Considering its remote location, Kongsfjorden is a well-studied system. For years, scientists have meticulously collected data on its physical, chemical, and biological properties, essentially providing snapshots of the fjord. Computer models enable us to simulate fjord dynamics between and beyond field campaigns.
Arild Sundfjord and Jack Kohler // Norwegian Polar Institute Jofrid Skarđhamar and Jon Albretsen // Institute of Marine Research
or more than two decades there have been annual cruises in Kongsfjorden to collect hy-drographic and biological data. Moorings at strategic locations have provided valuable data on seasonality in the outer part of the fjord. Many research groups have studied various aspects of the fjord’s marine life and biogeochemistry. All these data – valuable in themselves – can be leveraged to provide even greater understanding, through use of computer models.
Numerical circulation models can complement and integrate findings from short-term measurement cam-paigns and time series from separate locations, and help answer cross-discipline questions that cannot be properly addressed through field data alone.
CONSTRUCTING AND REFINING OUR MODELS We have established a new model for Kongsfjorden.
Building on existing tools, we have assembled new data on fjord bathymetry, glacial runoff and wind to produce multi-year simulations of conditions in the fjord and nearby shelf area. The results have been compared with existing measurements of salinity, temperature, currents and sea ice cover, and the new tools have been found suitable for further use.
F
Ongoing development will improve the model’s accuracy,particularly near the glacier fronts, and the model will be used for budget calculations for ocean acidification, simu-lations of lower trophic level ecosystems, and predictions of future circulation for scenarios when glaciers may have retreated further, thus changing meltwater runoff and circulation in the fjord.
Fram Centre partners (Norwegian Polar Institute, Institute of Marine Research, University Centre in Svalbard) have established a numerical ocean circulation model setup for Svalbard, based on the Regional Ocean Modeling System – (ROMS) a powerful modelling framework used by scien-tists all around the world. Kongsfjorden and the adjacent shelf have been set up on a high-resolution horizontal grid with cells of 160 × 160 metres and 35 vertical layers. The ROMS code allows for simulation of temperature, salinity, currents and sea ice cover. The model is forced with wind and other atmospheric forcing. At the off-shore bound-aries, the model is fed data from a regional model with 800 m resolution, and tides from a tidal prediction model.
We have also assembled a new data set on glacial runoff, based on measurements of glacier and land topography, glacier mass balance, and meteorological forcing. Taken together, this is the most detailed model of the area yet, and forced by the best available data sets.
Longitude
La titude Temper atur e [ ºC] D epth [m]
93
FRAM FORUM 2017 RESEARCH NOTES
Longitude
La titude Temper atur e [ ºC] D epth [m]
Salinit y
Longitude
La titude Salinit y
Longitude
La titude
Map showing part of the S800 model domain (left panel) with surface temperature for an arbitrary date in September 2009 (colour scale), and the area covered by the 160 m Kongsfjorden model (black box). The northward flowing West Spitsbergen Current and the fresher coast-al current are illustrated by red and blue flow fields, respectively, with embedded arrows indicating their flow direction. The right panel shows the domain of the 160 m model, with bathymetry (blue colour scale) and glacier runoff positions (black circles).
Modelled mean parameters at the surface (left panel) and at intermediate depth (100 m, right) for July-September 2005-2009 circulation is shown with arrows, and salinity in colours.
94
FRAM FORUM 2017TESTING MODELS BY COMPARING WITH REALITY We have assessed the quality of the simulations by comparing with available observational data from moorings, cruises and sea ice cover observations. The model reproduces seasonality in hydrography well, and the chief mechanisms of exchange between the main fjord and the shelf outside appear to be properly represented. Individual episodes of inflow are not al-ways simulated at the same time as in reality, and the resulting hydrography (freshwater content, stratifica-tion, minimum and maximum values for temperature and salinity) can therefore deviate from observations.
Nevertheless, we found that the model reproduces the overall picture well, and can be used, for example, to calculate the bulk flux of heat and other tracers car-ried with the water masses into and out of the fjord.
The analysis shows that there is significant circulation all the way into the interior fjord and towards the glacier fronts. This is seen not only in summer – when glacier meltwater drives surface outflow and water flows in at depth to compensate – but all year round.
Given that the ocean water is warmer in summer, the potential for ocean-driven glacier front melting is substantially larger in summer than in winter. But the freezing point of (salty) seawater is lower than that of freshwater, and the ocean heat and salinity carried into the fjord in winter will therefore contribute to delaying the establishment of an ice cover and can, periodically, drive melting of any sea ice that forms, even in periods when the seawater temperature is not high enough to lead to melt at the glacier fronts.
One of the main shortcomings of the present model system is that glacier runoff enters the domain as deep
“rivers” and not through sub-surface outlet tunnels at the base of the glacier fronts. In reality, as melting starts in late spring, the meltwater enters at the sur-face, through cracks and crevasses. But as the season progresses, a complex system of internal tunnels forms in the glacier; later in the melting season most of the freshwater comes out tens of metres below the surface at a very limited number of locations at each glacier front. Since freshwater has much lower density than seawater, the meltwater rises rapidly in “plumes”, and mixes with ambient water. This introduces different sub-surface dynamics and will set up compensating inflows at different depths throughout the melting season. Some of the discrepancies that were identified when we compared the model results with observa-tions likely stem from this simplification of the model.
ADDRESSING THE MODEL’S WEAKNESSES We are currently setting up a realistic stand-alone plume model and the plan is to use results from this to introduce a new parameterisation of meltwater runoff in the ROMS framework. Hopefully, this can be used to better assess the impact of glacier runoff in the fjord and along the outer coast, and also be applied in other fjords or coastal areas with tidewater glaciers. We are also setting up the model for a future scenario, build-ing on new radar measurements of sub-glacier topogra-phy, to simulate the fjord circulation and sea ice cover in a possible future Kongsfjorden with longer fjord arms and where meltwater enters the fjord only as The new model for Kongsfjorden builds on regional-scale
mod-elling done in an Arctic Ocean flagship project (ModOIE), where the Fram Strait was simulated with 800 m horizontal resolution.
The first phase of the work in Kongsfjorden was supported by the Fjord and Coast flagship (KongHiro project) and the present and planned work is funded by the Research Council of Norway (TIGRIF project).
RESEARCH NOTES
95
FRAM FORUM 2017
rivers flowing down from glaciers that have retreated fully onto land. A lower trophic level ecosystem model will be established, to assess nutrient dynamics and local versus imported productivity in the fjord.
FURTHER READING:
Sundfjord A, Albretsen J, Kasajima Y, Skogseth R, Kohler J, Nuth C, Skardhamar J, Cottier F, Nilsen F, Asplin L, Gerland S, Torsvik T (2017) Effects of glacier runoff and wind on surface layer dynamics and Atlantic Water exchange in Kongsfjorden, Svalbard;
a model study. Estuarine, Coastal and Shelf Science.
187: 260-272 DOI: 10.1016/j.ecss.2017.01.015
Calculated volume flux toward front of Kronebreen/Kongsvegen, for 2005-2010. Daily values in black and running mean in red. The model can be used to calculate volume fluxes, and thus flux-es of heat, carbon, salinity and other properties associated with water flux.
This is extremely difficult to do through observations.
RESEARCH NOTES