Observed Deep Cyclonic Eddies around Southern Greenland

Recent mooring measurements from the Overturning in the Subpolar North Atlantic Program have revealed abundant cyclonic eddies at both sides of Cape Farewell, the southern tip of Greenland. In this study, we present further observational evidence, from both Eulerian and Lagrangian perspectives, of deep cyclonic eddies with intense rotation (𝜁/f > 1) around southern Greenland and into the Labrador Sea. Most of the observed cyclones exhibit strongest rotation below the surface (700-1000 dbar), where maximum azimuthal velocities are ~30 cm/s at radii of ~10 km, with rotational periods of 2-3 days. The cyclonic rotation can extend to the deep overflow water layer (below 1800 dbar), albeit with weaker azimuthal velocities (~10 cm/s) and longer rotational periods of about one week. Within the mid-depth rotation cores, the cyclones are in near solid-body rotation and have the potential to trap and transport water. The first high-resolution hydrographic transect across such a cyclone indicates that it is characterized by a local (both vertically and horizontally) potential vorticity maximum in its core and cold, fresh anomalies in the overflow water layer, suggesting its source as the Denmark Strait outflow. Additionally, the propagation and evolution of the cyclonic eddies are illustrated with deep Lagrangian floats, including their detachments from the boundary currents to the basin interior. Taken together, the combined Eulerian and Lagrangian observations have provided new insights on the boundary current variability and boundary-interior exchange over a geographically large scale near southern Greenland, calling for further investigations on the (sub)mesoscale dynamics in the region.

Cyclonic eddies in the West Greenland Boundary Current System

The boundary current system in the Labrador Sea plays an integral role in modulating convection in the interior basin. Four years of mooring data from the eastern Labrador Sea reveal persistent mesoscale variability in the West Greenland boundary current. Between 2014 and 2018, 197 mid-depth intensified cyclones were identified that passed the array near the 2000 m isobath. In this study, we quantify these features and show that they are the downstream manifestation of Denmark Strait Overflow Water (DSOW) cyclones. A composite cyclone is constructed revealing an average radius of 9 km, maximum azimuthal speed of 24 cm/s, and a core propagation velocity of 27 cm/s. The core propagation velocity is significantly smaller than upstream near Denmark Strait, allowing them to trap more water. The cyclones transport a 200-m thick lens of dense water at the bottom of the water column, and increase the transport of DSOW in the West Greenland boundary current by 17% relative to the background flow. Only a portion of the features generated at Denmark Strait make it to the Labrador Sea, implying that the remainder are shed into the interior Irminger Sea, are retroflected at Cape Farewell, or dissipate. A synoptic shipboard survey east of Cape Farewell, conducted in summer 2020, captured two of these features which shed further light on their structure and timing. This is the first time DSOW cyclones have been observed in the Labrador Sea—a discovery that could have important implications for interior stratification.

The regional model of Subpolar Gyre based on NEMO 4

<p>A regional model of Subpolar Gyre in the North Atlantic is implemented. The NNATL12 model development aimed at a realistic representation of Subpolar Northern Atlantic's complex dynamics during the satellite era (starting from 1993 to nowadays) by using a high-resolution regional model that relies on the most up-to-date atmospheric and lateral forcing datasets and modeling techniques. Configuring this model, we focused on the representation of key processes in the Northern Atlantic, such as Irminger Rings, the boundary currents, deep convection, and convective eddies, dense waters cascading through the narrow straits between the Arctic and the Atlantic basins. NNATL12 model is based on NEMO4. The model domain covers the area between 47-70˚N and 84˚W-10˚E with a grid of 1/12˚ in horizontal and 75 vertical levels. In this region, the model is partially eddy-resolving. Three lateral open boundaries and initial conditions are set from the new GLORYS12 reanalysis (Lellouche et al., 2018). The surface forcing is provided by the new RAS NAAD dynamical hindcast based on the WRF model with a spatial resolution of 14 km (Gavrikov et al. 2020). The model adopted the most recent developments in the forced ocean modeling, such as upper boundary forcing schemes (Renault et al., 2020, Brodeau et al., 2016) and local-sigma vertical coordinate in the area of the overflows (Colombo et al., 2020). The model solution is sensitive to new parameterizations and vertical coordinate, which is demonstrated in various tests. The model provides a reliable estimate of the Subpolar North Atlantic circulation system at the surface and medium depth compared to observations. The model represents the ocean stratification at depths above 2000 m showing higher temperatures in the bottom of the Irminger Sea. At daily timescales, it is capable of representing the volume transport comparable to observed values. Irminger Rings TS-structure and dynamics are simulated consistent with the glider data. Comparing to the reanalysis model overestimates the March mixed layer depths and overextends the region of convection north. At the same time, the short-scale and decadal variability of MLD are reproduced by the model. Significant improvements of the deep stratification are obtained with the implementation of the local-sigma vertical coordinate. The model provides vertical profiles of temperature and salinity similar to the observed ones. However the Denmark Strait overflow waters are still too warm, but this is for a large part due to too warm waters at the sill. The high-frequency variability in the Denmark Strait is also in good accordance with the observations.</p>

