The Mid Depth Circulation in the Nordic Seas (2010)

G. Voet, D.Quadfasel (1) , K.A.Mork and H.Søiland (2)

  

Corresponding author : Gunnar Voet

(1) : University of Hamburg, Germany

(2) : University of Bergen, Norway

Full paper: Voet, G., D. Quadfasel, K. A. Mork, and H. Soiland, 2010: The mid-depth circulation of the Nordic Seas derived from profiling float observations. Tellus Series a-Dynamic Meteorology and Oceanography, 62, 516-529, doi:10.1111/j.1600-0870.2010.00444.x.

The trajectories of 61 autonomous ARGO floats deployed between 2001 to 2009 in the Nordic Seas (Greenland Sea, Norwegian and Lofoten Basins, Iceland Plateau) are used to analyse the mean circulation at mid-depth (1000-1500m) and its seasonal variability. The cyclonic mid-depth flow in all basins, tightly influenced by the topography in a weakly stratified region, is consistent with the surface flow (Jakobsen et al., 2003). The seasonal variability of the gyre circulation within the Greenland and the Norwegian basins is large and in magnitude comparable to that of the mean flow. The seasonal circulation is a barotropic response to the winds and baroclinic effects like convection only play a minor role for setting up the currents on the basin scale. This study is financially supported by Deutsche Forschungsgemeinschaft (SFB 512 E2) and the European Commission (Euro-Argo and Mersea).

Data and Methods

Within the Argo project, 61 profiling floats have been deployed in the four basins of the Nordic Seas since summer 2001, representing more than 4100 profiles through the last data update for this study in February 2009. Most of those floats were programmed for a 10-days cycle, and a parking depth at 1000 m except for seven floats in the Norwegian and the Lofoten Basin drifting at 1500-m depth. The positions of all profiles recorded by the floats and the corresponding growing data density are shown in figure 1.

 Figure 1.(left) : Positions of all profiles recorded by the floats in the Nordic Seas and (right) number of profiles per month recorded by the floats for each basin of the Nordic Seas.

The mid-depth circulation is estimated from the surface position data of the floats available in the trajectory files. The drift velocity is the distance calculated between the last surface position before its descent to the parking depth, and the first surface position after the ascent back to the surface, divided by the time interval. A mean circulation map at the float parking depth is obtained by averaging the data set over the period 2001-2009, and assigning each float observation to the nearest point of a rectangular grid with a spacing of 110 km.

The monthly mean wind stress curl for each basin of the Nordic Seas is calculated for the period when floats where present in the basin from NECP/NCAR reanalysis data (Kalnay et al., 1996). The ETOPO2 bathymetry with a two minutes resolution is used to assign bottom depths and topographic gradients to the float displacements.

Main Results

The first observation is that the mean flow in the Nordic Seas is topographically influenced. In general, the floats follow lines of constant bottom depth, and have the tendency to stay in the basin they were deployed. On average, 75% of all float positions stem from the deployment basin while the remaining 25% are located in one of the other basins.

The time-mean mid-depth circulation (figure 2) is dominated by cyclonic gyres in each of the subbasins, that are intensified at boundaries towards the edge of the basins. In the centre of the basins, the velocities are relatively small and more randomly directed. The circulation at mid-depth generally has the same pattern as the surface circulation shown in the study by Jakobsen et al.(2003), and is also very similar to the model results of Nøst and Isachsen (2003).

Figure 2. Time-mean mid-depth circulation of the Nordic Seas on a rectangular grid with a size of 110 km.Only mean values calculated from more than five observations are shown.

The float data set is used to estimate the strength of the seasonal cycle of the mid-depth circulation. The monthly mean gyre velocities have been thus calculated by averaging over all along bathymetry velocities recorded in the rim areas of the basin during one month. First results (figure 3) clearly show that the Greenland Basin has a seasonal cycle with higher velocities in winter, and a minimum velocity in late summer as has the Norwegian Basin. The observations over the Iceland Plateau also show lower velocities in summer than in winter, but the amplitude is small with only 0.5 cm/s (versus 3 cm/s for the Greenland Sea and 1.5 cm/s for the Norwegian Sea). This low amplitude matches with the low mean velocities found over the Iceland Plateau on figure 2. In the Lofoten Basin, there is no clear seasonal cycle, and low velocities occur in winter whereas maxima are seen in early summer and early winter with peak-to-peak amplitudes just over 1 cm/s.

