Figure 1.

Figure 2.

Figure 3.

Figure 4.

The temperate climate of Scandinavia and western Europe depends upon the warm waters of the northern North Atlantic to warm the westerly winds before they reach the European subcontinent. These waters, which originate in the Gulf Stream, flow northeast across the ocean and enter the Nordic Seas between Scotland and Iceland (see Fig. 1). There is considerable uncertainty about the pathways by which these waters make their way north. For example, the presence of a north-flowing, warm, salty current just west of Scotland was, for many years, thought to be the principal path by which Gulf Stream waters reached the Nordic Seas. This current can be seen in the famous 1909 Helland-Hansen figure (see Fig. 1), that shows the inflow through the Shetland and Faroe Channel and its continuation as the Norwegian Current, known popularly as the "Golf Strom," in Scandinavia. It has, therefore, come as a bit of a surprise to realize that the "Golf Strom" is not a simple extension north of the flow off Scotland, but comes from the west along a front that extends southeast from Iceland and passes just north of the Faroe Islands, also known as the Faroes. The existence of this front has been known for a long time and is quite visible in Figure 1, but its significance has largely gone unrecognized until just recently. In 1988, Professor Bogi Hansen, from the Fisheries Institute, Faroes, wrote a paper pointing out that a major part of the Atlantic inflow takes place over the Iceland-Faroe Ridge. The total inflow is about 8x10⁶m³/s (roughly eight times the total outflow of all the world's rivers) with more than half this flow entering between Iceland and the Faroes. These waters are somewhat cooler (6°C to 7°C) than the Faroe-Scotland waters, so in terms of heat transport, the two branches appear to contribute about the same amount to the Nordic Seas. To put this into perspective, the heat flux, about 1014 watts, corresponds to the energy output of roughly 100,000 major electric utility plants! The inflow appears to be highest in winter and lowest in summer. How stable are these branches of warm water that flow into the Nordic Seas? What happens if prevailing winds cause disturbances to the circulation in the northeastern Atlantic?
      While we know that much of the warm water must originate in the Gulf Stream, there is a good deal of uncertainty as to how it crosses the northeastern Atlantic. For example, Professor Wolfgang Krauss from the University of Kiel, Germany, noted that none of 200 satellite-tracked buoys deployed in the northern North Atlantic south of 55°N during 1981-89 made their way to the Nordic Seas. Why not? There is also debate about whether the waters crossing the Iceland-Faroe Ridge come from south of Iceland or from the Faroe Islands. Recent observations indicate that substantial amounts of warm water move north along the Reykjanes Ridge, the section of the mid-Atlantic Ridge south of Iceland. Some of these waters turn west toward Greenland, some flow northwest of Iceland, and some are thought to flow northeast into the Norwegian Sea (see Fig. 2). It is conceivable that part of our uncertainty about the mean circulation is due to the fact that these numerous branches vary, partly in response to changing wind patterns. Different studies have found different patterns of flow, suggesting that they change over time. We know, for example, that in the 1970s, the prevailing winds from Canada, south of Greenland and Iceland, were stronger and colder than in the 1960s, and the waters off Canada in the Labrador Sea were cooled and sank to great depths. These waters had to be replaced, perhaps by the warm, salty waters from the North Atlantic Current. We don't really know how, but perhaps when the winds are weaker, more of the waters continue northeast into the Nordic Seas. There is a compelling need for more information about the Atlantic currents, but it is prohibitively expensive to use research vessels for such extensive investigations. This is where a long-term program of current measurements from commercial vessels can be of tremendous assistance.
      Fortunately, there is a freighter, the MV Nuka Arctica of the Greenland shipping company, The Royal Arctic Line, that operates between Nuuk, on the west coast of Greenland, and Denmark, making one roundtrip every three weeks. The transit line between Cape Farewell and the Shetlands along 60°N (see Fig. 2) crosses all the waters that eventually move across the Greenland-Iceland-Faroes-Scotland Ridge or turn west towards Greenland and beyond. The ship owners have agreed to operate an acoustic Doppler current profiler (ADCP) on the vessel. This instrument can measure currents as deep as 400m depending upon the amount of backscattering zooplankton in the water. Fittings to accommodate the instrument were installed in the ship's hull during its regular drydocking in 1998, and the measurement program began in the spring of 1999.
      The current measurements will be used for many purposes. First, they will be used for statistical analyses to estimate the mean flow and its variability with time (particularly on seasonal and longer, interannual, time scales). This will provide basic descriptive information that is needed about the circulation of the North Atlantic. Second, the observations will be used for models and tests. For modelling studies of the Nordic Seas, knowledge of the currents along this route will serve as a boundary condition for numerical simulation of the currents in the Norwegian and Greenland Seas. Third, the ADCP measurements will be helpful for studies of circulation (including ice drift models) around Greenland. As our understanding of the upper ocean circulation improves, so will our understanding of the ocean's role in climate and climate change.
      Although the MV Nuka Arctica program hasn't started yet, the MV Nivi Ituk, another freighter operating along the same line, was equipped with expendable bathythermographs (XBTs), instruments that measure temperature to depths of about 450m. In the 1980s, the vessel took some 500 profiles (Figure 2 shows the ship's track across the subpolar North Atlantic). The mean temperature profiles for winter and summer, west and east of the Reykjanes Ridge, are shown in Figure 3 (spring and autumn profiles fall within these boundaries). The profiles measure the variations near the surface between the two seasons: warm in the summer and cold in the winter. The largest effect is confined to the first hundred meters. In winter, the temperature does not vary with depth; this suggests that the mixed layer in these two regions reaches to at least 400m. Even at those depths, the temperature gets warmer in summer, indicating that seasonal influences reach quite deep. These temperature changes could, in turn, affect the circulation in the area. It is very interesting to note the difference in temperatures between the two basins: the Irminger Sea is 2°C to 3°C colder than the Iceland Basin. These temperature differences imply a northward flow of water, stronger where the horizontal contrast is larger. Figure 4 shows a transect obtained from one of the ship's cruises. One can see clearly the front that separates the two different water masses on either side of the Reykjanes Ridge; cold Irminger Sea water to the west of the ridge, warmer Iceland Basin water to the east of it. This front, known as the Subarctic Front, is characterized by a narrow region with a steeply inclined temperature field, and a northward current flowing along the ridge. From the MV Nivi Ituk XBT data alone, we can address the stability of this front; a glance at different transects indicates that the front is generally found between 34°W and 26°W, more likely on the steep slopes of the Reykjanes Ridge, and that its strength varies on monthly time scales. This gives us some insight into changes in the flow patterns across the region. However, we cannot estimate currents and transports from this kind of information alone. This is why our knowledge of the circulation remains so tentative. New measurement technologies offer exciting prospects for major advancements in our understanding of the North Atlantic. When the ADCP becomes fully operational later this year, the Nuka Arctica will be equipped to measure currents down to depths of 300m to 400m, to obtain temperature profiles to approximately 750m depth (with an XBT system), and to measure temperature and salinity of the surface waters with a continuously recording thermosalinograph (CRT) which is already installed and operational. In addition, the Danish Meteorological Institute will take profiles of winds aloft every 12 hours by releasing weather balloons. Technicians at Nuuk, Greenland, and Aalborg, Denmark, will collect the data and service the instruments during port calls.