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Effect of freshwater from the West Greenland Current on the winter deep convection in the Labrador Sea
Abstract
The effect of mesoscale eddies on the deep convection in the Labrador Sea is examined by using a realistically configured eddy-resolving ice-ocean model. The near-surface boundary current flowing into the Labrador Sea is realistically simulated, namely the West Greenland Current which carries upper/onshore fresh and lower/offshore warm water, and eddies separating from these boundary currents with cold/fresh water atop warm/salty water are also well reproduced. The modeled convection is confined to the southwestern Labrador Sea as observed, and its depth and width are reproduced better than in previous modeling studies. Although previous modeling studies demonstrated only the importance of eddy-induced heat transport in inhibition of deep convection over the central to northern Labrador Sea, our study found that the eddy-induced transport of near-surface fresh water also significantly contributes.
... As Rossby's deformation radius is ~10 km around the Fram Strait, the effects of mesoscale eddies cannot be explicitly represented unless the horizontal grid size is significantly smaller than 10 km. As in the study of Kawasaki and Hasumi (2014;Section 7.5), special high resolution is applied to the region of interest by placing the poles of the general curvilinear horizontal coordinates close to the region, on Greenland and the Scandinavian Peninsula. The horizontal resolution is eddy resolving (2-3 km) in the Fram Strait and the Barents Sea Opening (BSO), and is eddy permitting (3-10 km) around the Nordic Seas, the Barents Sea and the Nansen Basin. ...
... The mesoscale eddies on the deep convection in the Labrador Sea are examined by Kawasaki and Hasumi (2014) using a realistically configured eddy-resolving ice-ocean model. The ice-ocean general circulation model employed in the study is COCO version 4 (Hasumi, 2015). ...
... This study directly points out that the contribution of freshwater to inhibiting convection is important in the northern Labrador Sea for the first time. Kawasaki and Hasumi (2014) studied the effect of both lateral transport and sea surface flux of heat and freshwater on the buoyancy in the central Labrador Sea, and the role of freshwater is not small than heat near the sea surface in the northern Labrador Sea. The existence of low salinity water near the sea surface would reduce oceanic heat low under cold air because strong neat surface stratification prevents heat uptake from the depth, and the sea surface is cooled immediately, as suggested by Gelderloos et al. (2012). ...
Under global warming due to anthropogenic increases in atmospheric carbon dioxide concentration, the surface air temperature in the Arctic is increasing with a speed that is more than double the global average, called “Arctic amplification (AA).” To grasp these changes in the Arctic, to understand the mechanism, to know the global influences, and to contribute to future climate projection, we conducted the Green Network of Excellence Program (GRENE) Arctic Climate Change Research Project “Rapid Change of the Arctic Climate System and its Global Influences” for five years between 2011 and 2016 (hereafter “GRENE Arctic”). To tackle four strategic targets presented, members from seven research themes, modeling, terrestrial ecosystem, atmosphere, cryosphere, greenhouse gases, marine ecosystem, and sea ice groups, worked together and reached to the outcomes. Here, the significant outcomes of the GRENE Arctic, presented in more than 100 research articles were compiled, reviewed, and synthesized. Among them, the notable key findings were highlighted in the final chapter. The GRENE Arctic was epoch-making as the first all-Japan comprehensive project incorporating multidisciplinary studies and collaboration between observation and modeling. The original synthetic report has been open at the GRENE home page (http://www.nipr.ac.jp/grene/).
... Yet, our understanding of the relative roles played by eddies and mean currents in the Labrador Sea balance of heat is incomplete and will be addressed here using a 1 /128 model forced with high-resolution (33 km, 3 h) atmospheric fields (see next section). Of particular relevance to this subject are the studies by Chanut et al. (2008) and more recently by Kawasaki and Hasumi (2014), who, among other insightful results, present integrated budgets of heat and buoyancy based on highresolution models forced with monthly climatology. In particular, Chanut et al. find that, on the basin scale, the heat required to balance its loss to the atmosphere is supplied by the large-scale circulation; it is then redistributed to the Labrador Sea interior almost exclusively by eddies. ...
... While observations seem to indicate that the structure of eddies in the Labrador Sea is dominated by temperature (Lilly and Rhines 2002;Pickart et al. 2002), the input of freshwater from the eddies, required to account for the spring restratification of convective areas, cannot be ignored (Hátún et al. 2007). Modeling studies do suggest that freshwater anomalies from the Labrador shelf (Myers 2005;McGeehan and Maslowski 2011) and from the region off the west coast of Greenland (Kawasaki and Hasumi 2014) can have a strong impact on the deep convection, particularly in sufficiently high-resolution models. It has also been observed that the waters entering and leaving the Labrador Sea on the same isopycnal level may have significantly different thermohaline properties (Palter et al. 2008). ...
... Consistent with Chanut et al. (2008, their Fig. 17) and with Kawasaki and Hasumi (2014, their Fig. 8), the total heat convergence due to the mean circulation and eddies [ Ð (M 1 E) dz] closely balances the loss of heat at the surface (Figs. 7a,b). ...
Deep convection in the Labrador Sea is an important component of the global ocean ventilation. The as-sociated loss of heat to the atmosphere from the interior of the sea is thought to be mostly supplied by me-soscale eddies, generated either remotely or as a result of convection itself—processes that are not resolved by low-resolution ocean climate models. The authors first employ a high-resolution (1 /128) ocean model forced with high-resolution (33 km, 3 h) atmospheric fields to further elaborate on the role of mesoscale eddies in maintaining the balance of heat and buoyancy in the Labrador Sea. In general agreement with previous studies, it is found that eddies remove heat along the coast and supply it to the interior. Some of the eddies that are generated because of the barotropic instability off the west coast of Greenland are recaptured by the boundary current. In the region of deep convection, the convergence of heat and buoyancy by eddies significantly in-creases with the deepening of the winter mixed layer. In addition, the vertical eddy flux plays an important part in the heat budget of the upper Labrador Sea, accounting for up to half of the heat loss to the atmosphere north of 608N. A low-resolution (18) model with parameterized eddies is then applied to show that it does capture, qualitatively, the general structure of eddy buoyancy advection along the Labrador Current. However, the 18 model is deficient in this regard in the most eddy active region off the west coast of Greenland, although some improvements can be made by forcing it with the high-resolution atmospheric fields.
... Many previous studies have investigated coastal circulation and transport mechanisms in the Labrador Sea. Modeling (Chanut et al., 2008;Den Toom et al., 2014;Kawasaki & Hasumi, 2014;Pennelly et al., 2019;Weijer et al., 2012) and observational studies with drifters (Cuny et al., 2002), moorings (Lilly et al., 2003), gliders (Hátún et al., 2007), and profiling floats (Prater, 2002) have shown that eddies can carry low-salinity water at the surface and high salinity water at depth, making important contributions to the cross-isobath transport of coastal water off western Greenland into the Labrador Sea and affecting the convection cycle (Chanut et al., 2008;Hátún et al., 2007). The temporal and spatial scales of the eddies are important, with eddies of short temporal and small spatial scales contributing the most (Chanut et al., 2008;McGeehan & Maslowski, 2011). ...
... Over the last decades, numerous studies have investigated freshwater content in the Labrador Sea (Houghton & Visbeck, 2002;Kawasaki & Hasumi, 2014;Myers & Deacu, 2004;Schmidt & Send, 2007;Straneo, 2006;Yashayaev, 2007), in part because of its potential impact on stratification and winter convection (Lazier, 1973;Pickart et al., 2002) and primary productivity (Arrigo et al., 2017;Oliver et al., 2018). The region has received increased attention in recent years, as record amounts of melting from the Greenland ice sheet have been observed and melting is projected to continue increasing over the next decades Tedesco et al., 2013). ...
The Greenland ice sheet is melting at increasing rates. Changes in freshwater input to the Labrador Sea can influence coastal circulation and biological processes, stratification, and potentially winter convection. Many recent studies have investigated freshwater variability in the region based on model simulations or observations with limited spatial/temporal coverage. Here, we use in situ (1990–2019) and satellite (2011–2017) observations of surface salinity to characterize freshwater content and to identify transport pathways in the Labrador Sea over multiple years. Large freshening is observed in coastal waters off southwest Greenland from July to November. Interannual variability in freshening near the coast seems to be at least partially related to variability in meltwater input, although the sparseness of in situ data precludes a quantitative assessment. The seasonal westward transport of freshwater is enhanced between 60°–62°N and especially between 63°–64.8°N from August to October, with the low-salinity waters circumnavigating the basin following the 1,000–2,000 m isobaths. That pathway coincides with intensifications in the component of the surface geostrophic flow that is directed offshore, highlighting the role played by the large-scale circulation on the westward transport of the freshwater. Low-salinity water can be transported toward the central Labrador Sea at synoptic scales, however, where it can potentially influence stratification. Consistent with previous modeling studies, offshore freshening is reduced in years with persistent downwelling-favorable wind conditions. Despite limitations under cold water conditions, satellite observations of surface salinity compare well with in situ data suggesting that they can be useful for monitoring freshwater content in high latitudes.
... Current patterns in the North Atlantic are also conducive to long-distance transport of litter. The Greenland current brings water from the North-East Atlantic around its southern tip, and north along the west coast (Brearley et al., 2012;Kawasaki and Hasumi, 2014;Pacini and Pickart, 2022). Modeling suggests that these currents can transport at least microplastics from several major European rivers north to the Barents Sea, Arctic Ocean and Greenland Sea, around Greenland, and north again along its west coast to a potential accumulation zone in Baffin Bay (Huserbråten et al., 2022). ...
Modeling studies illustrate the potential for long-range transport of plastics into the Arctic, although the degree to which this occurs remains relatively undocumented. We utilised a teaching exercise at a UArctic summer school graduate course in Nuuk, Greenland to conduct a preliminary in-depth analysis of beach litter sources in the Nuup Kangerlua fjord. Students and instructors collected and analysed 1800 litter items weighing 200 kg from one location in the fjord and another at its mouth. The results suggest a predominance of local sources to macrolitter, rather than long-range transport from Europe. Fisheries-related items and rope were common. Packaging which could be identified was largely suspected to be products distributed in Greenland, and soft plastics, which rarely disperse far from its source, were also common. The results suggest local measures to reduce mismanaged waste and emissions from fisheries are important for reducing marine litter in West Greenland.
... For a summary of these different eddy signatures, the reader is referred to Pacini and Pickart (2022), who also present the first observational evidence of BCEs along the west Greenland continental slope. Together, the IRs and BCEs transport cold, fresh water near the surface and warm, saline water at depth from the WGC into the basin, both of which can influence interior winter mixed layer development (Chanut et al., 2008;Katsman et al., 2004;Kawasaki & Hasumi, 2014;Lilly et al., 2003;Pennelly & Myers, 2020). While some studies are able to account for most of the necessary heat transport from the boundary current to the interior via eddy mechanisms (e.g., Katsman et al., 2004;Lilly et al., 2003), to date no studies can account for the freshwater transport needed to balance the interior Labrador Sea freshwater budget annually (Hátún et al., 2007;Lilly et al., 2003;Schulze Chretien & Frajka-Williams, 2018;Straneo, 2006). ...
Arctic‐origin and Greenland meltwaters circulate cyclonically in the boundary current system encircling the Labrador Sea. The ability of this freshwater to penetrate the interior basin has important consequences for dense water formation and the lower limb of the Atlantic Meridional Overturning Circulation. However, the precise mechanisms by which the freshwater is transported offshore, and the magnitude of this flux, remain uncertain. Here, we investigate wind‐driven upwelling northwest of Cape Farewell using 4 years of high‐resolution data from the Overturning in the Subpolar North Atlantic Program west Greenland mooring array, deployed from September 2014–2018, along with Argo, shipboard, and atmospheric reanalysis data. A total of 49 upwelling events were identified corresponding to enhanced northwesterly winds, followed by reduced along‐stream flow of the boundary current and anomalously dense water present on the outer shelf. The events occur during the development stage of forward Greenland tip jets. During the storms, a cross‐stream Ekman cell develops that transports freshwater offshore in the surface layer and warm, saline, Atlantic‐origin waters onshore at depth. The net fluxes of heat and freshwater for a representative storm are computed. Using a one‐dimensional mixing model, it is shown that the freshwater input resulting from the locus of winter storms could significantly limit the wintertime development of the mixed layer and hence the production of Labrador Sea Water in the southeastern part of the basin.
