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Heat Flux through Fram Strait calculated from high-resolution year-round measurements and from hindcasts with the NAOSIM model

Fram Strait is the only deep connection between the Arctic Ocean and Nordic Seas and represents the major gateway for the flux of warm water from mid latitudes to the Arctic Ocean. The oceanic heat imported from the North Atlantic has the potential to affect the ice cover in the Eurasian Arctic and to be released to the Arctic atmosphere. Thus, it is an important component for understanding Arctic climate, which is strongly linked with European climate, necessitating long-term measurements and simulations in a regional model.

Since 1997 a continuous time series of volume and heat flux through Fram Strait was derived from measurements with moored instruments. The moorings cover the cross section over the entire deep part of Fram Strait. Temperature and velocity are monitored, allowing to integrate heat fluxes and to distinguish between northward, southward and net fluxes. ASOF-N allowed continuing the time series, which is now sufficiently long to determine the variability of the oceanic fluxes through Fram Strait on inter-annual time scales - and also to approach the declared objective of the ASOF cluster to capture variability on decadal time scales.

The yearly averaged northward heat flow through Fram Strait increased dramatically, from about 38 to 60 Terrawatt (TW), during 1997-2000. In the following years the heat flow decreased slightly although summer temperatures of the inflow measured during ship surveys showed record high values in 2004 and 2005. While moorings record year round data and provide high temporal resolution, they still have a limited spatial resolution. Therefore, since 2001 they are complemented by ADCP (Acoustic Doppler Current Profiler) recordings during ship cruises that deliver high spatial resolution temperature and velocity data sets (typically two or three per year in the summer season). Yearly hydrographic measurements with high spatial resolution were also used to derive the flow field and heat flux with a third independent method.

Key innovative features:
1) The combination of these three observational methods, which to the best of our knowledge has never been published before, is one innovation made during the ASOF-N project in obtaining an as accurate estimate as possible of the heat transport.
2) The complicated topographic structure of Fram Strait leads to a splitting of the warm West Spitsbergen Current into various branches transporting water northward and eastward or recirculating immediately in Fram Strait. The size and strength of the different branches largely determine the input of oceanic heat to the inner Arctic Ocean and have to be distinguished. Therefore one key progress made during ASOF-N was recognizing the strong impact of areas of high recirculation on calculated heat and volume flow. By deploying additional moorings in the central part of Fram Strait the error of calculated heat and volume flow was considerably reduced.
3) A realistic model representation of these different branches was made possible by another innovation, the improved spatial resolution in the North Atlantic-Arctic Ocean-Sea Ice Model (NAOSIM) of AWI.

The NAOSIM models use meteorological data and simulate their influence on sea ice, currents, temperature and salinity in the ocean north of approximately 50°N. The improved model has a horizontal resolution of approximately 9km and a vertical resolution of down to 10m in upper ocean layers (with 50 depths layers in total). The modelling was done for the period from 1990 (using initial conditions in 1990 based on the coarser resolution version of the model) to 2005 using NCEP (National Centres for Environmental Predictions) reanalysis data for the integration.

This model version resulted in a number of improvements, for instance the far better reproduction of the recirculation of Atlantic waters in Fram Strait. The averaged northward volume transport increased to around 10Sverdrup (Sv) between 1995 and 2003. This fits observations much better than the around 3Sv of the previous model. The good agreement between the model and the observationally based estimates in Fram Strait makes it possible to use the model to relate changes in Fram Strait to large scale oceanic developments.

Potential users:
- The scientific community working in climate research (because the oceanic heat flux through Fram Strait is an important part in the North Atlantic and Arctic heat balance)
- Scientists working in Arctic Ocean research (influence of ocean temperature on sea ice, atmosphere, chemical & biological processes)
- Offshore technology and shipping in the Arctic (since the oceanic heat is expected to affect the Arctic ice cover)
- Commercial fishery (because of the temperature effect on the distribution of marine organisms)
- Advisory panels for national and international policies

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