Monitoring of oceanic fluxes across Fram Strait

U. Schauer,  W.-J. von Appen, B. Rabe, A. Beszczynska-Möller (formerly), E. Fahrbach (formerly) Alfred Wegener Institute (AWI), Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany

L. de Steur, P.A. Dodd, E. Hansen (formerly) Norwegian Polar Institute (NPI), Tromsø, Norway

The Fram Strait is only deep ocean connection between the Arctic Ocean and the Nordic Seas. It is about 600km wide of which approximately half is the 200m deep East Greenland shelf and the other half is up to 2500m deep. Deep water exchanges between the two basins pass through the Fram Strait. The inflow of warm and saline Atlantic Water into the Arctic happens across the Barents Sea and in the eastern side of Fram Strait. There, the West Spitsbergen Current transports warm water into the Arctic, but about half of the Atlantic Water transported to Fram Strait in the boundary current does not enter the Arctic, but rather recirculates across Fram Strait before joining the East Greenland Current flowing southward. The East Greenland Current is also the major export pathway for solid sea-ice and liquid freshwater out of the Arctic Ocean.

In order to monitor both the heat import to the Arctic Ocean and the sea-ice and freshwater export from the Arctic Ocean, the AWI and the NPI have been maintaining a mooring array across Fram Strait. While the array has experienced some changes over the years, the western side of the array has been operated by the NPI with a focus on the sea-ice and freshwater export while the eastern and central part of the array has been maintained by the AWI with a focus on the warm water import.

Figure caption: (a) Long-term mean (2002–2008) current vectors measured at the moored array at different depths, overlaid on the bottom topography, (b) long-term mean temperature (°C) measured by moored instruments, and (c) mean velocity (cm/s) of the cross section (meridional) current component in Fram Strait. (Figure from Beszczynska-Möller et al, ICES Journal of Marine Science 2012.)

References:
Beszczynska-Möller et al, 2012. ICES J. Mar. Sci. 69(5), doi: 10.1093/icesjms/fss056
De Steur et al., 2009. Geophys. Res. Lett. 36, doi: 10.1029/2009GL041278
De Steur et al., 2014. Deep-Sea Res. 92, doi: 10.1016/j.dsr.2014.05.018
Dodd, et al., 2012.  J. Geophys. Res., 117, C11005, doi:10.1029/2012JC008011
Rabe et al., 2013. Ocean Sci., 9, 91-109, doi:10.5194/os-9-91-2013

Atlantic inflow

The following results from the monitoring of the West Spitsbergen Current are discussed in Beszczynska-Möller et al, ICES Journal of Marine Science 2012:

The variability in Atlantic water temperature and volume transport in the West Spitsbergen Current (WSC), based on measurements by the array of moorings in Fram Strait (78°50′N) over the period 1997–2010 shows the following. The long-term mean net volume transport in the current of 6.6±0.4 Sv (directed northwards) delivered 3.0±0.2 Sv of Atlantic water (AW) warmer than 2°C. The mean temperature of the AW inflow was 3.1±0.1°C. On interannual time-scales, a nearly constant volume flux in the WSC core (long-term mean 1.8±0.1 Sv northwards, including 1.3±0.1 Sv of AW warmer than 2°C, and showing no seasonal variability) was accompanied by a highly variable transport of 2-6 Sv in the offshore branch (long-term mean of 5±0.4 Sv, strong seasonal variability, and 1-2 Sv of warm AW). Two warm anomalies were found in the AW passing through Fram Strait in 1999-2000 and 2005-2007. For the period 1997-2010, there was a positive linear trend in the AW mean temperature of 0.06°C/year, but no statistically significant trend was observed in the AW volume transport.