Climatic Importance of Large-Scale and Mesoscale Circulation in the Lofoten Basin Deduced from Lagrangian Observations

Abstract

The Nordic Seas (Norwegian, Iceland and Greenland Seas) is one of the regions that have been best covered and continuously monitored with hydrographic observations. The view of the large-scale ocean circulation in the Nordic Seas has traditionally been based on hydrography due to the relatively few direct current measurements. It has been known for a century that the ocean circulation in the Nordic Seas is influenced by the basin topography (Helland-Hansen and Nansen 1909). However, the large number of surface drifters that have been released during the last 10–15 years have increased our knowledge of the surface circulation in the Nordic Seas (Orvik and Niiler 2002). The main features of the upper circulation in the Nordic Seas are a northward flow of warm water on the eastern side and a cold current flowing southward on the western side (Helland-Hansen and Nansen 1909). The flow of warm waters into the Nordic Seas represents the final poleward transport of the global thermohaline circulation system before being transformed by cooling processes into intermediate and deep waters that flow back into the North Atlantic. As the Fig. 6.1 shows, two main branches of warm, saline Atlantic water of approximately equal magnitude enter the Norwegian Sea. The Norwegian Atlantic Current (NwAC) is revealed as a two branch current system through the entire Norwegian Sea (Poulain et al. 1996; Orvik and Niiler 2002). The eastern branch follows the shelf edge as a barotropic slope current while the western branch is a polar jet current associated with the Arctic Front (Orvik et al. 2001). While the inshore branch passes north against the Norwegian Continental Slope and is covered by current meter arrays in the Faroe–Shetland Channel, off Svinoy, across the Barents Sea Opening and in eastern Fram Strait, the offshore branch, passing north through the Norwegian Sea as a free jet, is unmeasured. Both will be involved in the spread of warmth to the Barents Sea and Arctic Ocean, and the issue of determining what might control this warm, saline