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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Nordic Seas Eddy Exchanges

Awarded: NOK 4.7 mill.

In NORSEE, we study oceanographic processes relevant for the Nordic Seas. The Nordic Seas are an important region in the global ocean, as this is a main site of water transformation for the large scale overturning circulation. The transformation involves complex processes, by which warmer waters adjacent to the coast are mixed with colder water in the interior by energetic eddies originating in the North Atlantic Current (NAC). The eddies also mix biological material, such as phytoplankton and fish larvae, and in addition pose challenges for subsurface activities in the offshore industry. In NORSEE, we conducted projects which shed light on these processes. Several years ago, we conducted a field experiment here, with 150 freely-drifting surface buoys. These drifters, tracked by satellite, revealed the complex pathways that water parcels take in traversing the Seas. But interpreting drifter trajectories can be subtle. So, to advance the methodology used, we also studied similar data sets from other parts of the world, in collaboration with international researchers. We studied the dispersion of the drifters in the Gulf of Mexico in the GLAD experiment, the largest drifter deployment to date in the world. The results were similar to those we obtained in the Nordic Seas, with the exception that the drifters, now tracked by GPS at higher resolution, exhibited distinct high frequency motion. Our work showed how this motion affects the overall dispersion. With other colleagues, we examined the dispersion of floats in the Southern Ocean, 1 km below the surface. These were deployed as part of a National Science Foundation experiment. The float trajectories revealed a wealth of information about the transfers occurring across the Antarctic Circumpolar Current (ACC), which in many ways resemble those occurring across the NAC. Another important question with regards to surface drifters is how they reflect the motion occurring beneath the surface. This is relevant for satellite measurements, as satellites resolve only surface fields. We developed an algorithm for projecting surface velocities down in the water column and tested it against simulations with full-complexity numerical ocean models. The algorithm yields accurate predictions down to nearly 1 km. This could potentially greatly improve forecasting ocean drift. In a follow-up study, we analyzed current meter records, from around the globe. The results suggested that our traditional concepts about the vertical structure of ocean eddies need to be modified. We explored that in a theoretical article, which considered how the vertical structure was affected by bottom topography. As a result, we must revisit a number of ocean theories, which assume the incorrect vertical structure. We have begun doing exactly that. In NORSEE we also studied the large scale overturning circulation, in particular how surface heating and cooling drive the flow. The equator to pole temperature gradient generates surface currents from the west to the east, which then turn north and south when encountering the eastern boundaries. The northward flow enters the Nordic Seas in the east and is then cooled---as in the observations. We conducted simulations with a global ocean model, forced only with surface heating and cooling. The resulting overturning was comparable to that observed in full models and in observations. The results help us understand how the overturning responds to global warming, which preferentially warms the Arctic. We have also studied the circulation and eddy processes in the Nordic Seas. We used a high-resolution numerical model to study how the Atlantic water recirculates in Fram Strait and returns south again. We found that most of the Atlantic Water recirculates further north than expected, and that eddies are important in mixing waters from the currents that flow into the Arctic and those that flow south along the east Greenland coast. Eddies are also important in the Lofoten Basin, where much of the cooling of the Atlantic Water occurs. We studied the how the steep continental slope off Northern Norway affects the eddying that spreads Atlantic Water offshore. Counter-intuitively, the steepest part of the continental slope is where most of the eddies are generated. In another study we focused on a particularly large eddy, the 'Lofoten Basin anticyclone', which resides in the middle of the basin. We found that the eddy is rather stable, which explains its pronounced longevity. Finally, using a radioactive nuclide as a 'tracer', we showed that transport paths and travel times through the Nordic Seas are strongly affected by eddy stirring along the path of the main currents.

The exchange of heat between the Nordic Seas and the atmosphere is an important and complex aspect of the oceanic thermohaline circulation and the climate system. The exchange is mediated by small scale eddies, which mix the warm waters from the North Atl antic with the cooler waters in the Nordic Seas. These eddies are poorly captured in climate models, with serious potential impacts for both the atmospheric circulation and the Arctic sea ice cover. The proposed work seeks to characterise these eddy proce sses, to define a benchmark by which models can be assessed so as to ultimately improve how the processes are represented in the models.

Funding scheme:

FRINATEK-Fri prosj.st. mat.,naturv.,tek