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We examine the trajectories of 168 drifters as a proxy for Atlantic water flowing through the Nordic Seas. The drifters were released at or passed through the Svinøy section, off the west coast of Norway. For comparison, we generate a set of synthetic trajectories using a stochastic model, with a range of diffusivities. With the both sets, we determine the transit times of both drifters and particles from Svinøy to Arctic gateways: the Barents Sea and Spitsbergen. The mean arrival times to these locations are roughly 200 and 500 days, respectively. This implies ample time for cooling of the surface waters, which increases densities and permits subduction before reaching the Arctic. However a range of transit times is seen; some parcels reach Fram Strait in only 4 months while others are still in the southern Nordic Seas after 2 years. The results do not support the idea that temperature or salinity anomalies can exist as coherent packets. The drifters passing Svinøy, when treated as a group, quickly spread over large distances, mixing with water in the Norwegian and Lofoten Basins. Thus an anomaly entering the Nordic Seas would quickly be obliterated. However, the velocity of the clusters center of mass is consistent with anomaly propagation speeds inferred previously from hydrographic measurements, suggesting the observed variability is advective in nature.

The Norwegian Atlantic Current (NwAC) and its eddy field are examined using data from surface drifters. The data set used spans nearly 20 years, from June 1991 to December 2009. The results are largely consistent with previous estimates, which were based on data from the first decade only. With our new data set, statistical analysis of the mean fields can be calculated with larger confidence. The two branches of the NwAC, one over the continental slope and a second further offshore, are clearly captured. The Norwegian Coastal Current is also resolved. In addition, we observe a semipermanent anticylonic eddy in the Lofoten Basin, a feature seen previously in hydrography and in models. The eddy kinetic energy (EKE) is intensified along the path of the NwAC, with the largest values occurring in the Lofoten Basin. The strongest currents, exceeding 100 cm s−1, occur west of Lofoten. Lateral diffusivities were computed in five domains and ranged from 1–5 × 107 cm2 s−1. The Lagrangian integral time and space scales are 1–2 days and 7–23 km, respectively. The data set allows studies of seasonal and interannual variations as well. The strongest seasonal signal is in the NwAC itself, as the mean flow strengthens by approximately 20% in winter. The EKE and diffusivities on the other hand do not exhibit consistent seasonality in the sampled regions. There are no consistent indications of changes in either the mean or fluctuating surface velocities between the 1990s and 2000s.

We compare two methods for estimating mean velocities and diffusivities from surface drifter observations, using data from the Nordic Seas. The first is the conventional method of grouping data into geographical bins. The second relies on a "clustering" algorithm, and groups velocity observations according to nearest-neighbor distance. Capturing the spatial variability of the mean velocity requires using bins with a length scale of ˜50km. However, because many bins have few observations, the statistical significance varies substantially between bins. Clustering yields sets with approximately the same number of observations, so the significance is more uniform. At the densely sampled Svinøy section, clusters can be used to construct the mean flow field with <=10km resolution. Clustering also excels at the estimation of eddy diffusivities, allowing resolution at the 20 km scale in the densely sampled regions. Taking bathymetry into account in the clustering process further improves mean estimates where the data is sparse. Clustering the available surface drifter data, extended by recent deployments from the POLEWARD project, reveals new features in the surface circulation. These are a large anticyclonic vortex in the center of the Lofoten Basin and two anticyclonic recirculations at the Svinøy section. Clustering also yields maps of the eddy diffusivities at unprecedented resolution. Diffusivities are suppressed at the core of the Norwegian Atlantic Current, while they are elevated in the Lofoten Basin and along the Polar Front.