Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Freshwater Flux through Fram Strait during 2003-2005 as measured during ASOF-N

Background and result description:
Fram Strait is the main source of freshwater for the Greenland and Iceland Seas. Along with Davis Strait it is also the main provider of freshwater for the Labrador Sea and North Atlantic. This freshwater export from the Arctic to subarctic seas has the potential to influence the northbound current systems by modifying the stratification of the receiving basins. This would alter the oceanic heat transport, which again would influence the climate of North Western Europe. The export of fresh water occurs in liquid (polar water) and solid (sea ice) phase. With the advent of ASOF-N we were able to continue and extend existing time series (solid phase freshwater flux since 1990 and liquid freshwater flux since 1997), and hence determine the seasonal and inter-annual variability of freshwater fluxes through Fram Strait.

The annual cycle of the liquid freshwater transport in the East Greenland Current (EGC) has a minimum in spring (~600km3/yr) and a maximum in late summer (~2000km3/yr). The mean of the transport time series is 900km3/yr. The anomalies with respect to the mean seasonal cycle have a magnitude of typically 500km3/yr. There is no general trend in the freshwater transport over the period 1997 to 2005. From the first ASOF-N winter time CTD section and the first mooring on the shelf it is clear that additional moorings are needed, as there is a potential for considerable freshwater transport across the wide shelf. During the May 2005 cruise low salinity water was found also east of the EGC, meaning that an additional freshwater transport could occur east of the existing mooring array.

The comparison between fresh water observations at 79 N, 74 N and 63 N so far shows no significant signal propagation of water mass characteristics along the EGC path, in contrast to what is observed along the Atlantic Water path in the east.

Key innovations:
Time series have been maintained using more basic observational arrays since 1997 but ASOF-N allowed using a full-scale version of the observational array facilitating key innovations to be made:
1) With the advent of ASOF-N we were able to tailor make the observational set-up to study fresh water transport, as far as the physical conditions allow. This includes near surface salinity sensors and tube moorings with ADCPs (Acoustic Doppler Current Profiler).
2) The wide shelf at this latitude was an open issue as due to extensive ice cover and icebergs drifting through the region, access is difficult and moorings are not likely to survive. Few observations have therefore been made in this area. Introducing tube moorings has provided our first direct measurements of salinity on the shelf at 79N.
3) The moorings provide year round point measurements in the vertical and horizontal. Annual cruises during summer provide high-resolution hydrography, which aids in the interpretation of the point measurements.

However, for the observation of freshwater fluxes we also need high-resolution spatial information on the seasonal cycle of the stratification. An extensive ice cover with heavy multiyear ice has prevented access to the region during winter. In 2005 a coastguard icebreaker was used to penetrate into the pack, serving as a base for helicopter CTD transects with portable equipment. This allowed ASOF-N to do the first high-resolution wintertime hydrographic transect.
4) For some years freshwater observations have been performed also at the 74 and 63 North latitudes. With ASOF-N we were able to do the first comparisons of freshwater observed at these different latitudes. The aim of this ongoing work is to draw conclusions on the fate and pathways of freshwater exiting the Arctic at 79N.
5) Model results from the high-resolution version of AWI's NAOSIM have been used to fill spatial and temporal gaps in the observations and to link the freshwater fluxes in the East Greenland Current with the large-scale oceanic circulation as well as with the meteorological forcing fields. The large liquid fresh water export event of the mid-1990s could thus be linked to changes in the Arctic Ocean freshwater distribution during previous years. These redistributions were forced by the strong positive NAO wind forcing during the early 1990s.

Current status:
The time series are now approaching a length, which will enable us to quantify variability on decadal time scales, a stated objective of the ASOF cluster.

Climate scientists, since the freshwater output from the Arctic is thought to influence the net densification at high latitudes, and hence the current systems governing the oceanic heat transport to northern regions. Oceanographers working in the Labrador and Nordic Seas, since the freshwater output will modify the stratification and hence the processes occurring here. Advisory panels for national and international policies, particularly when the link between Arctic freshwater output and the climate system is better established.

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