Observed Denmark Strait Overflow Cyclones around Greenland

<div> <p>Abundant cyclonic eddies are observed to travel along the Deep Western Boundary Current around Greenland by Lagrangian floats, hydrographic stations and moorings. Most of the cyclones have intensified rotations below the surface (700-1000 dbar), with maximum azimuthal velocities of ~30 cm/s at radii of ~10 km. The swift rotation and small radius lead to a relatively large Rossby number (~0.4), suggesting important contributions from the ageostrophic terms. The subsurface rotational core is also characterized with a local (both vertically and horizontally) potential vorticity (PV) maximum, which is associated with the pinching of isopycnals towards the mid-depths (i.e. high stratification). The PV structure suggests the origin of the cyclone as the Denmark Strait Overflow Cyclone. The latter is known to be formed by vortex stretching southwest of the Denmark Strait, where outflow waters with high PV from the sill descends the continental slope into the low PV Irminger Sea. Finally, we show that these cyclones can influence the boundary currents around Greenland by introducing property anomalies that originate from the Denmark Strait.</p> </div>

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

<p><em>Denmark Strait, the channel located between Greenland and Iceland, is a critical gateway between the Nordic Seas and the North Atlantic. </em><em>Mesoscale features crossing the strait regularly enhance the volume transport of the Denmark Strait overflow. They interact with the dense water masses descending into the subpolar North Atlantic and therefore are important for the Atlantic Meridional Overturning Circulation. Using a realistic numerical model, we find new evidence of the causal relationship between overflow surges (i.e., mesoscale features associated with high-transport events) and overflow cyclones observed downstream. Most of the cyclones form at the Denmark Strait sill during overflow surges and, because of potential vorticity conservation and stretching of the water column, grow as they move equatorward. A fraction of the cyclones form downstream of the sill, when anticyclonic vortices formed during high-transport events start collapsing. Regardless of their formation mechanism, the cyclones weaken starting roughly 150 km downstream of the sill, and potential vorticity is only materially conserved during the growth phase.</em></p>

Cyclonic eddies in the West Greenland boundary current system

<p>The Labrador Sea is an important site for deep convection, and the boundary current surrounding the Sea impacts the strength of this convection and the subsequent restratification. As part of the Overturning of the Subpolar North Atlantic Program, ten moorings have been maintained on the West Greenland shelf and slope that provide hourly, high-resolution renderings of the boundary current. These data reveal the presence and propagation of abundant mid-depth intensified cyclonic eddies, which have not previously been documented in the West Greenland boundary current system. This study quantifies these features and their structure and demonstrates that they are the downstream manifestation of Denmark Strait Overflow Water (DSOW) cyclones. Using the mooring data, the statistics of these features are presented, a composite eddy is constructed, and the velocity and transport structure are described. A synoptic survey of the region captured two of these features, and provides further insight into their structure and timing. This is the first time DSOW cyclones have been observed in the Labrador Sea, and their presence, propagation, and transport must be accounted for in order to assess their contribution to the heat and freshwater budgets of the Labrador Sea interior.</p>

Distribution and paleoecology of benthic foraminifera of Denmark Strait in Holocene and late Pleistocene

<p>Holocene and Pleistocene benthic foraminifera assemblage patterns studied from 63 samples in sediment core AMK-5890 collected from Iceland's western slope in the Denmark Strait during the 71th cruise of research vessel"Academic Mstislav Keldysh" in 2018 (Novigatsky A.N, et al., 2018). Core were sampled at 1 cm interval from Holocene and 5 cm from Pleistocene and washed through a 63 micron sieve.</p><p>In the first complex of deposits represented by Holocene deposits the total benthic foraminifera abundance reaches highest values 35 000 – 60 000 individuals/g of dry sediment (ind/g of dry sed). In the lower part of the complex, abundance decreases to 10,000 ind/g of dry sed. The species diversity ranges from 25 to 35 species in the sample. Trifarina angulosa is the dominant species (about 60%). The species Cibicides lobatulus is subdominant (25-30%) in the Holocene community which lived in areas with increased hydrodynamic characteristics (Lorenz, 2005). The small group benthic foraminifera (from 2 to 15%) includes Atlantic and boreal species Cassidulina laevigata, Cassidulina neoteretis and Uvigerina peregrina (Sejrup et al, 2004). This database of distribution and ecology of benthic foraminifera indicated that in Holocene favorable living environment (positive bottom temperatures and salinity, close to modern sea), increased productivity and wide influence of Atlantic waters to the north existed. The lower part of complex reflects the epoch of deglaciation.</p><p>There are short changes in all measures at the boundary of Holocene and Pleistocene: total benthic foraminifera abundance (1000-400 ind/g of dry sed) and species diversity (<20 species) decreases, and species assemblage is almost completely changes. It allows to identify the second complex that characterizes the transition to glacial deposits. At the top of the glacial complex, the peak of occurrence of Cibicidoides wuellerstorfi (about 25%) associates with a decrease in the influence of meltwater and active hydrodynamics (Struck, 2007). The glacial assemblage consists of two dominant species C. lobatulus (about 35%) and Cassidulina obusta (about 40%). Also, there are Cassidulina reniforme, Elphidium clavatum and Nonion labradoricum, which prefer cold waters and Arctic environmental conditions with the presence of ice cover.</p><p> Acknowledgments: Preparation, processing of samples and micropaleontological analysis was funded by RFBR, project number 20-35-90093. The expedition studies was funded by RPF, project number 14-50-00095, the primary lithological-mineralogical and geochemical studies was funded of the State assignment of the FANO of Russia (№ 0149-2019-0007).</p>

