Mesoscale Variability in Denmark Strait: The PV Outflow Hypothesis*

Abstract

The outflow through Denmark Strait shows remarkable mesoscale variability characterized by the continuous formation of intense mesoscale cyclones just south of the sill. These cyclones have a diameter of about 30 km and clear signatures at the sea surface and in currents measured near the bottom. They have a remnant of Arctic Intermediate Water (AIW) in their core. The authors’ hypothesis is that these cyclones are formed by stretching of the high potential vorticity (PV) water column that outflows through Denmark Strait. The light, upper layer of the outflow, the East Greenland Current, remains on the surface in the Irminger Sea, while the dense overflow water descends the east Greenland continental slope. The midlevel waters, mostly AIW, could thus be stretched by more than 100%, which would induce very strong cyclonic relative vorticity. The main test of this new hypothesis is by way of numerical experiments carried out with an isopycnal coordinate ocean model configured to have a marginal sea connected to a deep ocean basin by a shallow strait. An outflow is produced by imposing buoyancy forcing over the marginal sea. If the buoyancy forcing is such as to produce a single overflow layer (analogous to the overflows through the Strait of Gibraltar and the Faroe Bank Channel), then the resulting overflow is slightly time dependent. If the buoyancy forcing is such as to produce both a deep overflow and a midlevel outflow (analogous to the AIW), then the resulting outflow is highly time dependent and develops intense midlevel cyclones just south of the sill where the dense overflow water begins to descend the continental slope. The cyclones found in the numerical solutions have time and space scales set by the midlevel outflow transport, the bottom slope, and the deep stratification. Their scales and structure are roughly consistent with the cyclones observed south of the sill in Denmark Strait. High PV outflow through Denmark Strait is a result of the large-scale wind and buoyancy forcing over the Norwegian–Greenland Sea and Denmark Strait’s location on a western boundary. So far as we know, this configuration and this specific form of mesoscale variability are unique to Denmark Strait.