Ma. Spall et Jf. Price, MESOSCALE VARIABILITY IN DENMARK-STRAIT - THE PV OUTFLOW HYPOTHESIS, Journal of physical oceanography, 28(8), 1998, pp. 1598-1623
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.