Mechanisms linking the Labrador Sea with subtropical Atlantic overturning

<p>We analyze the causal chain linking sea surface buoyancy anomalies in the Labrador Sea and variability in the subtropical Atlantic meridional overturning circulation (AMOC) in the ECCO ocean state estimate on inter-annual timescales. Our study highlights the importance of Lower North Atlantic Deep Water (LNADW) for the north-south connectivity in the Atlantic Ocean. We identify important mechanisms that allow the Labrador Sea to impact the southward transport of LNADW. We show that NAC plays an essential role in the export of buoyancy anomalies from the Labrador Sea – and it furthermore exerts a positive feedback that amplifies these upper ocean anomalies in the eastern subpolar gyre – before they reach the denser water masses along the lower limb of the AMOC. Our results also highlight the contribution of the western Labrador Sea for the surface uptake of tracers that penetrate the LNADW near Denmark Strait, which has implications for the redistribution of ocean heat anomalies.</p>

Interpreting the observed variability of deep waters in the Irminger Sea

<p><strong>Temperature and salinity seasonal to interannual variability of Iceland Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) is investigated by combining two in-situ datasets in the Irminger Sea for the period 1997-2020: 12-yr of repeated hydrography (1997-2018) provided by the FOUREX, OVIDE and RREX sections and 4-yr of data (2016-2020) from 8 Deep Argo floats deployed in the region between 2016 and 2018. </strong></p><p><strong>In order to enable a consistent analysis of ocean temperature and salinity variability from unevenly distributed vertical profiles (both in space and time), it is necessary to estimate the appropriate regional climatology to be removed from every observation. Two independent procedures are followed to compute anomalies and quantify uncertainties related to the choice of climatology: First, the global 1°-resolution World Ocean Atlas 2018 (2005-2017 averages) climatology is retrieved from every observed profile (Deep Argo, hydrography). Second, </strong><span><strong>the well-known and sampled OVIDE transect (2002-2018 average) is used to build a reference section of geographical anomalies that are subsequently propagated along potential vorticity contours </strong></span><span><strong>in the Irminger Sea.</strong></span><strong> Neutral density surfaces 28.02 kgm</strong><sup><strong>-3 </strong></sup><strong>and 28.12 kgm</strong><sup><strong>-3</strong></sup><strong> are then chosen from mean OVIDE 2002-2018 gridded fields as representative of ISOW and DSOW levels, respectively. Significant decadal trends in water mass properties are revealed by repeated hydrography, whereas some striking boundary-interior spatial patterns are captured by Deep Argo floats. Property changes of ISOW and DSOW are discussed in terms of changes of source waters in the Nordic Seas, entrainment of Atlantic waters into the overflow waters and cascading events from the Greenland slope.</strong></p><p> </p>

Coastal Trapped Waves along the Southeast Greenland Coast in a realistic numerical simulation

<p>Ocean currents along the Southeast Greenland Coast play an important role in North Atlantic circulation and the global climate system. They carry dense water over the Denmark Strait sill, fresh water from the Arctic and the Greenland Ice Sheet into the subpolar ocean, and warm Atlantic water into Greenland’s fjords, where it can interact with outlet glaciers. Observational evidence from the OSNAP array and other mooring records shows that the circulation in this region displays substantial subinertial variability, typically with periods of several days. For the dense water flowing over the Denmark Strait sill, this variability augments the time-mean transport; on the shelf, the variability is large enough to occasionally reverse the mean transport direction of the coastal current, highlighting the importance of characterizing this variability when interpreting synoptic surveys. In this study, we used the output of a high-resolution realistic simulation to diagnose and characterize subinertial variability in sea surface height and velocity along the coast. The results show that the subinertial signals on the shelf and along the shelf break are coherent over hundreds of kilometers, and consistent with Coastal Trapped Waves in two subinertial frequency bands—at periods of 1–3 days and 5–18 days—portraying a combination of Mode I and higher modes waves. Furthermore, we find that northeasterly barrier winds may trigger the 5–18 day shelf waves, whereas the 1–3 day variability is linked to high wind speeds over Sermilik Deep.</p>