Kinematic Structure and Dynamics of the Denmark Strait Overflow

Peigen Lin1, Robert S Pickart1, Kerstin Jochumsen2, Martin Moritz2, Kent Moore3, Hedinn Valdimarsson4 and Tim Fristedt5, (1)Woods Hole Oceanographic Institution, Woods Hole, MA, United States, (2)Federal Maritime and Hydrographic Agency (BSH), Hamburg, Germany, (3)University of Toronto, Toronto, ON, Canada, (4)Marine Research Institute, Reykjavik, Iceland, (5)Swedish Defence Research Agency (FOI), Stockholm, Sweden
The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic Meridional Overturning Circulation. While its hydrographic structure is well documented, a full description of the velocity field across the strait remains incomplete. Here we analyze a set of 22 shipboard hydrographic and velocity sections occupied along the Látrabjarg transect at the Denmark Strait sill, obtained over the time period 1993-2018. The sections provide the first complete view of the kinematic components at the sill: the shelfbreak East Greenland Current (EGC), the combined flow of the Separated EGC and the North Icelandic Jet (NIJ), and the northward flowing North Icelandic Irminger Current (NIIC). We deconstruct the dense overflow in terms of water mass constituents and flow components, demonstrating that the combined EGC branches and NIJ transport comparable amounts. A strong cyclonic structure was present in two-thirds of the occupations, which is thought to be due to the combined effect of eddies and wind. Strong negative wind stress curl north of the strait intensifies the separated EGC, while the enhanced northerly winds under these conditions strengthen the NIIC and cause it to shift the west. Both the cyclonic and non-cyclonic flow states can be super-critical in different parts of the strait, leading to symmetric instability and enhanced mixing. A proxy is used to assess this condition in a larger set of shipboard crossings with hydrography only, elucidating the degree to which mesoscale features drive such mixing.