Regional settings and relevance for local and global climate trends
The Fram Strait builds a seaway from the North Atlantic to the Arctic Ocean. The passage is approx. 500 km wide seperating the northeast of Greenland from the Svalbard archipelago in the east. The broad regional setting is displayed in the Figures 1 and 2.
Figure 1 is snapped from the International Bathymetric Chart of the Arctic Ocean (IBCAO). The IBCAO covers the waters north of 64°N latitude by means of a DTM with 2.5 km horizontal grid spacing. The data set can be found on the Arctic Bathymetry Page of the National Geophysical Data Center (NGDC). The IBCAO so far was the most comprehensive data collection describing the sea floor in the entire Fram Strait.
Figure 2 shows the vicinity of the Fram Strait and the sheet index of the AWI Bathymetric Charts of the Fram Strait (AWI BCFS). The sheet numbering is based on the GEBCO 1:1,000,000 chart compilition.
Geologically the region is part of the Spitsbergen Transform connecting the Gakkel Ridge in the Arctic Ocean and the northernmost arm of the midatlantic ridge system, Knipovich Ridge, in the Norwegian-Greenland Sea. The passage opened 60 Ma ago. The plate tectonic movements led to a complex composition of transform faults and spreading centers along passive continental margins. The region is still tectonically active (Thiede et al. 1990, Crane et al. 1991).
The Fram Strait represents the unique deep water connection between the Arctic Ocean and the rest of the world ocean. Its bathymetry controls the exchange of water masses between the arctic basin and the north atlantic seas. The significant heat flux through water mass exchange and sea ice transport, i.e. transport of fresh water and sea ice southwards and transport of warm saline waters northwards, influences the thermohaline circulation at a global scale (Schmitz 1995, Gerdes & Schauer 1997).
Two main currents control the water mass exchange through Fram Strait. Arctic surface waters flow southward in the East Greenland Current along Fram Straits western border. Along the eastern margin Atlantic Waters flow northward in the West Spitsbergen Current. These major currents are seperated by a transition zone of denser upper-layer waters (Swift 1986). Results of recent modelling studies emphasise the importance of the Fram Strait for heat inflow to and freshwater export from the Arctic Ocean (Zhang & Zhang 2001, Meredith et al. 2001).
Related to the seabottom topography recirculation patterns exist which introduce cooled Atlantic Water to the East Greenland Current, preparing it for deep water formation in the Greenland Sea (Quadfasel et al. 1987, Rudels et al. 2000). Moreover water mass exchange and variation in the Fram Strait influences the formation of Norwegian Sea Overflow Water, contributing to the global ocean ventilation (Thiede et al. 1990, Aagaard et al. 1991). Particularly the Greenland Sea Deep Water formation, supported by the cold waters from the East Greenland Current, plays its role in the global thermohaline circulation. The cold deep waters from the Greenland Sea and Fram Strait mix with atlantic waters after overflowing the Greenland-Scotland Ridge. The new cold water mass mixes with water from the Labrador Sea to build the southward flowing North Atlantic Deep Water. This water mass is the atlantic contribution to the global oceanic circulation (Rudels et al. 2000, Boebel 2000).
An example on how the bathymetry controls the Fram Straits current systems is the persistent Eddy on top of Molloy Ridge. The cyclonic vortex has a diameter of approx. 100 km (Bourke et al. 1987, Thiede et al. 1990, Manley 1995). The eddy influences the heat flux via mixing of Atlantic waters from the West Spitsbergen Current and Arctic surface waters from the East Greenland Current. Moreover it has a strong impact on sea ice budgets as it transports ice away from the main ice edge causing a significant increase of the melt rate (Johannessen et al. 1986) estimated a melt rate increase by a factor of ten.
Kwok et al. (1998) estimated the winter sea ice transport through Fram Strait on the base of passive and active microwave satellite data. Analysis of an 18 year data record (1978 - 1996) resulted in an average areal winter sea ice flux of 670,000 km². The winter volume flux averaged to 1745 km³ for an observation period from October 1990 to May 1995. Vinje et al. (1998) estimated the mean annual ice export from the Arctic Ocean to be 2850 km³ for a time period from 1990 - 1996. Year to year volume variation turned out to be high. These recent studies emphasise that the sea ice stream through Fram Strait is the largest and most concentrated meridional ice flow worldwide, as stated by Vinje & Finnekaasa already in 1986.
There is some evidence that the North Atlantic Oscillation (NAO) influences the sea ice budget, particularly in wintertime. In the study mentioned above Kwok et al. (1998) found a significant correlation between the areal flux and the positive phase of the NAO for the months of December through March. On the basis of data from coupled sea-ice/ocean modelling Zhang et al. (2000) also state an increase of ice export during the high NAO period 1989 - 1996. However, Jung & Hilmer (2001) analyzed long term sea level pressure data (1908 - 1997) in combination with general circulation model output. On interannual and decadal time scales they found no significant link between the NAO and the sea ice export through Fram Strait.