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

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Structure and Variability of the Shelfbreak East Greenland Current North of Denmark Strait OCTOBER 2017 H Å VIK ET AL. 2631 Structure and Variability of the Shelfbreak East Greenland Current North of Denmark Strait a a b b c L. HÅVIK, K. VÅGE, R. S. PICKART, B. HARDEN, W.-J. VON APPEN, d e S. JÓNSSON, AND S. ØSTERHUS a Geophysical Institute, University of Bergen, and Bjerknes Centre for Climate Research, Bergen, Norway b Woods Hole Oceanographic Institution, Woods Hole, Massachusetts c Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany d Marine and Freshwater Research Institute, Reykjavik, and University of Akureyri, Akureyri, Iceland e Uni Research Climate, Bjerknes Centre for Climate Research, Bergen, Norway (Manuscript received 29 March 2017, in final form 9 August 2017) 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 shelf- break 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. 1. Introduction the deep limb of the Atlantic meridional overturning circulation. Roughly half of the total dense overflow The East Greenland Current (EGC) provides a direct across the Greenland–Scotland Ridge exits through connection between the Arctic Ocean and the North Denmark Strait (Dickson et al. 2008). Hence, Denmark Atlantic and is the main export pathway for both solid Strait is a key location for the exchange and coupling and liquid freshwater from the Arctic Ocean (Dickson between the Arctic Ocean, the Nordic Seas, and the et al. 2007). In addition to the freshwater transport, North Atlantic Ocean. dense water masses formed in the Nordic Seas and the North of Denmark Strait the circulation that sup- Arctic Ocean are carried south by the EGC toward plies the freshwater and dense overflow water to the Denmark Strait. Since the intermediate and deep water North Atlantic is complex and variable. As the EGC masses in the EGC are denser than the ambient water in approaches Denmark Strait, it bifurcates into two the North Atlantic, they sink toward the deep ocean as branches: the shelfbreak EGC and the separated EGC an overflow plume after crossing the Denmark Strait sill. (Våge et al. 2013)(Fig. 1), with the latter flowing along This overflow plume from the Nordic Seas contributes to the base of the Iceland continental slope toward the strait. Våge et al. (2013) proposed two different mechanisms that may be responsible for the formation of the sepa- Denotes content that is immediately available upon publica- rated EGC. Using a simplified model they argued that tion as open access. eddies shed from the shelfbreak EGC at the northern end of the Blosseville Basin migrate offshore toward Corresponding author:L.Håvik, [email protected] the Iceland slope where they coalesce and form a DOI: 10.1175/JPO-D-17-0062.1 Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). 2632 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 47 FIG. 1. Schematic overview of the currents in the vicinity of Denmark Strait. The shelfbreak EGC and the separated EGC transport both dense intermediate waters (purple lines) and light surface waters (light blue lines). The north Icelandic jet transports mostly dense intermediate waters from the Iceland Sea. The dashed line on the shelf represents the Polar Surface Water jet, which transports relatively fresh surface waters. The north Icelandic Irminger Current transports warm and saline Atlantic Water northward into the Iceland Sea along the coast of Iceland. The acronym BB rep- resents the Blosseville Basin. The black line indicates the Kögur transect, and the locations of the moorings are marked by black squares. Filled squares mark the moorings utilized in this study. coherent current. The other mechanism is associated with the Iceland continental slope (Jónsson 1999; Jónsson and negative wind stress curl over the Blosseville Basin. Valdimarsson 2004; Våge et al. 2011). This current is hy- Closed f/h contours (where f is the Coriolis frequency and pothesized to be the lower limb of a local overturning cell h is the depth) within the basin could lead to an anticy- in the Iceland Sea whose upper limb is the north Icelandic clonic circulation whose eastern branch is the separated Irminger Current (Våge et al. 2011). Unlike the EGC, the EGC. The observations used by Våge et al. (2013) came NIJ is not associated with any substantial freshwater from only four summertime shipboard occupations of the transport (de Steur et al. 2017). Kögur transect between the Iceland and Greenland Jochumsen et al. (2012) estimated a mean trans- shelves across the Blosseville Basin (Fig. 1). Year-long port of Denmark Strait Overflow Water (DSOW; poten- 23 6 3 21 time series from a mooring array deployed along the tial density su $ 27.8kgm ) of 3.4 Sv (1 Sv [ 10 m s ) same transect confirmed the existence of the two EGC at the Denmark Strait sill for the period 1996 to 2011, with branches, and Harden et al. (2016) found that the parti- no pronounced seasonal or interannual variability. How- tioning of transport between the two branches varied on a ever, on time scales of 2–10 days, the velocities and cor- weekly time scale because of the wind forcing. responding transports exhibited pronounced variability. Recently, Håvik et al. (2017) identified a third compo- Smith (1976) identified oscillations in the flow through nent of the boundary current system approaching Den- Denmark Strait on time scales of 2 days in current meter mark Strait: the Polar Surface Water (PSW) jet (Fig. 1). data and attributed this variability to baroclinic instability This current is situated on the east Greenland shelf, on- of the overflow. Recently, two features have been identi- shore of the shelfbreak branch, and accounts for a sizeable fied as the dominant sources of mesoscale variability at the fraction of the total southward freshwater transport of the Denmark Strait sill: boluses and pulses (Mastropole et al. EGC system (up to 55% in their sections). In addition to 2017; von Appen et al. 2017). The boluses are large the branches of the EGC, the north Icelandic jet (NIJ) lenses of weakly stratified overflow water associated with transports overflow water toward Denmark Strait along a modest strengthening of the flow. By contrast, pulses OCTOBER 2017 H Å VIK ET AL. 2633 correspond to a strong increase in velocity when the supplies on average 70% of the freshwater transport in overflow water is confined to a thin layer above the bottom. the EGC system to Denmark Strait (de Steur et al. On average, either of these features are present at the sill 2017). Our primary goal is to obtain a robust description every second day. of the mean state and variability of this branch and shed Presently it is unclear whether the variability in light on the dynamics that govern its flow. This is es- Denmark Strait arises from local processes or if it is a sential for understanding the interaction between result of upstream variability in the currents approach- the currents in this region, such as the bifurcation of the ing the strait. Most of our knowledge of both the water EGC or the time-varying compensation between the masses and the kinematic structure of the EGC farther two EGC branches described by Harden et al. (2016). north, between Fram Strait and Denmark Strait, is based Our dataset provides a unique opportunity to examine on analysis of synoptic summer sections (Rudels et al. the variability of the shelfbreak EGC throughout 1 2002, 2005; Jeansson et al. 2008; Nilsson et al. 2008; Våge year and how this in turn may influence both the flux of et al. 2013; Håvik et al. 2017). These studies generally freshwater and dense overflow water toward the North found that the EGC is a surface-intensified current that Atlantic. closely follows the topography of the shelf break toward Denmark Strait. However, such snapshots do not elu- 2. Data and methods cidate the variability throughout the year. Because of the presence of pack ice, observations from the rest of From September 2011 to August 2012 a densely the year have primarily been obtained by moorings. instrumented mooring array was deployed along the Strass et al. (1993) analyzed a year-long dataset from Kögur transect north of Denmark Strait from the Ice- four moorings deployed across the EGC close to 758Nin land shelf break to the east Greenland shelf break 1987–88. They showed that the current was highly vari- (Fig.
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