Anthony Bosse (OPLC) will present a seminar on February 3.

Vortex dynamics and biogeochemical implications in the Lofoten Basin of the Nordic seas

The warm Atlantic water flow throughout the Nordic Seas is the upper limb of the Atlantic overturning circulation, before it is transformed into intermediate and deeper waters through atmosphere-ocean interactions. In this region, the Lofoten Basin is the largest reservoir of heat, and is exposed to the largest surface buoyancy losses. An intense permanent anticyclone, the Lofoten Basin Eddy (LBE), impacts the hydrography, energetics, and the ecosystem dynamics in the basin. While the LBE characteristics were previously reported from opportunistic observations, the processes contributing to its longevity, the seasonal cycle of associated dynamics and energetics are not known. 

In a first part, we use observations from underwater gliders, subsurface floats and multiple cruises to describe the control mechanisms and the seasonality of the LBE. The lateral exchanges between this non-linear eddy and the surrounding are mediated by potential vorticity gradients. The 1200-m deep eddy core is highly coherent protected by a high strain rate at its periphery. Lateral exchanges are enhanced in early winter, when dynamical barrier weakens. An energy budget for the eddy reveals a timescale of dissipation of approximately 3 years, long enough to allow winter convection to restore the eddy’s potential energy reservoir and contribute to its longevity.

Secondly, we describe the impacts of the turbulence across the LBE on biogeochemical fluxes. Biogeochemical samples of nutrients and carbon were taken during a cruise in September 2017, while at the same time a glider equipped with turbulence and bio-optical sensors collected high-resolution data for 5 days across the LBE. The cross-section of biogeochemical variables is reconstructed along the glider trajectory. Combined with the microstructure glider data, vertical fluxes of nutrients due to turbulence are finally quantified.