Figure 3. Seasonal variability of the gyre velocity in the four basins. The gyre strength is defined as the mean velocity along the bathymetry at the rim of the gyre. Thin lines with markers give monthly mean values, thick lines show a low-pass filtered version (weighted three-point average) of the monthly values. The standard deviation for the monthly mean values (not shown in the figure) is about 4–6 cm s−1 for all months and basins. This statistical uncertainty reflects the mesoscale variability that is not resolved by the measurement cycle of the floats of around ten days.

The influence of the wind forcing on the seasonal variability of the flow field derived from the float data of figure 3 is then analysed. The mean wind forcing over the Nordic Seas looks cyclonic and has a clear seasonal cycle. The temporal change of the subbasin gyre velocity (fig. 4, in red) is calculated from the sum of NCEP wind forcing (fig.4, in green) and bottom friction (fig.4 in blue), and compared to the one seens in float observations (fig.4 in black).

Figure 4. Comparison of observed seasonal changes of the basin gyre circulation (black) and calculated changes from forcing terms (red). The wind forcing term is shown with the green line and the bottom friction with the blue line. The three-point weighted average version was used for all terms shown here.

  • For the Greeland Sea and the Norwegian Basin, the wind forcing explains a large part of the observed seasonal variability, with a correlation between observation and the sum of wind and bottom drag about 0.8 and time lags of 2 and 1 months respectively.
  • In the Lofoten Basin, this correlation is always below 0.5 and other processes than wind forcing are expected to play an important role in the observed change in the circulation on the seasonal cycle. The gyre circulation here contains a strong semi-annual component comparable in magnitude to the seasonal cycle, maybe explained by a weaker topographic control of the flow and a more important mesoscale variability generated by baroclinic instabilities of the flow in this subbasin.
  • Over the Iceland Plateau, the misfit between observation and the sum of wind forcing and bottom drag may be attributed either to a wrong velocity observation used in the analysis (because of unsufficient data) or to other oceanic processes influencing the mean flow in a basin of weaker topographic gradients.
Conclusion

The Nordic Seas are a region of intense water mass transformation. Here, through heat loss, freshwater input and melting and freezing of sea ice, buoyant Atlantic Water is transformed into buoyant (cold, low salinity) near surface waters and dense (cold, intermediate salinity) intermediate and deep waters. This transformation and the associated circulation set up and maintain fronts.

The Nordic Seas are exposed to strong momentum forcing, with the Greenland High and the Iceland Low being the permanent features of the atmospheric pressure pattern. Both, buoyancy and momentum forcing, drive the cyclonic circulation internal to the Nordic Seas that may be viewed as a northern extension of the North Atlantic’s Subpolar Gyre, albeit regional characteristics due to the limited exchange across the Greenland-Scotland Ridge. Momentum and buoyancy forcing are both characterized by large seasonal variability, like the cyclonic gyre circulation mainly responding to the winds.

References
  • Jakobsen, P. K., Ribergaard, M. H., Quadfasel, D., Schmith, T. and Hughes, C. W. 2003. Near-surface circulation in the northern North Atlantic as inferred from Lagrangian drifters: variability from the mesoscale to interannual. J. Geophys. Res. 108(C8), 3251.
  • Jonsson, S. 1991. Seasonal and interannual Variability of Wind Stress Curl Over the Nordic Seas. J. Geophys. Res. 96(C2), 2649–2659.
  • Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D. and co-authors. 1996. The NCEP/NCAR 40-Year Reanalysis Project. B.
  • Nøst and Isachsen : The Large-scale time-mean circulation of the Nordic Seas and Arctic Ocean from simplified dynamics Journal of Marine Research, Volume 61, Number 2, 1 March 2003 , pp. 175-210(36) 2003.