Surface circulation and eddy mixing in the Nordic Seas from clustered drifters

Lagrangian (current-following) instruments yield a unique insight into the structure of the oceanic circulation and its variability. However, obtaining a reliable estimate of spatially variable mean currents and eddy field from drifter and float observations is difficult, as these observations are irregularly distributed due to targeted deployments and flow variability. In this work we compare two methods for estimating mean velocities from surface drifter observations, using data from the Nordic Seas. The first is the conventional method of grouping data into geographical bins; and the bins need to have a spatial scale of 50km or less to capture the spatial variability of the mean currents. However, because many areas have few observations, the statistical significance varies substantially between bins questioning the reliability of the estimates. The second method relies on a "clustering" algorithm, and groups a chosen number of velocity observations according to nearest-neighbor distance (Koszalka and LaCasce, 2010). Clustering yields sets with approximately the same number of observations, so the significance is more uniform. At the densely sampled areas clusters can be used to construct the mean flow field with 10km resolution, revealing new features in the surface circulation: a large anticyclonic vortex in the center of the Lofoten Basin and two anticyclonic recirculations at the Svinoy section. Clustering also excels at the estimation of eddy diffusivities, a measure of eddy mixing, allowing resolution at the 20 km scale in the densely sampled regions. Diffusivities are suppressed at the core of the Norwegian Atlantic Current, while they are elevated in the Lofoten Basin and along the Polar Front.