Structure and Variability of the Shelfbreak East Greenland Current North of Denmark Strait

Abstract

Data from a mooring array deployed north of Denmark Strait from September 2011 to August 2012 are used to investigate the structure and variability of the shelfbreak East Greenland Current (EGC). The shelfbreak EGC is a surface-intensified current situated just offshore of the east Greenland shelf break flowing southward through Denmark Strait. This study identified two dominant spatial modes of variability within the current: a pulsing mode and a meandering mode, both of which were most pronounced in fall and winter. A particularly energetic event in November 2011 was related to a reversal of the current for nearly a month. In addition to the seasonal signal, the current was associated with periods of enhanced eddy kinetic energy and increased variability on shorter time scales. The data indicate that the current is, for the most part, barotropically stable but subject to baroclinic instability from September to March. By contrast, in summer the current is mainly confined to the shelf break with decreased eddy kinetic energy and minimal baroclinic conversion. No other region of the Nordic Seas displays higher levels of eddy kinetic energy than the shelfbreak EGC north of Denmark Strait during fall. This appears to be due to the large velocity variability on mesoscale time scales generated by the instabilities. The mesoscale variability documented here may be a source of the variability observed at the Denmark Strait sill.

Mesoscale Eddies Observed at the Denmark Strait Sill

“…Maximum EKE values are about 480 cm 2 /s 2 and correspond to anticyclones at the eastern moorings (DS1 and DS20) and cyclones at the western moorings (DS2 and DS21). Our EKE estimates are about 3 times larger than average near-surface EKE at the Kögur section (200 km north of the sill) presented in Håvik et al (2017). They also exceed estimates south of the sill (array A presented in Voet & Quadfasel, 2010).…”

Wind‐Driven Coastal Upwelling and Downwelling in the Shelfbreak East Greenland Current

“…We use data from the Kögur mooring array, deployed north of Denmark Strait from summer 2011 to summer 2012 (de Steur et al, ; Harden et al, ; Håvik, Våge, et al, ), to analyze density anomalies in the shelfbreak EGC and their relation to wind forcing. Twelve moorings were deployed between the east Greenland shelfbreak and the Iceland shelf (Figure ).…”

“…We defined a rotated coordinate system aligned with the mean along‐stream direction of the shelfbreak EGC (i.e., 139° counterclockwise from east; Håvik, Våge, et al, ). This results in a positive along‐stream direction toward the southwest (i.e., along shelf) and a positive cross‐stream direction toward the northwest (i.e., cross shelf).…”

“…Wind forcing has been hypothesized as an important mechanism causing this separation (Harden et al, ; Våge et al, ); however, the direct impact of the wind on the variability of the different branches remains unexplored. The variability of the shelfbreak EGC has been assessed in terms of internal ocean dynamics, and Håvik, Våge, et al () showed that throughout the year the current is both strengthening and weakening relative to its mean state as well as meandering offshore. The current was subject to baroclinic instabilities in winter and experienced high eddy kinetic energy associated with strong mesoscale variability.…”

“…North of Denmark Strait, at the Kögur section (Figure a), dense Denmark Strait Overflow Water (DSOW, σ θ >27.8 kg/m 3 ) is mostly situated offshore of the east Greenland shelfbreak (Håvik, Våge, et al, ). Across the Denmark Strait sill, however, an average of 111 hydrographic sections revealed that a significant portion of the shelf is covered by DSOW (Mastropole et al, ).…”