Stability properties of a barotropic surface-water jet observed in the Denmark Strait

Abstract

Observational data from a transect of the Denmark Strait have shown a rapid (1 m/s) southward-flowing barotropic surface jet. A linear stability analysis of the jet suggests that it may be unstable with respect to wavelike perturbations. The most unstable wave proved to be sinuous, i.e., meandering, with a growth rate of around 19 h (in terms of an e-folding time), a phase velocity of approximately 25 cm/s and wavelengths of around 40 km. Associated hydrographic data indicate that the local mixing of the surface waters predominantly is isopycnic. A possible explanation of this observed water-mass distribution could be that the jet initiated cross-channel water transports as the meandering instabilities grew with time. It must, however, be emphasised that since the stability properties of the jet are mainly locally determined, an overall description of the mixing processes in the area cannot be accomplished without invoking the along-stream variations of the flow.

Descent and Modification of the Overflow Plume in the Denmark Strait*

“…Since 1993 the Nordic World Ocean Circulation Experiment (WOCE) and European Community-Variability of Exchanges in Northern Seas (EC-VEINS) programs have undertaken a number of cruises to the Denmark Strait sill and nearby regions. Two of these cruises on the Finnish R/V Aranda are described by Fristedt et al (1999) and Rudels et al (1999). In recent years the Institut für Meereskunde (IFM), Kiel, has also undertaken a number of cruises with the R/V Poseidon and other German vessels to the region, some of which have been briefly described by Krauss and Käse (1998).…”

“…In this case, the inhomogeneities produced near or before the sill could become amplified during the descent of the slope. In this case, the models of Smith (1976) or Fristedt et al (1999) would be more applicable.…”

On the hydrography of Denmark Strait

“…It was demonstrated that the dominant period of variability in current meter records matched that of the most unstable baroclinic wave in a linear, idealized flow in Denmark Strait. An alternate theory was later proposed after the discovery of a weakly stratified southward-flowing current in Denmark Strait [Fristedt et al, 1999]. The current was shown to have unstable growth rates comparable to the baroclinic wave proposed by Smith [1976], that could be responsible for fluctuations in the overflow.…”

A new path for the Denmark Strait overflow water from the Iceland Sea to Denmark Strait

“…At this location the strongest current (up to 90 cm/s) was found over the Icelandic slope where the bottom depth was between 500–600 m supporting the continuation of the current observed at IS7 towards the sill. The structure of the overflow at the sill has been investigated using a vessel mounted ADCP (Acoustic Doppler Current Profiler) and it was observed that the current behaved like a barotropic jet [ Fristedt et al , 1999]. This was also the case at IS7 where currents were measured at 80, 500 and 750 m depth and they all showed a current of ca.…”