... While the respective contribution of these processes is regionally unknown, lateral eddy-driven heat exchanges-generated by the lateral density gradients and baroclinic instability-are widely acknowledged as important in densifying the boundary region, as seen for instance in the Labrador Sea (Katsman et al., 2004;Lilly et al., 2003). Lateral eddy-induced heat fluxes are required to balance the heat loss to the atmosphere and restratify open-ocean water columns following deep convection events (Chanut et al., 2008;Hátún et al., 2007;Katsman et al., 2004;Kawasaki & Hasumi, 2014;Tagklis et al., 2020). Additionally, eddies contribute to interior downwelling and boundary current barotropization through along-isopycnal water masses stirring and exchange between the boundary current and the interior Khatiwala & Visbeck, 2000). ...
A significant fraction of the Eulerian‐mean downwelling feeding the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) occurs along the subpolar North Atlantic continental slopes and is maintained by along‐boundary densification and large‐scale geostrophic balance. We here use Argo and shipboard hydrography data to map the 2002–2015 long‐term mean density field along the boundary via a dedicated optimal interpolation tool. The overall downstream densification implies an Eulerian‐mean downwelling of 2.12 ± 0.43 Sv at 1100 m depth between Denmark Strait and Flemish Cap. A clear regional pattern emerges with downwelling in the Irminger Sea and western Labrador Sea and upwelling along Greenland western continental slope. Comparisons with independent cross‐basin estimates confirm that vertical overturning transport across the marginal seas of the subpolar North Atlantic mainly occurs along the continental slopes, and suggest the usefulness of hydrographic data in providing quantitative information about the sinking branch of the AMOC.
... On the eastern side of Greenland, the East Greenland Current (EGC) carries southward outflow from the Arctic Ocean (liquid freshwater, sea ice melt and sea ice), mostly over the shelf, with little freshwater lost offshore (Dickson et al. 2007, de Steur et al. 2015 (Fig. 1a). Farther downstream, the West Greenland Current (WGC), which originates from the EGC, transports this freshwater northward across Davis Strait, with some offshore advection into the Labrador Sea (Myers 2005, Kawasaki & Hasumi 2014, Rykova et al. 2015 but see Rysgaard et al. 2020). A separate branch of Arctic Ocean outflow exits Baffin Bay through the western part of Davis Strait (Cuny et al. 2005, Curry et al. 2014, Haine et al. 2015 and joins the Labrador Current, mostly as a shelf-trapped current (Myers 2005, Fratantoni & McCartney 2007). ...
Greenland extends from 60˚ to 83˚ N, with 80% of its land mass covered by the Greenland Ice Sheet (GrIS). This extensive latitudinal gradient is associated with concomitant environmental gradients that impact the biogeochemical properties of its coastal waters. Although the Greenland fjords have been the subject of intense study, less is known of the productivity in the continental shelves, regions that support local fisheries and influence fjord oceanography. This study provides a large-scale overview of annual net primary production rates (NPP) and their spatial variability in 7 regional shelves around Greenland, over the last decade (2008-2017), with special emphasis on spring bloom initiation. NPP is estimated by two independent approaches already established as best for estimating Arctic productivity: a regional, physically-biologically coupled, regional 3D ocean model (SINMOD) and a spectrally-resolved, light-photosynthesis model of primary production (UQUAR-Takuvik model) that is applied to satellite observations of phytoplankton chlorophyll a, which is derived from ocean color remote sensing (OCRS). Both OCRS and SINMOD provide similar estimates of the timing and rates of productivity in Greenlandic waters, when compared with scarce field estimates. Bloom initiation shows a strong south-north gradient, beginning in April in the southern regions and late June in the Arctic Ocean. OCRS-modeled NPP highlights the effect of sea ice presence on bloom initiation; this method depicts the start of the bloom consistently later, by up to 13 days on average, than SINMOD-modeled NPP estimates. In contrast, numerical modeling is able to detect early phytoplankton growth in Greenland shelves, particularly underneath seasonal sea ice. Rates of annual NPP show a strong south-north gradient, with higher NPP rates observed in the North Atlantic water-influenced southern regions, with up to a factor of 3 decrease in NPP towards the north, in the western Eurasian Basin of the Arctic Ocean. Annual NPP varies from 78.3 ± 12.3 g C m-2 yr-1 and 80.3 ± 18.7 g C m-2 yr-1 in the southern regions to 24.7 ± 3.9 g C m-2 yr-1 in the Arctic Ocean. In each region, sea ice distribution and timing of formation and retreat affect location and timing of seasonal productivity with earlier and higher NPP offshore, moving inshore towards the summer. An average 55% to 75% of the annual production is estimated to be exported to depth, higher in Arctic and sub-Arctic regions, suggesting Greenland shelf waters have a potential for high carbon export to depth and relatively less carbon associated with the microbial food web.
... For about 50% of the years (since 1993) the MMLD is observed in April (see also Marshall and Schott, 1999;Kovalevsky, 2002;Fedorov et al., 2018;Bashmachnikov et al., 2018a). In late spring, the buoyancy fluxes restore the upper ocean stratification preventing the formation of the DC chimneys (Marshall and Schott, 1999;Kovalevsky, 2002;Kawasaki and Hasumi, 2014;Kovalevsky and Bashmachnikov, 2019). However, observations show that even several months after a DC chimney loses its connection with the sea-surface, water stability in the deep parts of the water column in the Greenland Sea stay low (Wadhams et al., 2004) or may continue to decrease (Kovalevsky, 2002). ...
This study investigates the physical processes and mechanisms driving the interannual variability of deep convective intensity in the Greenland Sea from 1993 to 2016. The intensity of deep convection is derived using the traditional Maximum Mixed Layer Depth, the total surface area with the monthly-mean mixed layer depth exceeding 800 m and various indices. All metrics show that the intensity of convection increased during the 2000s.
The analysis demonstrates that observed increases of the deep convective intensity in the Greenland Sea is associated with an increase in the upper ocean salinity. The long-term interannual variability of deep convection is mainly linked to the variation of the water salinity during the preceding summer and the current winter. In turn, the variability of the upper-ocean salinity is primarily related to the variability in the advection of Atlantic water into the region with the re-circulating branches of the West Spitsbergen Current and to a lesser degree, to the local sea ice melt. For only two winters during the study period did the sea ice contribute significantly to a weakening of the intensity of deep convection by substantially reducing oceanic heat loss to the atmosphere. The variability in the advected heat is effectively abated by the concurrent variations of oceanic heat release to the atmosphere. The interplay between the interannual variability of the oceanic heat advection and the winter air-sea net heat flux leads to a noticeable reduction of the interannual variability of both fluxes over the convective regions. As a result, the direct effect of the varying air-sea heat exchange did not have a pronounced direct effect on the interannual variation in the intensity of deep convection in the Greenland Sea, at least during the study period.
... This weakly stratified water mass helps to maintain the hydrographic structure of the subpolar North Atlantic (Talley and McCartney 1982;Sy et al. 1997;Rhein et al. 2002) and serves to sequester carbon at depth (Takahashi et al. 2009;Khatiwala et al. 2013). The ability of the rim current to flux heat and freshwater into the interior basin (e.g., Pickart 1992;Lilly et al. 1999;Kawasaki and Hasumi 2014) modulates the convection by influencing both the preconditioning and restratification process (Katsman et al. 2004;Chanut et al. 2008). ...
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.
... Another shortcoming is probably insufficient eddy activity in the Labrador Sea, so too little freshwater is transported from the West Greenland Current into the interior of the Labrador Sea (e.g. Eden and Böning, 2002;Kawasaki and Hasumi, 2014). ...
As a contribution towards improving the climate mean state of the atmosphere and the ocean in Earth system models (ESMs), we compare several coupled simulations conducted with the Max Planck Institute for Meteorology Earth System Model (MPI-ESM1.2) following the HighResMIP protocol. Our simulations allow to analyse the separate effects of increasing the horizontal resolution of the ocean (0.4 to 0.1∘) and atmosphere (T127 to T255) submodels, and the effects of substituting the Pacanowski and Philander (PP) vertical ocean mixing scheme with the K-profile parameterization (KPP).
The results show clearly distinguishable effects from all three factors. The high resolution in the ocean removes biases in the ocean interior and in the atmosphere. This leads to the important conclusion that a high-resolution ocean has a major impact on the mean state of the ocean and the atmosphere. The T255 atmosphere reduces the surface wind stress and improves ocean mixed layer depths in both hemispheres. The reduced wind forcing, in turn, slows the Antarctic Circumpolar Current (ACC), reducing it to observed values. In the North Atlantic, however, the reduced surface wind causes a weakening of the subpolar gyre and thus a slowing down of the Atlantic meridional overturning circulation (AMOC), when the PP scheme is used. The KPP scheme, on the other hand, causes stronger open-ocean convection which spins up the subpolar gyres, ultimately leading to a stronger and stable AMOC, even when coupled to the T255 atmosphere, thus retaining all the positive effects of a higher-resolved atmosphere.
... These eddies together with BCE are thought to reduce the depth of the convective events (Jones & Marshall, 1993 as they are very effective at transporting heat and freshwater into the convective patch playing a major role in the early stages of restratification throughout the patch. Recently, Kawasaki and Hasumi (2014) explored in a modeling study the importance of eddy-induced near surface freshwater transport for inhibiting deep convection. They concluded that lateral buoyancy transport caused by heat is actually significantly larger than that from freshwater, with the later contributing mainly to the near the surface buoyancy transport only in the northern Labrador Sea. ...
Labrador Sea Water (LSW) is one of the main contributors to the lower limb of the Atlantic Meridional Overturning Circulation. In this study, we explore the sensitivity of LSW formation to model resolution, Greenland melt, absence of high-frequency atmospheric phenomena, and changes in precipitation. We use five numerical model simulations at both (1/4)° and (1/12)° resolutions. A kinematic subduction approach is used to obtain the LSW formation rate over the period 2004 to 2016. The control simulation, with (1/4)° resolution, showed a mean annual production rate of 1.9 Sv (1 Sv = 10 ⁶ m ³ /s) in the density range of 27.68–27.80 kg/m ³ for the period 2004–2016. Deep convection events that occurred during 2008, 2012, and 2014–2016 were captured. We found that with (1/4)° resolution the LSW formation rate is 19% larger compared with its counterpart at (1/12)° resolution. The presence of Greenland melt and an increase in the precipitation impact the denser LSW layer replenishment but do not decrease the overall LSW formation rate nor the maximum convection depth. A dramatic response was found when filtering the atmospheric forcing, which induced a decrease of 44% in heat loss over the Labrador Sea, strong enough to halt the deep convection and decrease the LSW formation rate by 89%. Even if our experiment was extreme, a decrease in the storms crossing the Labrador Sea with a consequent reduction in the winter heat loss might be a bigger threat to deep convection and LSW formation in the future than the expected increases in the freshwater input.
... During summer, the warmest (~5°C), densest waters are found south of Davis Strait (Figure 9, orange circles) in the northern Labrador Sea. Moving northward, θ-S properties progressively cool and freshen at Davis Strait and to the north of Davis Strait (purple and blue circles); this may be due in part to eddy-driven mixing along the west Greenland slope (Kawasaki & Hasumi, 2014;Lilly et al., 2003) and strong shear and recirculation in eastern Davis Strait (Dunlap & Tang, 2006). Figure S3) reveal a thickening and warming of WGIW core waters during fall and winter. ...
Greenland fjords provide a pathway for the inflow of warm shelf waters to glacier termini and outflow of glacially modified waters to the coastal ocean. Characterizing the dominant modes of variability in fjord circulation, and how they vary over subannual and seasonal time scales, is critical for predicting ocean heat transport to the ice. Here we present a 2-year hydrographic record from a suite of moorings in Davis Strait and two neighboring west Greenland fjords that exhibit contrasting fjord and glacier geometry (Kangerdlugssuaq Sermerssua and Rink Isbræ). Hydrographic variability above the sill exhibits clear seasonality, with a progressive cooling of near-surface waters and shoaling of deep isotherms above the sill during winter to spring. Renewal of below-sill waters coincides with the arrival of dense waters at the fjord mouth; warm, salty Atlantic-origin water cascades into fjord basins from winter to midsummer. We then use Seaglider observations at Davis Strait, along with reanalysis of sea ice and wind stress in Baffin Bay, to explore the role of the West Greenland Current and local air-sea forcing in driving fjord renewal. These results demonstrate the importance of both remote and local processes in driving renewal of near-terminus waters, highlighting the need for sustained observations and improved ocean models that resolve the complete slope-trough-fjord-ice system.
