Kinematic Structure and Dynamics of the Denmark Strait Overflow from Ship-Based Observations

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 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). The total mean transport of overflow water is 3.54 ± 0.29 Sv, 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 cyclonic state and a non-cyclonic 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.

Wintertime Water Mass Transformation in the Western Iceland and Greenland Seas

“…Previous studies of the EGC, and the AtOW that it transports, have mainly focused on the regions close to Fram and Denmark Straits (de Steur et al., 2009, 2014; Fahrbach et al., 2001; Foldvik et al., 1988; Harden et al., 2016; Håvik, Våge, et al., 2017; Håvik & Våge, 2018; Lin et al., 2020). Based on a long‐term (1997–2009) mooring array deployed south of Fram Strait (orange line in Figure 1), de Steur et al.…”

“…By analyzing a set of shipboard hydrographic‐velocity sections along the transect at Denmark Strait, Lin et al. (2020) estimated the total mean transport of AtOW to be 1.49 ± 0.10 Sv. For the regions between the two straits, the EGC and AtOW have been only sparsely observed.…”

“…Previous studies of the EGC, and the AtOW that it transports, have mainly focused on the regions close to Fram and Denmark Straits (de Steur et al, 2009(de Steur et al, , 2014Fahrbach et al, 2001;Foldvik et al, 1988;Harden et al, 2016;Lin et al, 2020). Based on a long-term (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009) mooring array deployed south of Fram Strait (orange line in Figure 1), de Steur et al (2014) found that the annual mean transport of the EGC is 8.7 Sv (1 Sv = 10 6 m 3 /s) at 78.8°N (including 3 Sv transported by the recirculation of the West Spitsbergen Current).…”

“…Using a year-long mooring array deployed roughly 200 km north of Denmark Strait (purple line in Figure 1), Harden et al (2016) found that the annual transport of AtOW by the EGC is 2.54 ± 0.17 Sv. By analyzing a set of shipboard hydrographic-velocity sections along the transect at Denmark Strait, Lin et al (2020) estimated the total mean transport of AtOW to be 1.49 ± 0.10 Sv. For the regions between the two straits, the EGC and AtOW have been only sparsely observed.…”

Physical Controls on the Macrofaunal Benthic Biomass in Barrow Canyon, Chukchi Sea

“…Values of the shear Rossby number exceed −1, which together with the strong negative relative vorticity on the anti‐cyclonic side of the ACC, lead to a region of negative total potential vorticity (Figures 11a). Such a condition can lead to symmetric instability (e.g., Lin, Pickart, Jochumsen, et al., 2020), which can reach finite amplitude in a matter of hours (Brearley et al., 2012). This implies that, when the ACC is strong and the associated density front is sharp, the outflow of the canyon is subject to strong mixing.…”