The flow of Atlantic water to the Nordic Seas and Arctic Ocean

Abstract

The Nordic Seas, encompassing the Greenland, Iceland, Norwegian and Barents Seas, act as a buffer zone between the warm and saline waters of the North Atlantic Ocean, and the cold and fresh waters of the Arctic Ocean. The region, with its area of 4.1x106 km2 and a water volume of 4.5x106 km3, covers 1% and 0.3% of the World Ocean area and volume respectively (Jakobsson, 2002). Despite these modest numbers, the Nordic Seas with their complex topography, water mass distribution and flow regimes, is recognized as a key area for the conversion from light surface water to dense deep waters, having climatic impacts over large parts of the globe. The most important factor for the Nordic Seas marine climate is the amount of warm and saline Atlantic Water (AW) entering over the Greenland-Scotland Ridge, and flowing north in the eastern part of the Norwegian Sea eventually reaching the Arctic Ocean via the Barents Sea or through the Fram Strait between Greenland and Svalbard (Fig. 8.1). Its vital importance for the marine climate, water mass transformation and biomass production was recognized almost a century ago (Helland-Hansen and Nansen, 1909). Despite this, there is still no consensus on what is actually driving the inflow of AW to the Nordic Seas, or what are the main mechanisms for transforming the AW into the dense overflow water that is returning south over the Greenland-Scotland Ridge. This overflow water ventilates the world oceans as the deepest part of the Atlantic meridional overturning circulation (AMOC), the Atlantic part of what is commonly referred to as the 'great conveyor belt' (Broecker, 1991). Besides the heat transported by the ocean currents, the Nordic Seas and northwest Europe owe a very mild and favourable climate to the heat associated with the North Atlantic storm track. As a consequence of the dependency on heat transported into the region, the North Atlantic region has been particularly sensitive to climate changes in the past. Abrupt climate changes with ∼10°C temperature jumps over just a few decades occurred in the wake of the last glacial maximum. The last major climate perturbation in the region was the Younger Dryas termination some 11 600 years ago (Dansgaard et al., 1989). Observational evidence suggests that the abrupt changes in climate have been driven by sudden switches in the strength or positioning of the Gulf Stream and its extension into the Nordic Seas. These switches may have been caused by large ice discharges from inland glaciers or fresh water flushing from dammed lakes that have made the surface waters fresh enough to inhibit the deepwater formation (Clark et al., 2001). The findings in paleo records and early results based on relatively simple numerical climate models, have generated much debate on what may happen to the inflow of AW in a global warming scenario, and in particular if unpleasant surprises are laying ahead of us (Broecker, 1997; Rahmstorf, 1995). Despite the fact that our climate has been very stable for more than 10 000 years, and abrupt climate changes therefore may seem unlikely in present day climate, the release of greenhouse gases and the associated global warming may perturb the balance. Since the late '60s there has been no deep-reaching convection in the Greenland Sea (Dickson et al., 1996), the deep waters of the Nordic Seas have become warmer (Østerhus and Gammelsrod, 1999), the intermediate waters have become fresher (Blindheim et al., 2000), and there are indications of a reduced southward flow of cold, dense water over the Greenland-Scotland Ridge (Hansen et al., 2001). Most state-of-the-art climate models participating in the third assessment report of the Intergovernmental Panel of Climate Change (IPCC-TAR) suggested a 30-40% reduction in the strength of the AMOC during this century (Houghton et al., 2001). It is therefore tempting to ask if the recent observations in the Nordic Seas are indicating that such a reduction has already started. In this review we will focus on the inflow of AW to the Nordic Seas, the pathways and transformation of the AW within the Nordic Seas and Arctic Ocean, and the expected consequences of global warming. In section 2, we outline the main bathymetric features, the circulation patterns and the hydrographical distribution. In section 3, some of the large-scale changes thathave taken place in the Nordic Seas climate during the last couple of decades will be reviewed. In section 4, different forcing mechanisms for inflow and conversion of water masses are discussed in the context of the changes that have taken place. Finally the review is concluded and summarized in section 5.