... These boundary currents carry two distinct water masses: cold, fresh water that is relatively light and of Arctic origin (hereafter Polar Water), and warm, salty water that is denser and originates in the subtropics (hereafter Atlantic Water). During its transit in the boundary currents around Greenland, warm Atlantic Water progressively loses most of its heat through lateral mixing and exchange with the relatively colder offshore basins (Rignot et al., 2012a;Straneo et al., 2012;Kawasaki and Hasumi, 2014). ...
Melting of the Greenland Ice Sheet represents a major uncertainty in projecting future
rates of global sea-level rise. Much of this uncertainty is related to a lack of knowledge
about subsurface ocean hydrographic properties, particularly heat content, how these
properties are modified across the continental shelf, and the extent to which the ocean
interacts with glaciers. Early results from NASA’s five-year Oceans Melting Greenland
(OMG) mission, based on extensive hydrographic and bathymetric surveys, suggest that
many glaciers terminate in deep water and are hence vulnerable to increased melting due
to ocean-ice interaction. OMG will track ocean conditions and ice loss at glaciers around
Greenland through the year 2020, providing critical information about ocean-driven
Greenland ice mass loss in a warming climate.
... There are many other places in the ocean where local mesh refinement may contribute to increase model fidelity through increased realism in rendering topography and coastlines, reduced dissipation, or better representation of meso-scale processes such as lateral spreading or eddy fluxes. Learning about the impact of locally resolved dynamics on the general ocean circulation motivates a growing number of studies which use models formulated on nested or generalized curvilinear meshes to locally resolve eddy dynamics in regions of interest [see e. g., Durgadoo et al., 2013;Kawasaki and Hasumi, 2014;Talandier et al., 2014;Sein et al., 2015]. In these cases globally relevant regional dynamics can be simulated at a moderate computational cost compared to running a global eddy resolving model. ...
If unstructured meshes are refined to locally represent eddy dynamics in ocean circulation models, a practical question arises on how to vary the resolution and where to deploy the refinement. We propose to use the observed sea surface height variability as the refinement criterion. We explore the utility of this method (i) in a suite of idealized experiments simulating a wind-driven double gyre flow in a stratified circular basin and (ii) in simulations of global ocean circulation performed with FESOM. Two practical approaches of mesh refinement are compared. In the first approach the uniform refinement is confined within the areas where the observed variability exceeds a given threshold. In the second one the refinement varies linearly following the observed variability. The resolution is fixed in time. For the double gyre case it is shown that the variability obtained in a high-resolution reference run can be well captured on variable-resolution meshes if they are refined where the variability is high and additionally upstream the jet separation point. The second approach of mesh refinement proves to be more beneficial in terms of improvement downstream the mid-latitude jet. Similarly, in global ocean simulations the mesh refinement based on the observed variability helps the model to simulate high variability at correct locations. The refinement also leads to a reduced bias in the upper-ocean temperature. This article is protected by copyright. All rights reserved.
... The changes in the inflows and outflows have some important consequences for the freshwater accumulation in the subpolar gyre. Earlier observational and high-resolution model studies have shown that mixing from the West Greenland Current (source from Fram Strait via the East Greenland Current) toward the central Labrador sea is much larger than from the Labrador Current (source from CAA) [Myers, 2005;Schmidt and Send, 2007;Kawasaki and Hasumi, 2014]. This implies that a freshwater perturbation flowing out from the Arctic mainly via the Fram Strait (as in our simulations) has a larger impact on the Labrador Sea convection sites, subpolar gyre circulation, and the AMOC, than a freshwater perturbation flowing out via the CAA [see also Condron and Winsor, 2012, Figure 2]. ...
The Arctic Ocean has important freshwater sources including river runoff, low evaporation, and exchange with the Pacific Ocean. In the future, we expect even larger freshwater input as the global hydrological cycle accelerates, increasing high latitude precipitation and river runoff. Previous modelling studies show some robust responses to high latitude freshwater perturbations, including a strengthening of Arctic stratification and a weakening of the large-scale ocean circulation; some idealized modelling studies also document a stronger cyclonic circulation within the Arctic Ocean itself. With the broad range of scales and processes involved, the overall effect of increasing runoff requires an understanding of both the local processes and the broader linkages between the Arctic and surrounding oceans. Here, we adopt a more comprehensive modelling approach by increasing river runoff to the Arctic Ocean in a coupled ice–ocean general circulation model, and show contrasting responses in the polar and subpolar regions. Within the Arctic, the stratification strengthens, the halocline and Atlantic Water layer warm, and the cyclonic circulation spins up, in agreement with previous work. In the subpolar North Atlantic, the model simulates a colder and fresher water column with weaker barotropic circulation. In contrast to the estuarine circulation theory, the volume exchange between the Arctic Ocean and the surrounding oceans does not increase with increasing runoff. While these results are robust in our model, we require experiments with other model systems and more complete observational syntheses to better constrain the sensitivity of the climate system to high latitude freshwater perturbations. This article is protected by copyright. All rights reserved.
The amount of cross‐isobath freshwater exchange within the North Atlantic subpolar gyre is estimated from numerical modelling simulations. A regional configuration of the Nucleus for European Modelling of the Ocean model is used to carry out three simulations with horizontal resolutions of 1/4°, 1/12°, and 1/4° with a 1/12° nested domain. Freshwater transport is calculated across five isobaths in six regions for three distinct water masses. Fresh Polar Water is only transported offshore from the western coast of Greenland and the southern coast of Labrador; other regions have onshore transport of freshwater or little offshore transport. The salty water masses of Irminger and Labrador Sea Water typically have onshore transport, acting to promote subsurface freshening of the Labrador Sea. The freshwater transport via the Polar Water mass experiences a large degree of seasonal variability, while the Irminger and Labrador Sea Water masses do not. Decomposing the freshwater transport into the mean and turbulent components indicates that most regions and water masses have stronger freshwater transport associated with the mean flow while the turbulent flow in often the opposite direction. The only water mass and region where the mean and turbulent freshwater transport act in the same direction is Polar Water along the western margin of Greenland. Model resolution plays an important role in determining cross‐isobath exchange as our results from an identically forced simulation at 1/4° has reduced seasonal cycles, reduced transport, and sometimes transport in the opposite direction when compared against the 1/12° resolution simulations.
In this study, an idealized eddy-resolving model is employed to examine the interplay between the downwelling, ocean convection and mesoscale eddies in the Labrador Sea and the spreading of dense water masses. The model output demonstrates a good agreement with observations with regard to the eddy field and convection characteristics. It also displays a basin mean net downwelling of 3.0 Sv. Our analysis confirms that the downwelling occurs near the west Greenland coast and that the eddies spawned from the boundary current play a major role in controlling the dynamics of the downwelling. The magnitude of the downwelling is positively correlated to the magnitude of the applied surface heat loss. However, we argue that this connection is indirect: the heat fluxes affect the convection properties as well as the eddy field, while the latter governs the Eulerian downwelling. With a passive tracer analysis we show that dense water is transported from the interior towards the boundary, predominantly towards the Labrador coast in shallow layers and towards the Greenland coast in deeper layers. The latter transport is steered by the presence of the eddy field. The outcome that the characteristics of the downwelling in a marginal sea like the Labrador Sea depend crucially on the properties of the eddy field emphasizes that it is essential to resolve the eddies to properly represent the downwelling and overturning in the North Atlantic Ocean, and its response to changing environmental conditions.
A part of the global ocean conveyor belt (Atlantic thermohaline circulation), deep convection in the Greenland, Labrador and Irminger seas, is an important component of the Earth's climate system. In situ studies of interannual variability of deep convection is a challenge due to the small size of convective cells and to variations of their locations within the sea basins. In this work, for the first time the combined array of in situ and satellite data (ARMOR) is used for evaluation of the areas of the most frequent occurrence of deep convection in the North Atlantic. It is shown that in the Labrador and Irminger seas deep convection (>1000 m) can develop in almost any point of the water area of the seas. Within this area there are three sub-regions of the most frequent development of the deep convection. In the Greenland Sea, the most frequent development of deep convection takes place in two disconnected sub-regions. Histograms of the distribution of the number of episodes of the maximum convection development per month showed that in the Greenland Sea the most often convection reaches the maximum intensity in April, and in the Labrador and Irminger seas – in March.
Using EN4 data-set from 1950 to 2015, the areas of the most intense deep convective mixing are identified as the maximum depths of the upper mixed layer during the cold season. It has been shown that the areas with the maximum registered convection depth of 1500–2000 m are found in the Greenland basin (73°–76° N, 5° W-1° E) and the Boreas basin (77° N, 1–2.5° W). This refines the areas of the deep convection derived from in situ data and results of hydrodynamic modeling. It has been shown that the previously separated in literature areas of deep convection in the Labrador Sea (55–59° N 50–56° W) and the Irminger Sea (57–60° N, 35– 43° W), are in fact linked into one region by the episodic re-occurrence of the deep convection (1000 m and more) south of Greenland (between 56°–58° N). The intra-annual variability of deep convection was studied over the whole period of observations of 1950–2015. It is shown that the maximum depths of the upper mixed layer in all three seas was usually registered between December and May. The most often convection reaches the maximum depth in the Labrador and Irminger seas in March, and in the Greenland Sea — in April.
The Labrador Sea is one of a small number of deep convection sites in the North Atlantic that contribute to the meridional overturning circulation. Buoyancy is lost from surface waters during winter, allowing the formation of dense deep water. During the last few decades, mass loss from the Greenland ice sheet has accelerated, releasing freshwater into the high-latitude North Atlantic. This and the enhanced Arctic freshwater export in recent years have the potential to add buoyancy to surface waters, slowing or suppressing convection in the Labrador Sea. However, the impact of freshwater on convection is dependent on whether or not it can escape the shallow, topographically trapped boundary currents encircling the Labrador Sea. Previous studies have estimated the transport of freshwater into the central Labrador Sea by focusing on the role of eddies. Here, we use a Lagrangian approach by tracking particles in a global, eddy-permitting (1∕12°) ocean model to examine where and when freshwater in the surface 30m enters the Labrador Sea basin. We find that 60% of the total freshwater in the top 100m enters the basin in the top 30m along the eastern side. The year-to-year variability in freshwater transport from the shelves to the central Labrador Sea, as found by the model trajectories in the top 30m, is dominated by wind-driven Ekman transport rather than eddies transporting freshwater into the basin along the northeast.
Supplementary Figures 1-12, Supplementary Note 1, Supplementary Methods and Supplementary References.
The Greenland ice sheet has experienced increasing mass loss since the 1990s. The enhanced freshwater flux due to both surface melt and outlet glacier discharge is assuming an increasingly important role in the changing freshwater budget of the subarctic Atlantic. The sustained and increasing freshwater fluxes from Greenland to the surface ocean could lead to a suppression of deep winter convection in the Labrador Sea, with potential ramifications for the strength of the Atlantic meridional overturning circulation. Here we assess the impact of the increases in the freshwater fluxes, reconstructed with full spatial resolution, using a global ocean circulation model with a grid spacing fine enough to capture the small-scale, eddying transport processes in the subpolar North Atlantic. Our simulations suggest that the invasion of meltwater from the West Greenland shelf has initiated a gradual freshening trend at the surface of the Labrador Sea. Although the freshening is still smaller than the variability associated with the episodic â great salinity anomalies', the accumulation of meltwater may become large enough to progressively dampen the deep winter convection in the coming years. We conclude that the freshwater anomaly has not yet had a significant impact on the Atlantic meridional overturning circulation.
The heat influx of the Atlantic water and its interannual variability through the Fram Strait toward the Arctic Ocean are examined by using a realistically configured ice-ocean general circulation model. The modeled routes of the Atlantic water and high eddy activity around the Fram Strait are consistent with many observations. Two-thirds of the heat transported by the Atlantic water passing through the Fram Strait (78°N) is lost by the westward transport and the sea surface cooling, and the other one-third is injected to the Arctic Ocean. The contribution of oceanic eddy to the westward heat transport is 5% of that of mean current. The variability of sea level pressure anomaly centered at the Nordic Seas explains the interannual variability of the heat passing through the Fram Strait, transported westward, and cooled at the sea surface in the north of the Fram Strait. The interannual variabilities of these heat fluxes have significant correlations with the NAO. The interannual variability of heat transported by the Atlantic water and entering the Arctic Ocean is caused by the variability of the Siberian high.
