Near‐surface circulation of the Nordic seas as measured by Lagrangian drifters

Abstract

In the period June 1991 to August 1993, 107 Argos tracked, drifters drogued to 15 m depth, were released in the Nordic seas (or Greenland, Iceland, and Norwegian Seas). The drifter movements revealed the strong and spatially confined current systems along the surface salinity fronts of the Iceland‐Faroe Frontal zone and of the Norwegian coast and along the continental margins and their extensions to the Barents Sea and Spitsbergen. The Norwegian Atlantic Current is composed of three distinct streams (two continental margin and one coastal branches) which join into one single swift mean current west of the Lofoten and Vesterålen Islands, where the strongest measured currents are in excess of 110 cm s−1. In addition to the general cyclonic gyre circulation in the Nordic seas, the drifters indicate smaller cyclonic circulation patterns in all the major subbasins, i.e., the Iceland plateau, the Norwegian, the Lofoten, and the Greenland basins. No surface signature of the East Icelandic Current is disclosed by the drifters. Interpolated and low‐pass‐filtered position data were used to construct maps of 15‐m‐depth ensemble mean velocity, velocity variability, and residence time. Vigorous eddy fields are dominant in the strong currents and in the Lofoten basin. Eulerian correlations indicate that they tend to propagate to the west. In contrast, the Iceland plateau appears quiescent, both in the mean and eddy velocities. Single‐particle diffusivities are computed and are found to be in the range 1–7 × 107 cm2 s−1. The corresponding Lagrangian timescale and space scale are 1–3 days and 10–40 km, respectively. These Lagrangian drifter measurements compose the first basin‐scale, accurate near‐surface velocity data set of the Nordic seas.

The Crossover Time as an Evaluation of Ocean Models Against Persistence

“…The model forecast fails to ever reach a crossover time for the entire span of the available prediction. During the study period the average decorrelation times of the drifter velocities was found to be 1.2 days (not shown) in approximate agreement with Poulain et al (1996), LaCasce (2008), Döös et al (2013), and Lumpkin and Johnson (2013). This time scale is very similar to the e-folding time scale of persistence (black line in Figure 2a).…”

A Greenland Sea Perspective on the Dynamics of Postconvective Eddies*

“…The model also produces a mean broad anticyclone near the center of the Greenland Basin, which is a strong feature. Although float data (Poulain et al 1996) and current observations (Oliver and Heywood 2003) suggest that the local flow often opposes the largerscale mean circulation, it is doubtful that such a strong mean feature exists in the real Greenland Sea. Figure 7b shows that EKE is greatest where there are strong density gradients and weak topographic gradients.…”

“…Dashed arrows indicate currents that are either less well established by observations or less permanent. Studies on which the schematic is based include results from a tracer release experiment (Messias et al 2008), observations from floats (Poulain et al 1996;Gascard et al 2002;Jakobsen et al 2003), moored and lowered current meters (Woodgate et al 1999;Fahrbach et al 2001;Oliver and Heywood 2003), hydrographic observations (Quadfasel and Meincke 1987), and modeled reconstructions of the circulation of the Nordic seas (Nøst and Isachsen 2003;Eldevik et al 2005). …”

Deep ocean exchange with west-European shelf seas

“…The NCC transports 1-2 Sv northwards from inflow across 61 • N, with variability also 1-2 Sv -current variance is 0.01-0.02 m 2 s −2 (Poulain et al, 1996). NCC salinity (reduced by Baltic outflow) is less than 33 at 61 • N. Despite much riverine freshwater (greatly exceeding precipitation-evaporation), the salinity increases to about 34 off northern Norway as Atlantic water is entrained.…”