MESOSCALE VARIABILITY IN DENMARK-STRAIT - THE PV OUTFLOW HYPOTHESIS

The outflow through Denmark Strait shows remarkable mesoscale variabil ity characterized by the continuous formation of intense mesoscale cyc lones 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 (AI W) in their core. The authors' hypothesis is that these cyclones are f ormed by stretching of the high potential vorticity (PV) water column that outflows through Denmark Strait. The light, upper layer of the ou tflow, the East Greenland Current, remains on the surface in the Irmin ger Sea, while the dense overflow water descends the east Greenland co ntinental slope. The midlevel waters, mostly AIW, could thus be stretc hed by more than 100%, which would induce very strong cyclonic relativ e vorticity. The main test of this new hypothesis is by way of numeric al experiments carried out with an isopycnal coordinate ocean model co nfigured to have a marginal sea connected to a deep ocean basin by a s hallow strait. An outflow is produced by imposing buoyancy forcing ove r the marginal sea. If the buoyancy forcing is such as to produce a si ngle 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), t hen the resulting outflow is highly time dependent and develops intens e midlevel cyclones just south of the sill where the dense overflow wa ter begins to descend the continental slope. The cyclones found in the numerical solutions have time and space scales set by the midlevel ou tflow 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 t he 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.