This atlas consists of a description of data analysis procedures and horizontal maps of annual, seasonal, and monthly climatological distribution fields of salinity at selected standard depth levels of the world ocean on a one-degree latitude-longitude grid. The aim of the maps is to illustrate large-scale characteristics of the distribution of ocean salinity. The fields used to generate these climatological maps were computed by objective analysis of all scientifically quality-controlled historical salinity data in the World Ocean Database 2009. Maps are presented for climatological composite periods (annual, seasonal, monthly, seasonal and monthly difference fields from the annual mean field, and the number of observations) at selected
standard depths.
We estimate the winter sea ice export through the Fram Strait using ice
motion from satellite passive microwave data. Sea ice motion
(October-May) is obtained by tracking the displacement of common
features in sequential 85 and 37 GHz brightness temperature fields. The
average winter area flux over the 18-year record (1978-1996) is 670,000
km2, ˜7% of the area of the Arctic Ocean. The winter
area flux ranges from a minimum of 450,000 km2 in 1984 to a
maximum of 906,000 km2 in 1995. The daily, monthly, and
interannual variabilities of the ice area flux are high. There is an
upward trend in the ice area flux over the 18 year record. The average
winter volume flux over the winters of October 1990 through May 1995 is
1745 km3 ranging from a low of 1375 km3 in the
1990 flux to a high of 2791 km3 in 1994. The sea level
pressure gradient across the Fram Strait explains more than 80% of the
variance in the ice flux over the 18 year record. We use the
coefficients from the regression of the time series of area flux versus
pressure gradient across the Fram Strait and ice thickness data to
estimate the summer area and volume flux. The average 12 month area flux
and volume flux are 919,000 km2 and 2366 km3. We
find a significant correlation (R = 0.86) between the area flux and
positive phases of the North Atlantic Oscillation (NAO) index over the
months of December-March. Correlation between our 5 years of volume flux
estimates and the NAO index gives R = 0.56. During the high NAO years a
more intense Icelandic low increases the gradient in the sea level
pressure by almost 1 mbar across the Fram Strait, thus increasing the
atmospheric forcing on ice transport. Correlation is reduced during the
negative NAO years because of decreased dominance of this large-scale
atmospheric pattern on the sea level pressure gradient across the Fram
Strait.
An improved model for the oceanic boundary layer is presented in view of
the recent observation of the microstructure of the upper ocean
including the high dissipation rate near the sea surface. In the new
model the surface boundary conditions for both the turbulent kinetic
energy flux and the roughness length scale are modified. The
parameterization of stratification effects on turbulence is improved,
and the convective process is reformulated on the basis of the
observation of uniform temperature and velocity profiles within the
convective mixed layer. Evolutions of the profiles of both the
dissipation rate and temperature of the observation data Patches
Experiment as well as the time series of the sea surface temperature
over the observation days, are successfully simulated during a diurnal
cycle for the first time. It is also shown that the model reproduces
various important features of the oceanic boundary layer, for example,
the formation of a diurnal thermocline, the profiles of buoyancy flux,
and the magnitudes of the buoyancy gradients both within the mixed layer
and at the diurnal thermocline. Performance of the model is compared
with that of the widely used Mellor-Yamada model.
The dynamics of a quasigeostrophic flow confined in a two-layer channel over variable topography on the beta plane is numerically investigated. The topography slopes uniformly upward in the north-south direction (in the beta sense) and is a smooth function of the zonal coordinate. The bottom slope controls the local supercriticality and is configured to destabilize the flow only in a central interval of limited zonal extent. Linearized solutions indicate that, for a wide enough channel, unstable modes exist for an arbitrary short interval of instability, confirming previous analysis on disturbances with no meridional variation. For small local maximum supercriticality, the instability is maintained by a short bottom-trapped wave localized at the downstream edge of the unstable region and oscillating in phase with the upper-layer disturbance. When nonlinearity is retained in the problem, the equilibration of the bottom-trapped wave is associated with the formation of coherent vortices. Both cyclones and anticyclones are formed continuously at the northeastern edge of the unstable interval. Through vortex stretching mechanisms, dipoles inside the interval of instability can split upon reaching the northern wall: Anticyclones move downstream along the north wall and propagate into the downstream stable region, while cyclonic structures tend to remain trapped inside the interval of instability. The authors suggest the relevance of their results to the observed eddy field of the Labrador Sea.
During June-November 1994, a mooring in the central Labrador Sea near the former Ocean Weather Station Bravo recorded a half-dozen anomalous events that prove to be two different types of coherent eddies. Comparisons with simple analytical models are used to classify these events as coherent eddies on the basis of their velocity signatures. The first clear examples of long-lived convectively generated eddies are reported. These four small (radius ~5-15 km) eddies are exclusively anticyclonic, with cold, fresh middepth potential temperature (χ) and salinity (S) cores surrounded by azimuthal currents of ~15 cm s-1. Their χ/S properties identify them unambiguously as the products of wintertime deep convection in the interior Labrador Sea. Compared with eddies in other regions, these anticyclones are unusual for their strong surface expressions and composite χ/S cores. Two warm cyclones are also seen: these are larger (radius ~15 km) than the anticyclones and about as energetic (currents ~15 cm s-1). Their χ/S and potential vorticity properties suggest that they are created by stretching a column of water from the Irminger Current, which surrounds the Labrador Sea on three sides.
The occurrence and extent of deep convection in the Labrador Sea in winters 1996/97 and 1997/98 is investigated from measurements of over 200 neutrally buoyant subsurface Profiling Autonomous Lagrangian Circulation Explorer (PALACE) and Sounding Oceanographic Lagrangian Observer (SOLO) floats. In addition to providing drift velocity data and vertical profiles of temperature and salinity, 55 floats are equipped with vertical current meters (VCMs). Time series of vertical velocity (derived from measured pressure and vertical flow past the float) and temperature are obtained from the VCM floats. Mixed layer depths estimated from profile measurements indicate that convection reached depths greater than 1300 m in 1997, but no deeper than 1000 m in 1998. Deep mixed layers were concentrated in the western basin, although a number of deep mixed layers were observed southwest of Cape Farewell and also north of 60°N. The highest variance in vertical velocity and the lowest mean temperatures were found in the western basin, suggesting that this area is the main site of deep convection. Deep mixed layers and large vertical velocities were observed as late as April and May, despite the fact that surface forcing appears to have ceased. Estimates of mean vertical velocity appear to be affected by a float sampling bias, whereby floats preferentially sample convergent regions. The effect of this bias, which is dependent on the float depth within the convective layer, is to sample upward flow in early winter and downward flow in late winter when the convective layer has deepened. A one-dimensional heat balance model is examined, whereby the winter surface heat flux, estimated from temperature profiles, is balanced by the turbulent vertical heat flux associated with deep convection, estimated from time series measurements. The plume-scale vertical heat flux can only account for roughly -80 of -350 W m -2 measured at 400-m depth. The vertical heat flux at longer timescales is investigated, but cannot be resolved with this dataset. Failure to balance the surface heat flux by plume-scale motions, combined with an observed high variance of w and T at low frequencies, suggests that motion at these longer timescales contributes to the one-dimensional heat budget in winter.
A decade of weak convection in the Labrador Sea associated with decreasing water mass transformation, in combination with advective and eddy fluxes into the convection area, caused significant warming of the deep waters in both the central Labrador Sea and boundary current system along the Labrador shelf break. The connection to the export of Deep Water was studied based on moored current meter stations between 1997 and 2009 at the exit of the Labrador Sea, near the shelf break at 53°N. More than 100 year-long current meter records spanning the full water column have been analyzed with respect to high frequency variability, decaying from the surface to the bottom layer, and for the annual mean flow, showing intra- to interannual variability but no detectable decadal trend in the strength of the deep and near-bottom flow out of the Labrador Sea.
Mooring records collected in the central Labrador Sea are evaluated regarding the variability of the hydrographic properties of newly formed Labrador Sea Water (LSW) between 1994 and 2005. This time series is longer and of significantly higher temporal resolution than any discussed before in the context of decreasing convection activity. For the upper 1500 m depth range two distinct warming periods are identified from 1997 to 1999 and from 2003 to 2005 leading to a substantial temperature increase of 0.6°C over the recent decade. The time series of LSW source water properties suggest that ocean transport of heat and freshwater anomalies play a significant role in determining the ultimate convection depth. In 2005 the LSW temperature and salinity had reached high values comparable to those from the early 1970's, shortly after the passage of the Great Salinity Anomaly.
A significant fraction of the lateral heat transport into the Labrador Sea's interior, needed to balance the net heat loss to the atmosphere, is attributed to the Irminger Current Anticyclones. These mesoscale eddies advect warm, salty boundary current water, of subtropical origin, from the boundary current to the interior— but when or how they release their anomalous heat content has not been previously investigated. In this study, we discuss the seasonal and interannual evolution of these anticyclones as inferred from the analysis of hydrographic data from the Labrador Sea from 1990 to 2004. The 29 identified anticyclones fall into two categories, which we refer to as unconvected and convected. Unconvected anticyclones have properties that are close to those of the boundary current, including a fresh surface layer, and they are found near the boundaries and never observed in winter. Convected anticyclones, on the other hand, contain a mixed layer, lack a freshwater cap and are observed throughout the year. Using a one-dimensional mixing model, it is shown that the convected eddies are those Irminger Current Anticyclones that have been modified by the large winter buoyancy loss of the region. This provides evidence that such eddies can survive the strong winter buoyancy loss in the Labrador Sea and that their anomalous heat and salt content is not trivially mixed into the Sea's interior. Finally, we observe a clear trend in the eddies' properties toward warmer and saltier conditions after 1997 reflecting changes in the source waters and the reduced atmospheric forcing over the Labrador Sea.
The cycle of open ocean deep convection in the Labrador Sea is studied in a realistic, high-resolution (4 km) regional model, embedded in a coarser (1/3°) North Atlantic setup. This configuration allows the simultaneous generation and evolution of three different eddy types that are distinguished by their source region, generation mechanism, and dynamics. Very energetic Irminger Rings (IRs) are generated by barotropic instability of the West Greenland and Irminger Currents (WGC/ IC) off Cape Desolation and are characterized by a warm, salty subsurface core. They densely populate the basin north of 58°N, where their eddy kinetic energy (EKE) matches the signal observed by satellite altimetry. Significant levels of EKE are also found offshore of the West Greenland and Labrador coasts, where boundary current eddies (BCEs) are spawned by weakly energetic instabilities all along the boundary current system (BCS). Baroclinic instability of the steep isopycnal slopes that result from a deep convective overturning event produces convective eddies (CEs) of 20-30 km in diameter, as observed and produced in more idealized models, with a distinct seasonal cycle of EKE peaking in April. Sensitivity experiments show that each of these eddy types plays a distinct role in the heat budget of the central Labrador Sea, hence in the convection cycle. As observed in nature, deep convective mixing is limited to areas where adequate preconditioning can occur, that is, to a small region in the southwestern quadrant of the central basin. To the east, west, and south, BCEs flux heat from the BCS at a rate sufficient to counteract air-sea buoyancy loss. To the north, this eddy flux alone is not enough, but when combined with the effects of Irminger Rings, preconditioning is effectively inhibited here too. Following a deep convective mixing event, the homogeneous convection patch reaches as deep as 2000 m and a horizontal scale on the order of 200 km, as has been observed. Both CEs and BCEs are found to play critical roles in the lateral mixing phase, when the patch restratifies and transforms into Labrador Sea Water (LSW). BCEs extract the necessary heat from the BCS and transport it to the deep convection site, where it fluxed into convective patches by CEs during the initial phase. Later in the phase, BCE heat flux maintains and strengthens the restratification throughout the column, while solar heating establishes a near-surface seasonal stratification. In contrast, IRs appear to rarely enter the deep convection region. However, by virtue of their control on the surface area preconditioned for deep convection and the interannual variability of the associated barotropic instability, they could have an important role in the variability of LSW.
