Mesoscale Eddies Observed at the Denmark Strait Sill

Abstract

The Denmark Strait overflow is the major export route of dense water from the Arctic Mediterranean into the North Atlantic. At the Strait's shallow sill, the overflow is a bottom-intensified cold and dense plume, bound to the east by a thermal front formed with the warmer, northward flowing North Icelandic Irminger Current. More than two decades of observations at the sill show strong fluctuations of volume flux on daily time scales. To better understand the source of this variability, a five-mooring array was installed at the sill, capturing nearly 1 year of velocity and bottom temperature measurements at a high temporal and spatial resolution. Bottom temperature fluctuations that exceed 4 • C indicate a meandering of the front between the plume and the North Icelandic Irminger Current. Current vector rotation shows trains of alternating cyclones and anticyclones at the sill. An eddy crosses the sill every 3 to 6 days with a mean velocity of 0.4 m/s and a typical diameter of 30 to 40 km. The results suggest that anticyclones, with centers passing through the deepest part of the sill, may be responsible for periods of increased volume flux-also referred to as boluses and pulses in previous studies. Although the relationship between eddies, pulses, and boluses is still unclear, the results show that eddies are directly linked to fluctuations in the strength, thickness, and position of the overflow plume. Plain Language SummaryThe southward flow of dense water from the Arctic ocean plays a crucial role in global ocean circulation but is almost immediately constrained on its way south by a submarine ridge that connects Greenland, Iceland, the Faroe Islands, and Scotland. The southward flow is therefore forced to pass through several straits and up and over relatively shallow sills. Most of the flow passes over a sill in the Denmark Strait, located between Greenland and Iceland. In this study, we present observations from an array of instruments, which measure the southward flow as it passes over the Denmark Strait sill. The flow is characterised by trains of eddies (vortices), with an alternating sense of rotation; meaning a counterclockwise eddy is usually followed by a clockwise eddy. The eddies are 30 to 40 km wide and need about 1 day to pass over the sill. These eddies help to explain pronounced changes in the flow across the sill, as they can help either to speed up the flow or slow it down. The results of this study contribute to understanding mesoscale fluctuations, which influence local mixing processes and water mass transports.

Mechanisms of Heat Flux Across the Southern Greenland Continental Shelf in 1/10° and 1/12° Ocean/Sea Ice Simulations

“…This EKE patch corresponds to a region of large sea surface height anomalies observed by AVISO (Trodahl & Isachsen, 2018) and is also present in the drifter‐derived EKE estimate (Figure 5). Heightened EKE near the Denmark Strait is also consistent with observations of mesoscale eddies and boluses formed at the Denmark Strait overflow (Moritz et al., 2019). The 2005–2009 average EKE in the defined box just south of the Denmark Strait (outlined in black in Figures 4c and 4d) is similar in POP (139 cm 2 s −2 ) and HYCOM (131.7 cm 2 s −2 ), but the maximum EKE in POP (968 cm 2 s −2 ) is twice the maximum in HYCOM (397 cm 2 s −2 ).…”

“…The high‐frequency signals in the Hovmöller diagrams (Figures 10a and 9a) originating to the south of the Denmark Strait are comparable to the topographically trapped Rossby waves (Münchow et al., 2020) in a trough near the Fram Strait, to the cyclonic eddies formed at the Denmark Strait (Moritz et al., 2019), and to the coastally trapped shelf waves in this region (Gelderloos et al., 2021). In the previous section, we found these high‐frequency signals to be consistent with DSO eddies traveling along the shelf break.…”

Lagrangian Perspective on the Origins of Denmark Strait Overflow

“…The formation, sources, and pathways of the DSO have also been studied (Rudels et al 2002;Köhl et al 2007). However, despite various proposed circulation schemes based on hydrographic transects (Harden et al 2016;Våge et al 2013), the sources and details of the DSO upstream pathways are still uncertain due to mesoscale eddy features, short time scale (3-5 day) variability, mixing, and water mass transformation (Almansi et al 2017;Spall et al 2019;Moritz et al 2019).…”

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

“…Using the mooring data from the aforementioned Blosseville Basin array, Huang et al (2019) demonstrated that high-frequency variability is driven by mean-to-eddy baroclinic conversion at the shoreward edge of the NIJ. Using a yearlong mooring array in Denmark Strait, Moritz et al (2019) resolved the passage of eddies, finding more anticyclones in the deepest part of the strait and more cyclones west of this. Satellite altimetry data have revealed enhanced levels of surface eddy kinetic energy in the vicinity of the strait (Høyer and Quadfasel 2001;Håvik et al 2017b).…”

“…Bathymetry contours are in meters. investigated by Moritz et al (2019). While these measurements have enhanced our understanding of the flow components in Denmark Strait, they are limited in cross-strait coverage and only have near-bottom temperature and salinity information.…”