Turbulence north of Svalbard in summer 2018: mixed layer evolution and Atlantic Water layer dynamics

Zoé Koenig1, Eivind Kolås2, Ilker Fer2, Ragnheid Skogseth3, Marika Marnela4 and Frank Nilsen5, (1)University of Bergen / NPI, Bergen, Norway, (2)University of Bergen, Bergen, Norway, (3)UNIS, Longyearbyen, Norway, (4)UNIS, Norway, (5)The University Centre in Svalbard, Longyearbyen, Norway
North of Svalbard is a key region for the Arctic Ocean heat and salt budget as it is the gateway for one of the main branches of Atlantic Water in the Arctic. As the Atlantic Water layer advences into the Arctic Ocean, its core deepens from about 250 m depth around the Yermak Plateau to 350 m in the Laptev Sea, and gets colder and less saline due to mixing with surrounding waters. The complex topography in the region facilitates vertical and horizontal exchanges between the water masses and, together with strong shear and tidal forcing driving increased mixing rates, impacts the heat and salt content of the Atlantic Water layer that will circulate the Arctic Ocean.

In summer 2018, turbulence structure in the Atlantic Water boundary current was measured north of Svalbard in two different periods (July and September), within the framework of the Nansen Legacy project. Turbulence profiles were collected using a Vertical Microstructure Profiler (Rockland Scientific) in both cruises (total of 240 profiles) and a Microrider (Rockland Scientific) mounted on a Slocum glider (4 days of data, 54 dives and 108 profiles) in September.

We investigated changes in the mixed layer through the summer and the sources of vertical mixing in the water column. In the mixed layer, depth-integrated turbulent dissipation rate is about 10-4 W m-2. Variations in the turbulent heat, salinity and buoyancy fluxes are strong, and hypothesized to be affected by the evolution of the surface meltwater layer through summer. When integrated over the Atlantic Water layer, the turbulent dissipation rate is about 3.10-3 W m-2. Whilst the wind work exerted in the mixed layer accounts for most of the variability in the mixed layer, tidal forcing plays an important role in setting the dissipation rates deeper in the water column.