The authors propose a parameterization for restratification by mixed layer eddies that develop from baroclinic instabilities of ocean fronts. The parameterization is cast as an overturning streamfunction that is proportional to the product of horizontal buoyancy gradient, mixed layer depth, and inertial period. The parameterization has remarkable skill for an extremely wide range of mixed layer depths, rotation rates, and vertical and horizontal stratifications. In this paper a coarse resolution prognostic model of the parameterization is compared with submesoscale mixed layer eddy resolving simulations. The parameterization proves accurate in predicting changes to the buoyancy. The climate implications of the proposed parameterization are estimated by applying the restratification scaling to observations: the mixed layer depth is estimated from climatology, and the buoyancy gradients are from satellite altimetry. The vertical fluxes are comparable to monthly mean air–sea fluxes in large areas of the ocean and suggest that restratification by mixed layer eddies is a leading order process in the upper ocean. Critical regions for ocean–atmosphere interaction, such as deep, intermediate, and mode water formation sites, are particularly affected.
A hierarchy of coarse-resolution World Ocean experiments were integrated with a view to determining the most appropriate representation of the global-scale water masses in OGCMs. The largest-scale response of the simulated ocean to the prescribed forcing in each model run is described. The World Ocean model eventually has a realistic approximation of continental outlines and bottom bathymetry. -from Author
Time series of hydrographic and transient tracer ( 3H and 3He) observations from the central Labrador Sea collected between 1991 and 1996 are presented to document the complex changes in the tracer fields as a result of variations in convective activity during the 1990s. Between 1991 and 1993, as atmospheric forcing intensified, convection penetrated to progressively increasing depths, reaching ;2300 m in the winter of 1993. Over that period the potential temperature (u)/salinity (S) properties of Labrador Sea Water stayed nearly constant as surface cooling and downward mixing of freshwater was balanced by excavating and upward mixing of the warmer and saltier Northeast Atlantic Deep Water. It is shown that the net change in heat content of the water column (150-2500 m) between 1991 and 1993 was negligible compared to the estimated mean heat loss over that period (110 W m22), implying that the lateral convergence of heat into the central Labrador Sea nearly balances the atmospheric cooling on a surprisingly short timescale. Interestingly, the 3H-3He age of Labrador Sea Water increased during this period of intensifying convection. Starting in 1995, winters were milder and convection was restricted to the upper 800 m. Between 1994 and 1996, the evolution of 3H-3He age is similar to that of a stagnant water body. In contrast, the increase in u and S over that period implies exchange of tracers with the boundaries via both an eddy-induced overturning circulation and along-isopycnal stirring by eddies (with an exchange coefficient of O(500 m2 s 21)). The authors construct a freshwater budget for the Labrador Sea and quantitatively demonstrate that sea ice meltwater is the dominant cause of the large annual cycle of salinity in the Labrador Sea, both on the shelf and the interior. It is shown that the transport of freshwater by eddies into the central Labrador Sea ( ;140 cm between March and September) can readily account for the observed seasonal freshening. Finally, the authors discuss the role of the eddy-induced overturning circulation with regard to transport and dispersal of the newly ventilated Labrador Sea Water to the boundary current system and compare its strength (2-3 Sv) to the diagnosed buoyancy-forced formation rate of Labrador Sea Water.
A 12-month mooring record (May 1994-June 1995), together with accompanying PALACE float data, is used to describe an annual cycle of deep convection and restratification in the Labrador Sea. The mooring is located at 56.75°N, 52.5°W, near the former site of Ocean Weather Station Bravo, in water of ∼3500 m depth. This is a pilot experiment for climate monitoring, and also for studies of deep-convection dynamics. Mooring measurements include temperature (T), salinity (S), horizontal and vertical velocity, and acoustic measurement of surface winds. The floats made weekly temperature-salinity profiles between their drift level (near 1500 m) and the surface. With moderately strong cooling to the atmosphere (∼300 W m-2 averaged from November to March), wintertime convection penetrated from the surface to about 1750 m, overcoming the stabilizing effect of upperocean low-salinity water. The water column restratifies rapidly after brief vertical homogenization (in potential density, salinity, and potential temperature). Both the rapid restratification and the energetic high-frequency variations of T and S observed at the mooring are suggestive of a convection depth that varies greatly with location. Lateral variations in T and S exist down to very small scales, and these remnants of convection decay (with e-folding time ∼170 day) after convection ceases. Lateral variability at the scale of 100 km is verified by PALACE profiles. The Eulerian mooring effectively samples the convection in a mesoscale region of ocean as eddies sweep past it; the Lagrangian PALACE floats are complementary in sampling the geography of deep convection more widely. This laterally variable convection leaves the water column with significant vertical gradients most of the year. Convection followed by lateral mixing gives vertical salinity profiles the (misleading) appearance that a one-dimensional diffusive process is fluxing freshwater downward. During spring, summer, and fall the salinity, temperature, and buoyancy rise steadily with time throughout most of the water column. This is likely the result of mixing with the encircling boundary currents, compensating for the escape of Labrador Sea Water from the region. Low-salinity water mixes into the gyre only near the surface. The water-column heat balance is in satisfactory agreement with meteorological assimilation models. Directly observed subsurface calorimetry may be the more reliable indication of the annual-mean air-sea heat flux. Acoustic instrumentation on the mooring gave a surprisingly good time series of the vector surface wind. The three-dimensional velocity field consists of convective plumes of width ∼200 to 1000 m, vertical velocities of 2 to 8 cm s-1, and Rossby numbers of order unity, embedded in stronger (∼20 cm s-1) lateral currents associated with mesoscale eddies. Horizontal currents with timescales of several days to several months are strongly barotropic. They are suddenly energized as convection reaches great depth in early March, and develop toward a barotropic state, as also seen in models of convectively driven geostrophic turbulence in a weakly stratified, high-latitude ocean. Currents decay through the summer and autumn, apart from some persistent isolated eddies. These coherent, isolated, cold anticyclones carry cores of pure convected water long after the end of winter. Boundary currents nearby interact with the Labrador Sea gyre and provide an additional source of eddies in the interior Labrador Sea. An earlier study of the pulsation of the boundary currents is supported by observations of sudden ejection of floats from the central gyre into the boundary currents (and sudden ingestion of boundary current floats into the gyre interior), in what may be a mechanism for exchange between Labrador Sea Water and the World Ocean.
Ageostrophic baroclinic instabilities develop within the surface mixed layer of the ocean at horizontal fronts and efficiently restratify the upper ocean. In this paper a parameterization for the restratification driven by finite-amplitude baroclinic instabilities of the mixed layer is proposed in terms of an overturning streamfunction that tilts isopycnals from the vertical to the horizontal. The streamfunction is proportional to the product of the horizontal density gradient, the mixed layer depth squared, and the inertial period. Hence restratification proceeds faster at strong fronts in deep mixed layers with a weak latitude dependence. In this paper the parameterization is theoretically motivated, confirmed to perform well for a wide range of mixed layer depths, rotation rates, and vertical and horizontal stratifications. It is shown to be superior to alternative extant parameterizations of baroclinic instability for the problem of mixed layer restratification. Two companion papers discuss the numerical implementation and the climate impacts of this parameterization.
The depth of winter convection in the central Labrador Sea is strongly influenced by the prevailing stratification in late summer. For this late summer stratification salinity is as important as temperature, and in the upper water layers salinity even dominates. To analyze the source of the spring and summer freshening in the central region, seasonal freshwater cycles have been constructed for the interior Labrador Sea, the West Greenland Current, and the Labrador Current. It is shown that none of the local freshwater sources is responsible for the spring-summer freshening in the interior, which appears to occur in two separate events in April to May and July to September. Comparing the timing and volume estimates of the seasonal freshwater cycles of the boundary currents with the central Labrador Sea helps in understanding the origin of the interior freshwater signals. The first smaller pulse cannot be attributed clearly to either of the boundary currents. The second one is about three times stronger and supplies 60% of the seasonal summer freshwater. Transport estimates and calculated mixing properties provide evidence that its source is the West Greenland Current. The finding implies a connection also on interannual time scales between Labrador Sea surface salinity and freshwater sources in the West Greenland Current and farther upstream in the East Greenland Current. The freshwater input from the West Greenland Current thus also is the likely pathway for the known modulation of Labrador Sea Water mass formation by freshwater export from the Arctic (via the East Greenland Current), which implies some predictability on longer time scales.
Temperature, salinity and other property distributions observed across the central Labrador Sea in the early summers between 1990 and 2000 reveal a 4-year period of exceptionally intense convection followed by 5 years of restratification. The intense convection led, in the centre of the Sea, to mixed layers increasing in density and depth to a maximum of 2300 m thereby creating a fresh deep pool of Labrador Sea Water (LSW). In the second half of the decade, warmer winter weather limited the depth of convection to ∼1000 m. The shallower convection isolated the deep reservoir of homogeneous LSW between 1000 and 2000 m from renewal: this reservoir slowly diminished in volume as the layer became more stratified. In addition, the mean temperature and salinity of the 1000–2000 m layer increased by 0.4°C and 0.025 as warmer more saline water was mixed into the central region from the boundaries. In the upper layer between 150 and 1000 m the restratification processes led to an increase in temperature of 0.6°C but no significant change in salinity. The upper 150 m also showed no discernible trends in salinity but did participate in the warming trend. Interannual variability in local atmospheric forcing accounts for much of the observed change in heat content in the convectively overturned part of the water column during both the convection and restratification phases. It is proposed that constant horizontal fluxes transport heat and salt from the boundaries into the centre of the Sea. When the heat loss from the sea surface is greater than the horizontal flux the mixed layer becomes colder and denser and the depth of convection increases. When the heat loss is less than the horizontal flux and the convection remains shallow the temperature rises in both the 0–1000 m and the 1000–2000 m layers and salinity increases in the deeper layer. In both situations salinity in the upper 1000 m remains roughly constant as the horizontal salinity flux approximately offsets the annual input of fresh water of 60±10 cm into the surface layer.
We summarize 24 years (1978–2002) of ice export estimates and examine, over a 9-year record, the associated variability in the time-varying upward-looking sonar (ULS) thickness distributions of the Fram Strait. A more thorough assessment of the PMW (passive microwave) ice motion with 5 years of synthetic aperture radar (SAR) observations shows the uncertainties to be consistent with that found by Kwok and Rothrock [1999], giving greater confidence to the record of ice flux calculations. Interesting details of the cross-strait motion profiles and ice cover characteristics revealed by high-resolution SAR imagery are discussed. The average annual ice area flux over the period is 866,000 km2/yr. Between the 1980s and 1990s, the decadal difference in the net exported ice area is ∼400,000 km2, approximately half the annual average. Except for the years with extreme negative NAO, correlation of winter ice area export with the NAO index remains high (R2 = 0.62). With thickness estimates from ULS moorings, we estimate the average annual ice volume flux (8 years) to be ∼2218 km3/yr (∼0.07 Sv). Over the ∼9-year ULS ice thickness data set, there is an overall decrease of 0.45 m in the mean ice thickness over the entire time series and a decrease of 0.23 m over the winter months (December through March). Correspondingly, the mode of the MY ice thickness exhibits an overall decrease of 0.55 m and a winter decrease of 0.42 m. These are significant trends. Whether these trends are indicative of the thickness trends of the Arctic Ocean is examined, as the time-varying behavior of the monthly ULS thickness distributions can be related not only to the seasonal cycle in the basal growth and melt, but also to the magnitude and pattern of ice motion in the Arctic Ocean, and the proximity of the ULS moorings to the ice edge.
Salinity stratification is critical to the vertical circulation of the high-latitude ocean. We here examine the control of the vertical circulation in the northern seas, and the potential for altering it, by considering the budgets and storage of fresh water in the Artic Ocean and in the convective regions to the south. We find that the present-day Greenland and Iceland seas, and probably also the Labrador Sea, are rather delicately poised with respect to their ability to sustain convection. Small variations in the fresh water supplied to the convective gyres from the Arctic Ocean via the East Greenland Current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past. The North Atlantic salinity anomaly of the 1960s and 1970s is a recent example; it must have had its origin in an increased fresh water discharge from the Arctic Ocean. Similarly, the freshing and cooling of the deep North Atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the Artic Ocean into the convective gyres. Finally, we note that because of the temperature dependence of compressibility, a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean.
