NOVEMBER 2020 
LIN ET AL. 
3735 
Kinematic Structure and Dynamics of the Denmark Strait Overflow from 
Ship-Based Observations 
PEIGEN LIN,* ROBERT S. PICKART,* KERSTIN JOCHUMSEN,® G. W. K. MOORE,“ HEÖINN VALDIMARSSON, 4 
TIM FRISTEDT,“ AND LAWRENCE J. PRATT* 
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts; ° Federal Maritime and Hydrographic Agency (BSH) 
Hamburg, Germany; © Department of Physics, University of Toronto, Toronto, Ontario, Canada; * Marine and Freshwater 
Research Institute, Reykjavik, Iceland; © Department of Marine Technology, Multiconsult Norway AS, Troms@, Norway 
(Manuscript received 6 May 2020, in final form 26 August 2020) 
ABSTRACT: The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic 
meridional overturning circulation, yet a description of the full velocity field across the strait remains incomplete. Here 
we analyze a set of 22 shipboard hydrographic-velocity sections occupied along the Lätrabjarg 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). The total mean transport of overflow water is 3.54 
+ 0.29 Sv (1Sv = 10°m*s7'), comparable to previous estimates. The dense overflow is partitioned in terms of water mass 
constituents and flow components. The mean transports of the two types of overflow water—Atlantic-origin Overflow Water 
and Arctic-origin Overflow Water—are comparable in Denmark Strait, while the merged NIJ-separated EGC transports 55 % 
more water than the shelfbreak EGC. A significant degree of water mass exchange takes place between the branches as they 
converge in Denmark Strait. There are two dominant time-varying configurations of the flow that are characterized as a Cy- 
zlonic state and a noncyclonic state. These appear to be wind-driven. A potential vorticity analysis indicates that the flow 
through Denmark Strait is subject to symmetric instability. This occurs at the top of the overflow layer, implying that the 
mixing/entrainment process that modifies the overflow water begins at the sill. 
KEYWORDS: Currents; Instability; Ocean circulation; Ocean dynamics; Potential vorticity; Transport 
1. Introduction 
The dense water formed in the Nordic Seas is the main 
source of lower North Atlantic Deep Water that plays an es- 
sential role in the Atlantic meridional overturning circulation 
(AMOC) (Dickson and Brown 1994). Studies have now dem- 
onstrated that the dominant contribution to the AMOC is as- 
sociated with the warm-to-cold transformation that occurs in 
:he Nordic Seas as opposed to that which takes place in the 
Labrador Sea (Pickart and Spall 2007; Holte and Straneo 2017; 
Lozier et al. 2019). Denmark Strait is one of the key passages 
(hrough which the dense water from the Nordic domain enters 
ıhe North Atlantic Ocean. The so-called Denmark Strait 
Overflow Water (DSOW) accounts for roughly half of the total 
dense water flowing over the Greenland-Scotland Ridge. The 
mean transport of DSOW at the sill, which is typically defined 
as water denser than 27.8 kg m, is estimated to be 3.2-3.5 Sv 
(Harden et al. 2016; Jochumsen et al. 2017). The other main 
passage of overflow water is the Iceland-Scotland ridge, 
accounting for —35Sv, including approximately 1Sv via 
Iceland-Faroe Ridge, and approximately 2 Sv via the Faroe 
Bank Channel (Österhus et al. 2008). 
There are three different pathways that advect the dense 
water into Denmark Strait from the north, supplying the 
overflow water (Fig. 1): the shelfbreak East Greenland 
Current (EGC); the separated EGC; and the North Icelandic 
Jet (NIT). The EGC emanates from Fram Strait and is a 
surface-intensified flow transporting Atlantic-origin Overflow 
Water at depth. At these latitudes it is composed of a 
shelfbreak branch and an offshore slope branch (Hävik et al. 
2017a). Together they advect a combination of warm, salty 
water that has been modified along the rim current system of 
‚he Nordic Seas (Mauritzen 1996) and also in the high Arctic 
Rudels et al. 2005). The shelfbreak EGC transport decreases 
as it progresses southward, while the slope branch appears to 
Je diverted eastward into the interior north of the Iceland Sea 
(Hävik et al. 2017a). When the shelfbreak EGC reaches the 
1orthern end of the Blosseville Basin it bifurcates to form the 
zeparated EGC (Fig. 1). Väge et al. (2013) attribute the bi- 
[urcation to local wind stress curl and topography, as well as 
oaroclinic instability of the shelfbreak current. 
The NIJ is a middepth-intensified current on the north 
celand slope that transports Arctic-origin Overflow Water 
3quatorward. This is water that has been modified in the in- 
‚erior basins of the western Nordic Seas, and is colder, fresher, 
and denser than the Atlantic-origin Overflow Water. It was 
aypothesized by Väge et al. (2011) that the NIT is part of a local 
averturning loop in the Iceland Sea whereby the subtropical- 
arigin water transported northward by the North Icelandic 
‚rminger Current (NIIC, Fig. 1) is fluxed into the interior of the 
5asiın and converted to overflow water by wintertime air-sea 
21eat loss. The dense water then progresses back to the Iceland 
slope where it sinks and feeds the NLJ. However, it has since 
een demonstrated that the bulk of the Arctic-origin water 
must originate from farther north where the wintertime 
mixed layers are denser (Väge et al. 2015). Recent analysis 
Corresponding author: Peigen Lin, plinwhoi@gmail.com 
DOI: 10.1175/T7PO-D-20-0095.1 
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