No abstract

As the Arctic sea ice declines over the last decades, understanding and monitoring the Atlantic Water (AW), the main source of heat and salt of the Arctic Ocean, has become increasingly more important (Carmack et al., 2015;Polyakov et al., 2017). AW enters the Arctic Ocean through both the Barents Sea and Fram Strait. The Fram Strait branch enters the Arctic Ocean with the West Spitsbergen Current (WSC), on the continental slope west of Svalbard. On the southern tip of the Yermak Plateau, the WSC splits in different branches as the isobaths diverge:

The Winter Process Cruise (WPC) aboard RV Kronprins Haakon (KH2021702) conducted observations on processes that control the position and variability of the polar front in the Northern Barents Sea and the distribution of Arctic and Atlantic water masses. Moreover, the WPC serviced 2 gateway moorings sites (M1 and M4) and collected complementary hydrographic, microstructure and current profiles to detect the winter circulation pattern and the layering structures between the two competing water masses. Meteorological measurements were also made.

<p>On the continental slope north of Svalbard, Atlantic Water is transported eastward as a part of the Arctic Circumpolar Boundary Current. As inflow of Atlantic Water through the Fram Strait is the largest oceanic heat source to the Arctic Ocean, it is important to improve our knowledge about the dynamics and processes that govern the heat exchange between Atlantic Water and water masses of Arctic origin. This includes processes that enable lateral exchange across the shelf break or into the interior of the deep basin. Here, we study the vorticity dynamics on the slope and its contribution to the water mass modifications and heat exchange. Focusing on topographically trapped waves – sub-inertial oscillations trapped to follow the continental slope – we establish their existence and properties on the northern slope of Svalbard using a free baroclinic wave model. Their dependence on background stratification and current properties is explored in sensitivity analysis. Next, we discuss their contribution to lateral exchange from the boundary current on the slope to the continental shelf, troughs, and the deep Nansen Basin in the Arctic Ocean, including exchange associated with instabilities and resulting eddy shedding off the vorticity waves. Hydrographic and current time series from 2018-19 at two mooring arrays crossing the slope north of Svalbard (The Nansen Legacy project) are used to associate the observed physical environment with model-predicted topographic waves. Analysis of the in-situ data will determine which wave mode that can exist over the sloping seafloor and the observed hydrography and flow, and the model will give the corresponding spatial characteristics for the given frequencies and wave numbers. Energetic oscillations present in the observations are analyzed in light of the model results. Of special interest are the seasonal variability in hydrography and current strength and the resulting modification of the wave characteristics. Moreover, the interaction between the vorticity waves and tidal oscillations in the diurnal band is emphasized.</p>

<p></p><p>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 advances 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.</p><p></p><p>In summer 2018, 6 moorings organized in 2 arrays were deployed across the Atlantic Water Boundary current for a year, within the framework of the Nansen Legacy project. In parallel, turbulence structure in the Atlantic Water boundary current was measured north of Svalbard in two different periods (July and September), using a Vertical Microstructure Profiler (Rockland Scientific) in both cruises and a Microrider (Rockland Scientific) mounted on a Slocum glider in September.</p><p></p><p>Using mooring observations, we investigated the background properties of the Atlantic Water boundary current (transport, vertical structure, seasonal variations) and the possible sources of the low-frequency variations (period of more than 2 weeks).</p><p></p><p> Using observations during the cruise periods, 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<sup>-4</sup> W m<sup>-2</sup>. 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<sup>-3</sup> W m<sup>-2</sup>. 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.</p><p></p>