The authors present the first quantitative comparison between new velocity datasets and high-resolution models in the North Atlantic subpolar gyre [ 1 ⁄10° Parallel Ocean Program model (POPNA10), Miami Isopycnic Coordinate Ocean Model (MICOM), 1 ⁄6° Atlantic model (ATL6), and Family of Linked Atlantic Ocean Model Experiments (FLAME)]. At the surface, the model velocities agree generally well with World Ocean Circulation Experiment (WOCE) drifter data. Two noticeable exceptions are the weakness of the East Greenland coastal current in models and the presence in the surface layers of a strong southwestward East Reykjanes Ridge Current. At depths, the most prominent feature of the circulation is the boundary current following the continental slope. In this narrow flow, it is found that gridded float datasets cannot be used for a quantitative comparison with models. The models have very different patterns of deep convec-tion, and it is suggested that this could be related to the differences in their barotropic transport at Cape Farewell. Models show a large drift in watermass properties with a salinization of the Labrador Sea Water. The authors believe that the main cause is related to horizontal transports of salt because models with different forcing and vertical mixing share the same salinization problem. A remarkable feature of the model solutions is the large westward transport over Reykjanes Ridge [10 Sv (Sv 10 6 m 3 s 1) or more].
Deepwater formation, the process whereby surface water is actively converted into deep water through heat and freshwater exchange at the air-sea interface, is known to occur in the North Atlantic but not in the North Pacific. As such, the thermohaline circulation is fundamentally different in these two regions. In this review we provide a description of this circulation and outline a number of reasons as to why deep water is formed in the North Atlantic but not in the North Pacific. Special emphasis is given to the role of interactions with the Arctic Ocean. We extend our analysis to discuss the observational evidence and current theories for decadal-interdecadal climate variability in each region, with particular focus on the role of the ocean. Differences between the North Atlantic and North Pacific are highlighted.
The Labrador Sea has exhibited significant temperature and salinity variations over the past five decades. The whole basin was extremely warm and salty between the mid-1960s and early 1970s, and fresh and cold between the late 1980s and mid-1990s. The full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s. The freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the Labrador Sea sustained throughout the subsequent years. It took only a decade for the whole water column to lose most of its excessive freshwater, reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s. If the recent tendencies persist, the basin’s storages of salt and heat will fairly soon, likely by 2008, exceed their historic highs.
This paper discusses a numerical closure, motivated from the ideas of Smagorinsky, for use with a biharmonic operator. The result is a highly scale-selective, state-dependent friction operator for use in eddy-permitting geophysical fluid models. This friction should prove most useful for large-scale ocean models in which there are multiple regimes of geostrophic turbulence. Examples are provided from primitive equation geopotential and isopycnal-coordinate ocean models. 1.
We review what is known about the convec-tive process in the open ocean, in which the properties of large volumes of water are changed by intermittent, deep-reaching convection, triggered by winter storms. Observational, laboratory, and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales, the large-scale circulation of the ocean, and the prevailing meteorology. Two aspects make ocean convection interesting from a theoretical point of view. First, the timescales of the convective process in the ocean are sufficiently long that it may be modified by the Earth's rotation; second, the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability. Moreover, the convec-tive and geostrophic scales are not very disparate from one another. Detailed observations of the process in the Labrador, Greenland, and Mediterranean Seas are de-scribed, which were made possible by new observing technology. When interpreted in terms of underlying dynamics and theory and the context provided by labo-ratory and numerical experiments of rotating convec-tion, great progress in our description and understand-ing of the processes at work is being made.
From 1969 to 1971 convection in the Labrador Sea shut down, thus interrupting the formation of the intermediate/ dense water masses. The shutdown has been attributed to the surface freshening induced by the Great SalinityAnomaly (GSA), a freshwater anomaly in the subpolarNorth Atlantic. The abrupt resumption of convection in 1972, in contrast, is attributed to the extreme atmospheric forcing of that winter. Here oceanic and atmospheric data collected in the Labrador Sea atOceanWeather Station Bravo and a one-dimensional mixed layer model are used to examine the causes of the shutdown and resumption of convection in detail. These results highlight the tight coupling of the ocean and atmosphere in convection regions and the need to resolve both components to correctly represent convective processes in the ocean. They are also relevant to present-day conditions given the increased ice melt in the Arctic Ocean and from the Greenland Ice Sheet. The analysis herein shows that the shutdown was initiated by theGSA-induced freshening as well as themild 1968/69 winter. After the shutdown had begun, however, the continuing lateral freshwater flux as well as two positive feedbacks [both associated with the sea surface temperature (SST) decrease due to lack of convectivemixing with warmer subsurface water] further inhibited convection. First, the SST decrease reduced the heat flux to the atmosphere by reducing the air-sea temperature gradient. Second, it further reduced the surface buoyancy loss by reducing the thermal expansion coefficient of the surfacewater. In 1972 convection resumed because of both the extreme atmospheric forcing and advection of saltier waters into the convection region.
Freshwater exiting the Arctic Ocean through the Canadian Arctic Archipelago (CAA) has been shown to affect meridional overturning circulation and thereby the global climate system. However, because of constraints of spatial resolution in most global ocean models, neither the flow of low salinity water through the CAA to the Labrador Sea nor the eddy activity that may transport freshwater from the shelf to areas of open ocean convection can be directly simulated. To address these issues, this study uses a high-resolution ice- ocean model of the pan-Arctic region with a realistic CAA and forced with realistic atmospheric data. This model resolves conditions in the Arctic Ocean upstream of the Labrador Sea and is coupled to a thermodynamic- dynamic sea ice model that responds to the atmospheric forcing. The major shelf-basin exchange of liquid freshwater occurs south of Hamilton Bank, whereas the largest ice flux occurs in the northwest of the basin. Freshwater flux anomalies entering the Labrador Sea through Davis Strait do not immediately affect deep convection. Instead, eddies acting on shorter time scales can move freshwater to locations of active convection and halt the process. Convection is modulated by the position of the ice edge, highlighting the critical need for a coupled ice-ocean model. Finally, the size of eddies and the short duration of events demonstrate the need for high resolution, both spatial and temporal.
Restratification after deep convection is one of the key factors in determining the temporal variability of dense water formation in the Labrador Sea. In the subsurface, it is primarily governed by lateral buoyancy fluxes during early spring. The roles of three different eddy types in this process are assessed using an idealized model of the Labrador Sea that simulates the restratification season. The first eddy type, warm-core Irminger rings, is shed from the boundary current along the west coast of Greenland. All along the coastline, the boundary current forms boundary current eddies. The third type, convective eddies, arises directly around the convection area. In the model, the latter two eddy types are together responsible for replenishing 30% of the winter heat loss within 6 months. Irminger rings add another 45% to this number. The authors' results thus confirm that the presence of Irminger rings is essential for a realistic amount of restratification in this area. The model results are compared to observations using theoretical estimates of restratification time scales derived for the three eddy types. The time scales are also used to explain contradicting conclusions in previous studies on their respective roles.
A Reynolds stress-based model is used to derive algebraic expressions for the vertical diffusivities K α(α = m, h, s) for momentum, heat, and salt. The diffusivities are expressed as K a(R ρ, N, Ri T, ∈) in terms of the density ratio R p = α,∂S/∂z(α 1∂T/∂z) -1, the Brunt-Väisälä frequency N 2 = -gρ -1o∂p/∂z, the Richardson number Ri T = N 2/Σ∑ 2 (∑ is the shear), and the dissipation rate of kinetic energy ∈. The model is valid both in the mixed layer (ML) and below it. Here R p and N are computed everywhere using the large-scale field from an ocean general circulation model while Ri T is contributed by resolved and unresolved shear. In the ML, the wind-generated large-scale shear dominates and can be computed within an OGCM. Below the ML, the wind is no longer felt and small-scale shear dominates. In this region, the model provides a new relation Ri T = cf(R ρ) with c ≈ l in lieu of Munk's suggestion Ri T ≈ c. Thus, below the ML, the K α become functions of R p, N, and ∈. The dissipation ∈ representing the physical processes responsible for the mixing, which are different in different parts of the ocean, must also be expressed in terms of the large-scale fields. In the ML, the main source of stirring is the wind but below the ML there is more than one possible source of stirring. For regions away from topography, one can compute ∈ using a model for internal waves. On the other hand, near topography, one must employ different expressions for ∈. In agreement with the data, the resulting diffusivities are location dependent rather than universal values. Using North Atlantic Tracer Release Experiment (NATRE) data, the authors test the new diffusivities with and without an OGCM. The measured diffusivities are well reproduced. Also, a set of global T and S profiles is computed using this model and the KPP model. The profiles are compared with Levitus data. In the North Atlantic, at 24°N, the meridional overturning is close to the measured values of 17 ∓ 4 Sv and 16 ∓ 5 Sv (Sv e5 10 6 m 3 S -1). The polar heat transport for the North Atlantic Ocean, the Indo-Pacific Ocean, and the global ocean are generally lowered by double diffusion. The freshwater budget is computed and compared with available data.
This paper is an observational study of small-scale coherent eddies in the Labrador Sea, a region of dense water formation thought to be of considerable importance to the North Atlantic overturning circulation. Numerical studies of deep convection emphasize coherent eddies as a mechanism for the lateral transport of heat, yet their small size has hindered observational progress. A large part of this paper is therefore devoted to developing new methods for identifying and describing coherent eddies in two observational platforms, current meter moorings and satellite altimetry. Details of the current and water mass structure of individual eddy events, as they are swept past by an advecting flow, can then be extracted from the mooring data. A transition is seen during mid-1997, with long-lived boundary current eddies dominating the central Labrador Sea year-round after this time, and convectively formed eddies similar to those seen in deep convection modeling studies apparent prior to this time. The TOPEX / Poseidon altimeter covers the Labrador Sea with a loose “net” of observations, through which coherent eddies can seem to appear and disappear. By concentrating on locating and describing anomalous events in individual altimeter tracks, a portrait of the spatial and temporal variability of the underlying eddy field can be constructed. The altimeter results reveal an annual “pulsation” of energy and of coherent eddies originating during the late fall at a particular location in the boundary current, pinpointing the time and place of the boundary current-type eddy formation. The interannual variability seen at the mooring is reproduced, but the mooring site is found to be within a localized region of greatly enhanced eddy activity. Notably lacking in both the annual cycle and interannual variability is a clear relationship between the eddies or eddy energy and the intensity of wintertime cooling. These eddy observations, as well as hydrographic evidence, suggest an active role for boundary current dynamics in shaping the energetics and water mass properties of the interior region.
The hydrographic structure of the Labrador Sea during wintertime convection is described. The cruise, part of the Deep Convection Experiment, took place in February-March 1997 amidst an extended period of strong forcing in an otherwise moderate winter. Because the water column was preconditioned by previous strong winters, the limited forcing was enough to cause convection to approximately 1500 m. The change in heat storage along a transbasin section, relative to an occupation done the previous October, gives an average heat loss that is consistent with calibrated National Centers for Environmental Prediction surface heat fluxes over that time period (~200 W m -2). Deep overturning was observed both seaward of the western continental slope (which was expected), as well as within the western boundary current itself-something that had not been directly observed previously. These two geographical regions, separated by roughly the 3000-m isobath, produce separate water mass products. The offshore water mass is the familiar cold/fresh/dense classical Labrador Sea Water (LSW). The boundary current water mass is a somewhat warmer, saltier, lighter vintage of classical LSW (though in the far field it would be difficult to distinguish these products). The offshore product was formed within the cyclonic recirculating gyre measured by Lavender et al. in a region that is limited to the north, most likely by an eddy flux of buoyant water from the eastern boundary current. The velocity measurements taken during the cruise provide a transport estimate of the boundary current "throughput" and offshore "recirculation." Finally, the overall trends in stratification of the observed mixed layers are described.
Time series of profiles of potential temperature, salinity, dissolved oxygen, and planetary potential vorticity at intermediate depths in the Labrador Sea, the Irminger Sea, and the Iceland Basin have been constructed by combining the hydrographic sections crossing the sub-arctic gyre of the North Atlantic Ocean from the coast of Labrador to Europe, occupied nearly annually since 1990, and historic hydrographic data from the preceding years since 1950. The temperature data of the last 60 years mainly reflect a multi-decadal variability, with a characteristic time scale of about 50 years. With the use of a highly simplified heat budget model it was shown that this long-term temperature variability in the Labrador Sea mainly reflects the long-term variation of the net heat flux to the atmosphere. However, the analysis of the data on dissolved oxygen and planetary potential vorticity show that convective ventilation events, during which successive classes of Labrador Sea Water (LSW) are formed, occurring on decadal or shorter time scales. These convective ventilation events have performed the role of vertical mixing in the heat budget model, homogenising the properties of the intermediate layers (e.g. temperature) for significant periods of time. Both the long-term and the near-decadal temperature signals at a pressure of 1500dbar are connected with successive deep LSW classes, emphasising the leading role of Labrador Sea convection in running the variability of the intermediate depth layers of the North Atlantic. These signals are advected to the neighbouring Irminger Sea and Iceland Basin. Advection time scales, estimated from the 60 year time series, are slightly shorter or of the same order as most earlier estimates, which were mainly based on the feature tracking of the spreading of the LSW94 class formed in the period 1989–1994 in the Labrador Sea.
