in terms of heat and salt). When the numbers, reported here, are compared to the latest estimates for the inflow through the Faroe-Shetland Channel (Turrell el al., 1999), the Iceland-Faroes Gap volume flux of Atlantic water ranges from somewhat below up to about the same values as those in the Faroe-Shetland Channel. In terms of heat and salt import to the Nordic Seas, all the estimates indicate that the Faroe-Shetland Channel inflow dominates over the inflow through the Iceland-Faroes Gap.

References

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Hansen, B., Larsen, K. M. H. 1999. Traditional current meter observations in Faroese offshore waters 1977 - 1994. Data Report. The Faroese Fisheries Laboratory. Technical Report 99-01, 193 pp.

Hansen, B., Larsen, K. M. H., 0sterhus, S., Turrell, B., and Jonsson, S. 1999. The Atlantic Water inflow to the Nordic Seas. The International WOCE Newsletter 35,33-35.

Hansen, B., 0sterhus, S., Gould., W. J., and Rickards, L. J. 1998. North Atlantic - Norwegian Sea Exchanges The ICES NANSEN Project. ICES Cooperative Resear~h Report, No. 225, 3-82.

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Simonsen, K. and Haugan, P. M. 1996. Heat budgets of the Arctic Mediterranean and sea surface heat flux parameterizations for the Nordic Seas. Journal of Geophysical Research, 101 (C3), 6553-6576.

Turrell, W. R., Hansen, B., 0sterhus, S., Hughes, S., Ewart, K., and Hamilton, J. 1999. Direct Observations of Inflow to the Nordic Seas through the Faroe Shetland Channel 1994-1998. ICES CM 1999/L:OI, 16 pp.

Worthington, L. V. 1970. The Norwegian Sea as a mediterranean basin. Deep-Sea Research, 17,77-84.

0sterhus, S. and Hansen, B. 1995. The Nordic WOCE Trawl-resistant ADCP System. Proceedings of the IEEE Fifth Working Conference on: Current Measurement, Feb. 7-9, 1995,.