Description: We examine the relative dispersion of surface drifters deployed in the POLEWARD experiment in the Nordic Seas during 2007–2008. The drifters were launched in pairs and triplets, yielding 67 pairs with an initial separation of 2 km or less. There were 26 additional pairs from drifters which subsequently came near one another. As these produced statistically identical dispersion to the original pairs, we used them as well, yielding 93 pairs. The relative dispersion exhibits three phases. The first occurs during the first two days, at spatial scales less than 10 km. The dispersion increases approximately exponentially during this period, with an e-folding time of roughly half a day. During the second phase, from 2 to roughly 10 days and scales of 10 to roughly 100 km, the dispersion increases as a power law, with r2 α t3. At the largest spatial and temporal scales, the dispersion increases linearly in time and the pair velocities are uncorrelated, consistent with diffusive spreading. We use a stochastic model with a representative mean flow to test the effect of the mean shear on dispersion. The model produces dispersion comparable to the observed during the second and third phases but fails to capture other statistics, such as the PDFs of the displacements. These statistics are instead suggestive of an inverse energy cascade, from the deformation scale up to 100 km.

Description: 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.

Description: During the 4th International Polar Year 2007–2009 (IPY), it has become increasingly obvious that we need to prepare for a new era in the Arctic. IPY occurred during the time of the largest retreat of Arctic sea ice since satellite observations started in 1979. This minimum in September sea ice coverage was accompanied by other signs of a changing Arctic, including the unexpectedly rapid transpolar drift of the Tara schooner, a general thinning of Arctic sea ice and a double-dip minimum of the Arctic Oscillation at the end of 2009. Thanks to the lucky timing of the IPY, those recent phenomena are well documented as they have been scrutinized by the international research community, taking advantage of the dedicated observing systems that were deployed during IPY. However, understanding changes in the Arctic System likely requires monitoring over decades, not years. Many IPY projects have contributed to the pilot phase of a future, sustained, observing system for the Arctic. We now know that many of the technical challenges can be overcome. The Norwegian projects iAOOS-Norway, POLEWARD and MEOP were significant ocean monitoring/research contributions during the IPY. A large variety of techniques were used in these programs, ranging from oceanographic cruises to animal-borne platforms, autonomous gliders, helicopter surveys, surface drifters and current meter arrays. Our research approach was interdisciplinary from the outset, merging ocean dynamics, hydrography, biology, sea ice studies, as well as forecasting. The datasets are tremendously rich, and they will surely yield numerous findings in the years to come. Here, we present a status report at the end of the official period for IPY. Highlights of the research include: a quantification of the Meridional Overturning Circulation in the Nordic Seas (“the loop”) in thermal space, based on a set of up to 15-year-long series of current measurements; a detailed map of the surface circulation as well as characterization of eddy dispersion based on drifter data; transport monitoring of Atlantic Water using gliders; a view of the water mass exchanges in the Norwegian Atlantic Current from both Eulerian and Lagrangian data; an integrated physical–biological view of the ice-influenced ecosystem in the East Greenland Current, showing for instance nutrient-limited primary production as a consequence of decreasing ice cover for larger regions of the Arctic Ocean. Our sea ice studies show that the albedo of snow on ice is lower when snow cover is thinner and suggest that reductions in sea ice thickness, without changes in sea ice extent, will have a significant impact on the arctic atmosphere. We present up-to-date freshwater transport numbers for the East Greenland Current in the Fram Strait, as well as the first map of the annual cycle of freshwater layer thickness in the East Greenland Current along the east coast of Greenland, from data obtained by CTDs mounted on seals that traveled back and forth across the Nordic Seas. We have taken advantage of the real-time transmission of some of these platforms and demonstrate the use of ice-tethered profilers in validating satellite products of sea ice motion, as well as the use of Seagliders in validating ocean forecasts, and we present a sea ice drift product – significantly improved both in space and time – for use in operational ice-forecasting applications. We consider real-time acquisition of data from the ocean interior to be a vital component of a sustained Arctic Ocean Observing System, and we conclude by presenting an outline for an observing system for the European sector of the Arctic Ocean.

Description: 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.