This chapter reviews the progress in the methods for determining ocean heat transport and reassesses the direct estimation of ocean heat transports from the World Ocean Circulation Experiment (WOCE) observations. The role of the oceans in maintaining the global heat balance is evaluated largely by three methods—namely, (1) the traditional method, (2) the residual method, and (3) the direct method. The advantage of the direct method is that it deals with ocean circulation and the mechanisms of ocean heat transport, while its disadvantage has been that direct estimates could only be made at a few locations where high-quality observations are available. One of the goals of the WOCE was to remedy the principal disadvantage of the direct method, i.e. a consistent goal in designing the WOCE observational program was to make the necessary measurements so that ocean heat transport could be directly determined as a function of latitude in each ocean basin. These transports could then be used as a constraint on air-sea heat fluxes and as a rigorous test of ocean and coupled atmosphere-ocean general circulation models. Direct estimates of ocean heat transport are usually made using zonal coast-to-coast hydrographic sections.
Sea level anomalies measured by the altimeters aboard the TOPEX/Poseidon and ERS satellites for the periods 1993–2001 and 1997–2001, respectively, are used to investigate the eddy field in the subpolar North Atlantic and in the Labrador Sea. A quadratic correction of the obtained eddy kinetic energy (EKE) with respect to significant wave height is applied that led to an increased correlation between moored and altimetric EKE in the central Labrador Sea. The mean EKE field shows higher levels associated with the main currents and a strong seasonality in the Labrador Sea. The annual cycle of the EKE shows a propagation of West Greenland Current (WGC) EKE into the central Labrador Sea with a mean southward propagation speed of about 3 cm s−1, while the EKE maximum in the Labrador Current is well separated from the interior by local EKE minima. The interannual variability of the EKE in the Labrador Sea shows distinct regional differences. In the WGC region, strong early winter maxima are found during 1993 and 1997–1999. In the central Labrador Sea, maxima are found during March/April 1993–1995 and 1997. Variations in the annual cycle of the WGC EKE are observed: While there is a weak annual cycle in the WGC region during 1994–1996 with more continuous EKE generation, during 1997–2000, there is a strong seasonal cycle with maximum EKE during January and particularly low EKE during summer. The propagation of WGC EKE into the central Labrador Sea is enhanced during 1997–2000, leading to a long persistence of EKE in the central Labrador Sea. During 1993–1995 and 1997 the central Labrador Sea EKE almost instantaneously increased during March/April, followed, in the earlier years, by a relatively fast destruction of the winterly generated EKE.
Data from profiling RAFOS floats, TOPEX/Poseidon altimetry, and the alongtrack scanning radiometer (ATSR) aboard ERS-I have been used to describe the spatial and seasonal patterns of eddy variability in the Labrador Sea. Peaks in sea surface height (SSH) variability appear in two regions: off the west Greenland shelf near 61.5°N, 52°W where the 3000-m isobath separates from the shelf, and in the center of the basin at 58°N, 52°W. Both locations show seasonal ranges in SSH variability of up to 40 mm, with the Greenland site, having largest variability in January-March, leading the central site by 50 days. A sea surface temperature image from the ATSR at the Greenland site shows numerous eddies, both cyclonic and anticyclonic, being formed by injection of West Greenland Current water into the Labrador Sea interior. Data from profiling RAFOS floats launched in 1997 as part of the Labrador Sea Deep Convection Experiment are used to describe three of the West Greenland Current eddies in detail. One of the sampled eddies was anticyclonic, while the other two were cyclonic. The eddies contained various mixtures of Irminger Sea Water. Peak azimuthal velocities ranged from 22 to 42 cm s-1, and diameters from 20 to 50 km. Although the floats were at a depth of 375 m, the surface elevations derived from cyclogeostrophy agreed with those obtained from TOPEX/Poseidon. The temporal and spatial patterns in SSH variability are thought to be caused primarily by seasonal variations in the strength and stability of the West Greenland Current and, less likely, by eddy formation following deep convection in the basin interior.
In the northern high latitudes, where deep water formation occurs,
horizontal freshwater transport in the form of seawater and sea ice is a
major component of the freshwater budget. The horizontal freshwater
transport controls the surface salinity in this region, and thus has a
significant impact on the deep water formation process and the resultant
Atlantic deep circulation. In this study, we focus on the freshwater
transport through the Canadian Archipelago, and investigate how the
intensity of the Atlantic deep circulation depend on opening and closing
of the Canadian Archipelago. An ice-ocean coupled model is used; it
consists of the oceanic component COCO3 (CCSR Ocean Component Model
version 3) and the sea ice component including dynamics and
thermodynamics. The horizontal resolution is 1 degree. Restoring surface
salinity to observed data is not employed. When the Canadian Archipelago
is opened, the Atlantic deep circulation strengthens by 21 %. The deep
water formation in the northern North Atlantic is responsible for the
enhancement. The flow of a low salinity water through the Canadian
Archipelago does not directly affect the deep water formation in the
northern North Atlantic, since it flows only in the western part of the
Labrador Sea. The confinement of the flow in the western Labrador Sea is
caused by the reproduction of the cyclonic circulation there. Instead,
the surface salinity in the deep water formation region is affected by
the East Greenland Current, which flows from the Fram Strait along the
east coast of Greenland and increases its salinity by opening the
Canadian Archipelago. Consequently, the deep water formation is
activated there and the Atlantic deep circulation strengthens. Thus, it
is suggested that the Canadian Archipelago throughflow does not directly
suppress the deep water formation in the northern North Atlantic, but
indirectly activate it by the increase in salinity of the East Greenland
Current.
An important, yet poorly understood, aspect of the water mass transformation process in the ocean is the manner in which the convected fluid, once formed, is accommodated and drawn into the general circulation. Following "violent mixing" in the open ocean that creates a deep homogenous body of fluid, restratification of the surface (∼500 m) layer is observed to occur rapidly, sealing over the convection patch. Recent hydrographic casts and tomography inversions in the Gulf of Lions by Send et al., for example, show that very quickly, within a week or so of the cessation of cooling, a stratified near-surface layer develops on top of the mixed patch. This restratification occurs much more rapidly than can be accounted for by air-sea fluxes. By analytical and numerical study the authors argue that advection by geostrophic eddies spawned by the baroclinic instability of the mixed patch is likely to be a principal mechanism by which restratification occurs. A restratification timescale, τ restrat ≈ 56r/(Nh), where r is the radius of the patch of mixed water, h its depth, and N the ambient stratification, can be deduced from the magnitude of the lateral buoyancy flux associated with the geostrophic eddy field. This formula finds support from numerical results and is in broad agreement with the observations. Finally the results of the study are used to interpret recent field observations in the Labrador and Mediterranean Seas.
Objectively analyzed surface hydrographic fields and NCEP-NCAR reanalysis fluxes are used to estimate water mass transformation and formation rates in the Labrador Sea, focusing on Labrador Sea Water (LSW). The authors estimate a mean long-term transformation of between 2.1 ± 0.2 and 3.9 ± 0.3 Sv (Sv = 106 m3 s-1) over the years 1960-99 to water with densities greater than σ = 27.65 kg m-3, depending on the correction used for the latent and sensible heat fluxes. Mean long-term formation rates are found between 0.9 ± 0.2 and 1.7 ± 0.3 Sv for for σ = 27.675 - 27.725 kg m-3 and 1.2 ± 0.2 and 2.0 ± 0.3 Sv for σ > 27.725 kg m-3. There is tremendous variability associated with these formation rates with years of strong water formation (5.7-6.6 ± 0.5-0.7 or 9.5-10.8 ± 0.7-1.1 Sv) mixed with years of little or no formation in the given density ranges. The North Atlantic Oscillation (NAO) is linked (correlation coefficient of 0.45, significant at the 99% level) with the overall formation rate for σ > 27.625 kg m-3. The observed long-term increase in net precipitation over the Labrador Sea does not seem to have had any significant effect on LSW, potentially reducing LSW transformation rates by 0.1 Sv. A reduction in surface salinity leads to formation occurring at a reduced density, but with little change in the amount of water transformed.
Experiments with a suite of North Atlantic general circulation models are used to examine the sources of eddy kinetic energy (EKE) in the Labrador Sea. A high-resolution model version (1/12°) quantitatively reproduces the observed signature. A particular feature of the EKE in the Labrador Sea is its pronounced seasonal cycle, with a maximum intensity in early winter, as already found in earlier studies based on altimeter data. In contrast to a previously advanced hypothesis, the seasonally varying eddy field is not related to a forcing by high-frequency wind variations but can be explained by a seasonally modulated instability of the West Greenland Current (WGC). The main source of EKE in the Labrador Sea is an energy transfer due to Reynolds interaction work (barotropic instability) in a confined region near Cape Desolation where the WGC adjust to a change in the topographic slope: Geostrophic contours tend to converge upstream of Cape Desolation, such that the topographically guided WGC narrows as well and becomes barotropically unstable. The eddies spawned from the WGC instability area, dominating the EKE in the interior Labrador Sea, are predominantly anticyclonic with warm and saline cores in the upper kilometer of the water column, while the few cyclones originating as well from the instability area show a more depth-independent structure. Companion experiments with a 1/3° model exhibit the strength of the WGC, influenced by either changes in the wind stress or heat flux forcing, as a leading factor determining seasonal to interannual changes of EKE in the Labrador Sea.
Intense, buoyant anticyclonic eddies spawned from the west Greenland boundary current were observed with high-resolution autonomous Seaglider hydrography and satellite altimetry as they entered the Labrador Sea interior. Surveys of their internal structure establish the transport of both low-salinity water in the upper ocean and warm, saline Irminger water at depth. The observed eddies can contribute significantly to the rapid restratification of the Labrador Sea interior following wintertime deep convection. These eddies have saline cores between 200 and 1000 m, low-salinity cores above 200 m, and a velocity field that penetrates to at least 1000 m, with 0-1000-m average speeds exceeding 40 cm s-1. Their trajectory, together with earlier estimates of the gyre circulation, suggests why the observed region of deep convection is so small and does not occur where wintertime cooling by the atmosphere is most intense. The cyclostrophic surface velocity field of the anticylones from satellite altimetry matched well with in situ dynamic height baroclinic velocity calculations.
The general circulation of the Labrador Sea is studied with a dataset of 53 surface drifters drogued at 15 m and several hydrographic sections done in May 1997. Surface drifters indicate three distinct speed regimes: fast boundary currents, a slower crossover from Greenland to Labrador, and a slow, eddy-dominated flow in the basin interior. Mean Eulerian velocity maps show several recirculation cells located offshore of the main currents, in addition to the cyclonic circulation of the Labrador Sea. Above the northern slope of the basin, the surface drifters have two preferential paths: one between the 1000-m and 2000-m isobaths and the other close to the 3000-m isobath. The vertical shear estimated from CTD data supports the presence of two distinct currents around the basin. One current, more baroclinic, flows between the 1000-m and 2000-m isobaths. The other one, more barotropic, flows above the lower continental slope. The Irminger Sea Water carried by the boundary currents is altered as it travels around the basin. Profiling Autonomous Lagrangian Circulation Explorer (PALACE) floats that followed approximately the Irminger Sea Water in the Labrador Sea show signs of isopycnal mixing between the interior and the boundary current in summer-fall and convection across the path of the Irminger Sea Water in winter-spring.
