Watermass transformation processes and vortex dynamics in the Lofoten Basin of the Norwegian Sea

ProVoLo is a coordinated, basic research study to observe, understand, and quantify the fundamental processes that shape the oceanographic structure of the Lofoten Basin. The Lofoten Basin, situated in the northern Norwegian Sea, is emerging as a fundamental player in our climate and fisheries. In this area the warm and salty Atlantic Water is subject to the greatest heat losses anywhere in the Nordic Seas. The Lofoten Basin is energetic: it stands out as a hotspot in the maps of eddy kinetic energy including the slope region associated with eddies shed from the Norwegian slope current, and a maximum at the center of the basin associated with a long-lived, deep and large eddy (Lofoten Basin Eddy, LBE). The study addresses the water transformation processes in the three distinct and important regions of the Lofoten Basin: the steep Norwegian continental slope, the basin pooling the warm Atlantic Water, and the frontal region over the rough Mohn Ridge. The main approach is a combination of theory, numerical modelling and an observational programme. The field component includes dedicated process cruises in summer and in winter coordinated with deployments of moorings, underwater gliders, and deep floats. Specifically, we address the hypotheses - the slope and the front currents bordering the Lofoten Basin each contribute significantly to the variability of energetics, water properties and their mixing in the basin; - eddy-induced transport from the instability of the slope current, and related dynamics are critical for the heat and salt budget of the basin; - the LBE is a crucial component of the water mass transformations in the basin; - the stability and lifetime of the LBE are affected by substantial vertical and horizontal mixing across the eddy; - mixing across the front over the Mohn Ridge is substantial. The first scientific cruise was conducted in June 2016. Surveys and process studies were made in the LBE in the central basin as well as near the Mohn Ridge. Operations included ocean current, temperature, salinity and microstructure profiling, moorings, subsurface and surface drifters, and ocean gliders. Gliders, moorings and subsurface drifters (RAFOS floats) collected data from summer 2016 to fall 2017. The summer cruise in 2016 was followed by a winter process study in March 2017, and the last PROVOLO cruise in September 2017. In this last cruise, we recovered all instruments and collected new data. Seagliders are underwater robots, which profile the upper 1000 m of the water column and send the data directly to your office. The missions can be controlled by satellite communication when the glider is at the surface. One glider concentrated in the Lofoten Basin and the LBE. A second glider collected transects across the Mohn Ridge. Neutrally buoyant RAFOS floats drift freely through the ocean and trace out the movement of waters. A total of 18 RAFOS floats were deployed in the LBE, at the centre, at the radius of max speed (15 km) and at a radius of 30 km, with target depths 200, 500 and 800 m. Another set of 7 RAFOS floats were deployed across the Mohn Ridge. Using this unique data set we described the circulation patterns, dynamics and water mass transformation processes in the Lofoten Basin. We first analysed 3 years of data from Seagliders deployed in the LBE. The core water properties show substantial interannual variability. Our observations make a step change in the available data sets from the region and in our understanding of the year-to-year and seasonal variability, and the mechanisms affecting the horizontal and vertical exchanges and the stability and lifetime of the eddy. We next gathered data sets from different sources in the period between 2000 and 2017. A detailed analysis shows that the western Lofoten Basin is an important location for Atlantic water transformation. Microstructure measurements across the eddy showed a turbulent core relative to outer stations. Particularly there was strong shear and mixing below the deep velocity maximum. Energy and waves, generated at inertial time scales determined by the Earth's rotation, were trapped by the eddy's rotation, and provided an additional source for turbulence. Based on summer observations, we estimate a time scale of 14 years to drain the total energy of the eddy. We studied the pathways of the Atlantic Water in the region using numerical simulations. We analyzed the trajectories of 1.5 mill. numerical particles introduced uniformly in Lofoten Basin at several depths, to study how their paths and characteristics (temperature, salinity etc) evolve. The relatively warm Atlantic Water mainly enters the Lofoten Basin as a slab from the south at surface and at deeper levels from the continental slope in the southeast, implying that the highly energetic continental slope can have a larger impact on the flow at deeper levels than at the surface.

The project delivered results that contribute to the understanding of the dynamics of components of the ocean circulation in the region, with applications to nutrient distribution, marine ecosystems, ocean carbon uptake and acidification. Furthermore large heat losses from the Lofoten Basin impacts local weather. The region is characterized by extreme weather events and polar lows. A bottle-neck for progress in accurate predictions, of extreme events in particular, is the procurement of detailed observations to constrain the parameterizations and their uncertainties. Our observations provide new insight into oceanographic processes and dynamical mechanisms in a region where the warm Atlantic Water goes through substantial transformations.

ProVoLo is a coordinated, basic research proposal to support a comprehensive programme to observe, understand, and quantify the fundamental processes that shape the oceanographic structure of the Lofoten Basin (LB). The LB situated in the northern Norwegian Sea is emerging as a fundamental player in our climate and fisheries. It is remarkably energetic and is home to the largest and deepest pool of warm Atlantic Water. The role played by the LB in watermass transformation is increasingly being recognized, yet the understanding of the processes and pathways of energy transfer and mixing is incomplete. ProVoLo is unique because 1) it addresses the three connected geographical regions of the LB (the Norwegian slope, the central basin with its persistent LBE, and the Mohn Ridge), covers 2) summer and wintertime conditions, and 3) the entire water column from spatial scale of turbulence to mesoscale. ProVoLo builds on a strong national team and international network. The main approach is a combination of theory, numerical modelling and an ambitious, but feasible, observational programme. The field component includes dedicated process cruises in summer and in winter coordinated with deployments of moorings, gliders and Lagrangian floats. We hypothesize that - the slope and the front currents bordering the LB each contribute significantly to the variability of EKE, water properties and their mixing in the basin; - eddy-induced transport from the instability of the slope current, and sub-mesoscale dynamics are critical for constraining the lateral heat and salt fluxes; - the LBE is a crucial component of the watermass transformations and mode water formation in the LB; - the stability and lifetime of the LBE are affected by substantial isopycnal and diapycnal mixing across the rim of the LBE; - submesoscale processes lead to substantial isopycnal/diapycnal mixing across the front over the Mohn Ridge.