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The Iceland-Faroe Inflow of Atlantic Water to the Nordic Seas

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The Iceland-Faroe Inflow of Atlantic Water to the Nordic Seas ICES Annnal Science Conference 1999 CM 1999/L:21 Nordic Seas Exchanges The Iceland-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. Norway 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 Greenland-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 Atlantic Ocean across the Greenland-Scotland Ridge into the Nordic Seas and the Arctic Ocean 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.
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