Data obtained in the western Labrador Sea during March 1976 by Hudson are analysed to show that new Labrador Sea Water was being formed at this time. On the basis of hydrographic and moored current-meter data, it is hypothesized that a 200 km scale cyclonic gyre forms in winter in the western Labrador Sea and that this gyre retains the developing deep mixed layers in this general area long enough for the transformation to Labrador Sea Water to take place. Using a model, it is demonstrated that water columns found along the western boundary of the Labrador Sea can be modified by cooling, evaporation and mixing to form deep mixed layers with the properties of Labrador Sea Water. Approximately 105 km3 of new Labrador Sea Water was formed in 1976, an estimate that is consistent with earlier estimates of mean annual production rates. This water, 2.9°C, 34.84‰, is some 0.6°C cooler and 0.06‰ fresher than that defined by Lazier (1973) from his data collected in 1966. The variation of Labrador Sea Water ...
A process study is conducted on the evolution of boundary currents in a two-layer quasigeostrophic model on the f plane. These currents are composed of two strips of uniform potential vorticity (PV), one in each layer, and both hugging the coast. Coastal water separation (“detrainment”) through baroclinic instability and topographic perturbation is examined. It is shown that the key characteristics of the flow finite-amplitude destabilization can be explained with the help of a linear quantity—the critical amplitude Ac—that refers to the location of the line (often called critical layer) where the phase speed of the growing perturbation is equal to the unperturbed flow velocity. Notably, prediction on PV front breaking location is made possible. Different detrainment regimes (i.e., the way fragments of the boundary current are isolated and detached from the initially rectilinear core—e.g., filament formation, eddy shedding) are also identified, related to various Ac value ranges, and compared with observed oceanic events.
Monthly averages of water temperature, salinity, and density anomaly at 11 depths between 10 and 1500 m, at Ocean Weather Ship Bravo (56°30'N, 51°00'W), between 1964 and 1974 are presented. Near‐surface salinity values between 1967 and 1971 were significantly lower than those between 1964–1967. Coincident with the lower salinity values, the winter‐time heat losses were less than normal. The combination of increased stratification with the low heat losses tended to limit the convectively mixed upper layer in winter to unusually shallow depths. It is suggested that the low salinity condition was indirectly due to an anomalously high atmospheric pressure cell over Greenland. This cell increased the anticylonic air flow around Greenland causing an increase in the proportion of low‐salinity polar water in the east Greenland and Labrador Currents and subsequently in the interior of the Labrador Sea.
Two climatologies, one using an isopycnic approach and the other employing a more classical geopotential approach, are produced for the Labrador Sea region. These differ from existing climatologies through the use of smaller search radii, more data and a carefully chosen depth dependent correction scheme. This results in the preservation of the strong fronts that exist between cold, fresh boundary currents and warmer, more saline interior waters and, in general, less smoothing of features. The waters of the West Greenland Current, the Labrador Current and the interior are well represented, especially Labrador Sea Water and the Deep Western Boundary Current. We consider that our ‘best’ results are produced by the isopycnal climatology. Isopycnal averaging gives more realistic results by reducing artificial mixing of water properties and preserving the baroclinicity of the flow. We estimate the total transport, using the results from the isopycnal climatology in a diagnostic model driven by climatological winds. For the Labrador Current/subpolar gyre at 53°N we find a transport of 46.6 Sv southward, with 9.7 Sv of that being Labrador Sea Water, 12.1 Sv being Gibbs Fracture Zone Water and 8.0 Sv being Denmark Strait Overflow Water. Transport into the Labrador Sea is 41.2 Sv with 6.6 Sv of Labrador Sea Water exported back to the Irminger Sea. Total southward freshwater transport by the Labrador Current (including slope and ‘gyre’ branches) is 239 mSv at 53°N, with almost 60% of this carried in the upper layer. Import of fresh water to the Labrador Sea from the east in the East Greenland Current is 129 mSv, which is divided almost equally among all layers. Our estimate of the long‐term mean formation rate of Labrador Sea Water is between 3.6 and 3.8 Sv.
The variability of the surface eddy kinetic energy (EKE) in the Labrador Sea is investigated with a suite of numerical integrations using a regional ocean model. Simulations are performed over the period 1980–2001 and are compared to satellite observations over the last 9 years. The surface EKE pattern in the basin is dominated by a region along the West coast of Greenland where eddies, mainly anticyclonic, are formed by instability of the main currents flowing over the continental slope, consistent with previous idealized results. Here the interannual changes are linked to the shear of the incoming boundary current system imposed as boundary condition to the model domain. The highly variable strength of the East Greenland current at the northeast boundary, derived from the Simple Ocean Data Assimilation (SODA) reanalysis, strongly influences the vortex formation.In the center of the Labrador Sea, where deep convection occurs, a statistically significant portion of the modeled interannual surface EKE variability is correlated with the local atmospheric forcing, and both heat and wind fluxes play an important role and can be adopted as predictors at a lag of 2–3months. The Arctic Oscillation index can also be used as a remote indicator of the atmospheric fluxes, but with lower skill than local measurements. In contrast the North Atlantic Oscillation index does not correlate significantly with the surface EKE at intraseasonal and interannual scales. The analysis of altimeter data over the 1993–2001 supports the existence of this asymmetry between the regime locally forced by the atmosphere in the central basin, and the regime remotely forced by the incoming boundary current along the west Greenland coast. Those results have important implications for monitoring and predicting the surface eddy kinetic energy variability in the Labrador Sea.
The seasonal and interannual variations in the export of Labrador Sea Water (LSW), and in the heat and freshwater transport through the central Labrador Sea, are examined for two different periods: from 1964 to 1974, using Ocean Weather Station Bravo data, and from 1996 to 2000, using data collected from profiling floats. A typical seasonal cycle involves a 300-m thickening of LSW (convection) followed by an equivalent thinning (restratification). Restratification is characterized by a drift of properties toward boundary current values that is indicative of a vigorous lateral exchange. The net result is a convergence of heat and salt, between 200 and 700 m, that balances the net surface heat loss to the atmosphere and partially offsets the surface freshwater accumulation due to surface, lateral exchange. Interannual variations in the export of LSW can be explained by taking into account changes in the central Labrador Sea–boundary current density gradient, which governs the lateral exchange. Interannual variations in how much heat is converged into the region, on the other hand, mostly reflect changes in the temperature of LSW. This only partly explains, however, the increased convergence of heat that occurs during the late 1990s. In years in which convection does not occur, restratification trends continue throughout the entire year, albeit at a reduced rate.
The North Atlantic is a peculiarly convective ocean. The convective renewal of intermediate and deep waters in the Labrador Sea and Greenland/Iceland Sea both contribute significantly to the production and export of North Atlantic Deep Water, thus helping to drive the global thermohaline circulation, while the formation and spreading of 18-Degree Water at shallow-to-intermediate depths off the US eastern seaboard is a major element in the circulation and hydrographic character of the west Atlantic. For as long as time-series of adequate precision have been available to us, it has been apparent that the intensity of convection at each of these sites, and the hydrographic character of their products have been subject to major interannual change, as shown by Aagaard (1968), Clarke, Swift, Reid and Koltermann (1990), and Meincke, Jonsson and Swift (1992) for the Greenland Sea, in the OWS BRAVO record from the Labrador Sea, (egLazier, 1980 et seq.), and at the Panulirus / Hydrostation “S” site in the Northern Sargasso off Bermuda (eg Jenkins, 1982, Talley and Raymer, 1982). This paper reviews the recent history of these changes showing that the major convective centres of the Greenland and Labrador Seas are currently at opposite convective extrema in our postwar record, with vertical exchange at the former site limited to 1000 m or so, but with Labrador Sea convection reaching deeper than previously observed, to over 2300 m. As a result, the deep water of the Greenland Sea has become progressively warmer and more saline since the early '70s as a result of increased horizontal exchange with the Arctic Ocean through Fram Strait, while the Labrador Sea Water has become progressively colder and fresher over the same period through increased vertical exchange; most recently, convection has become deep enough there to reach into the more saline NADW which underlies it, so that cooler, but now saltier and denser LSW has resulted. The horizontal spreading of these changing watermasses in the northern gyre is described from the hydrographic record. The theory is advanced that the scales of atmospheric forcing have imposed a degree of synchrony on convective behaviour at all three sites over the present century, with ventilation at the Sargasso and Greenland Sea sites undergoing a parallel multi-decadal evolution to reach a long term maximum in the 1960s, driven by the twin cells of the North Atlantic Oscillation (NAO). During the NAO minimum of the 1960s, with an extreme Greenland ridge feeding record amounts of fresh water into the northern gyre in the form of the Great Salinity Anomaly, and its partner cell over the Southeast USA causing a southwestward retraction of storm activity (Dickson and Namias, 1976), the surface freshening and postwar minimum in storm activity in the intervening area of the Labrador Sea also brought a progressive reduction, and ultimately a cessation, of wintertime convection there during the 1960s. In other words, the evolution of winter convective activity during the century was in phase but of different sign at the three sites. In these events, we see strong evidence of a direct impact of the shifting atmospheric circulation on the ocean; while this certainly does not rule out either feedbacks from anomalous ice and SST conditions on the atmosphere, or autonomous oscillations of the ocean's overturning circulation, it does tend to minimise them.
Two experiments are run with an eddy-permitting model of the sub-polar North Atlantic, using full and partial cell formulations of the underlying bottom topography. Although there are numerous improvements in the circulation in the partial cell experiment, there is also a serious degrading of the hydrography, through an unexpected and unrealistic increase in the salinity within the Labrador Sea. An analysis of the freshwater and heat budgets of the Labrador Sea in the model experiments show significant changes in the partial cell experiment, with respect to the full cell experiment, associated with a net flux of freshwater out of the Labrador Sea. The main components of this increased flux is the greater import of high salinity water from the North Atlantic current and the enhanced export of Labrador Sea Water directly to the Irminger Sea. Both of these fluxes are primarily related to the presence of a strong Labrador Sea counter-current in the partial cell simulation. One striking feature associated with the generation of the counter-current is a very large amount of eddy activity along the Labrador slope. A brief energy analysis, including examining the time-rates of conversion between the different types of energies, is given for both experiments. This analysis suggests that the increased production of eddies in the partial cell experiment is at least partly due to enhanced baroclinic instability occurring in frontal regions. Additionally, a net conversion of eddy kinetic energy into mean kinetic energy that only occurs in the partial cell experiment suggests that the Labrador Sea counter-current is in part driven by these eddies.
Observations of deep ocean temperature and salinity in the Labrador and Greenland Seas indicate that there is negative correlation between the activities of deep convection in these two sites. A previous study suggests that this negative correlation is controlled by the North Atlantic Oscillation (NAO). In this study, we discuss this deep convection seesaw by using a coupled atmosphere and ocean general circulation model. In this simulation, the deep convection is realistically simulated in both the Labrador and Greenland Seas and their negative correlation is also recognized. Regression of sea level pressure to wintertime mixed layer depth in the Labrador Sea reveals strong correlation between the convection and the NAO as previous studies suggest, but a significant portion of their variability is not correlated. On the other hand, the convection in the Greenland Sea is not directly related to the NAO, and its variability is in phase with changes in the freshwater budget in the GIN Seas. The deep convection seesaw found in the model is controlled by freshwater transport through the Denmark Strait. When this transport is larger, more freshwater flows to the Labrador Sea and less to the Greenland Sea. This leads to lower upper-ocean surface salinity in the Labrador Sea and higher salinity in the Greenland Sea, which produces negative correlation between these two deep convective activities. The deep convection seesaw observed in the recent decades could be interpreted as induced by the changes in the freshwater transport through the Denmark Strait, whose role has not been discussed so far.
Model drift in the Labrador Sea in eddy permitting model simulations is examined using a series of configurations based on the NEMO numerical framework. There are two phases of the drift that we can identify, beginning with an initial rapid 3-year period, associated with the adjustment of the model from its initial conditions followed by an extended model drift/adjustment that continued for at least another decade. The drift controlled the model salinity in the Labrador Sea, over-riding the variability. Thus, during this initial period, similar behavior was observed between the inter-annually forced experiments as with perpetual year forcing. The results also did not depend on whether the configuration was global, or regional North Atlantic Ocean. The inclusion of an explicit sea-ice component did not seem to have a significant impact on the interior drift. Clear cut evidence for the drift having an advective nature was shown, based on two separate currents/flow regimes. We find, as expected, the representation of freshwater in the sub-polar gyre’s boundary currents important. But this study also points out another, equally important process and pathway: the input of high salinity mode water from the subtropical North Atlantic. The advective regime is dependent on the details of the model, such as the representation of the freshwater transport in the model’s East Greenland Current being very sensitive to the strength of the local sea surface salinity restoring (and the underlying field that the model is being restored to).