High-Frequency Variability in the Circulation and Hydrography of the Denmark Strait Overflow from a High-Resolution Numerical Model

Abstract

Initial results are presented from a yearlong, high-resolution (~2 km) numerical simulation covering the east Greenland shelf and the Iceland and Irminger Seas. The model hydrography and circulation in the vicinity of Denmark Strait show good agreement with available observational datasets. This study focuses on the variability of the Denmark Strait overflow (DSO) by detecting and characterizing boluses and pulses, which are the two dominant mesoscale features in the strait. The authors estimate that the yearly mean southward volume flux of the DSO is about 30% greater in the presence of boluses and pulses. On average, boluses (pulses) are 57.1 (27.5) h long, occur every 3.2 (5.5) days, and are more frequent during the summer (winter). Boluses (pulses) increase (decrease) the overflow cross-sectional area, and temperatures around the overflow interface are colder (warmer) by about 2.6°C (1.8°C). The lateral extent of the boluses is much greater than that of the pulses. In both cases the along-strait equatorward flow of dense water is enhanced but more so for pulses. The sea surface height (SSH) rises by 4–10 cm during boluses and by up to 5 cm during pulses. The SSH anomaly contours form a bowl (dome) during boluses (pulses), and the two features cross the strait with a slightly different orientation. The cross streamflow changes direction; boluses (pulses) are associated with veering (backing) of the horizontal current. The model indicates that boluses and pulses play a major role in controlling the variability of the DSO transport into the Irminger Sea.

Evolution of Denmark Strait Overflow Cyclones and Their Relationship to Overflow Surges

“…Finally, anticyclones coincide with an enhanced cross‐sectional flow toward Greenland within the overflow layer (Figure h). All of these anomalies in the composites of cyclones and anticyclones are consistent with the anomalies associated with boluses and pulses, respectively (Almansi et al, ; Mastropole et al, ; von Appen et al, ).…”

“…The additional fields shown in this paper can be reproduced using OceanSpy v0.1 (Almansi et al, ). The model is particularly well suited to study mesoscale features occurring on short time scales (1–10 days; Almansi et al, ; Spall et al, ). Haine et al () found that the spatial resolution of the atmospheric fields used to force high‐resolution models significantly affects the ocean circulation in Denmark Strait.…”

“…The model solutions have been also compared to data collected upstream of Denmark Strait by Håvik et al (). Vertical sections of hydrographic and velocity fields are consistent with the validation performed by Almansi et al (). Overall, the agreement between the model and the available observations around Denmark Strait is excellent.…”

Mesoscale Eddies Observed at the Denmark Strait Sill

“…Eddies are one potential source of variability at the sill, and while cyclones and anticyclones are documented north and south of the sill from observations and models, they have not been clearly identified in the passage or at the sill (e.g., Bruce, 1995;Harden et al, 2016;Jungclaus et al, 2001;Krauss & Käse, 1998;Lundrigan & Demirov, 2019;Shi et al, 2001;Voet & Quadfasel, 2010). Recent observational and model studies describe the variability in terms of a background flow disturbed by boluses or pulses passing over the sill (Almansi et al, 2017;Mastropole et al, 2017;von Appen et al, 2017). Boluses are defined as cold, weakly stratified lenses that are accompanied by increased current speeds and a thickening of the plume.…”

“…Recent observational and model studies describe the variability in terms of a background flow disturbed by boluses or pulses passing over the sill (Almansi et al, ; Mastropole et al, ; von Appen et al, ). Boluses are defined as cold, weakly stratified lenses that are accompanied by increased current speeds and a thickening of the plume.…”