West Greenland Current Research Articles

Variability of Eddy Formation off the West Greenland Coast from a 1/60° Model

Abstract Eddies generated off the west Greenland coast modulate the deep convection in the Labrador Sea, while there are still open questions related to their formation mechanisms. Using 11 years (2008–18) of output from a NEMO model configured with a 1/60° nest in the Labrador Sea, we present the patterns of baroclinic and barotropic instability off the west Greenland coast. We highlight the generation of Irminger Rings at Cape Desolation and boundary current eddies at the location of the Overturning in the Subpolar North Atlantic Program (OSNAP) West section. In between these formation sites, eddy energy attenuation occurs along the West Greenland Current (WGC). Overall, baroclinic instability dominates in the upper 1000 m and is twice as strong as the barotropic instability. Seasonally, the instabilities are generally twice as strong in winter compared to summer. Interannually from 2008 to 2018, the instabilities generally show a strengthening trend, with values in 2018 two to three times as strong as those in 2008. We found that on an interannual time scale, the strengthening of WGC and the steepening of its velocity contours enhance the barotropic instability, and the intrusion of the upper Irminger Sea Intermediate Water (uISIW) on the Irminger Water enhances the baroclinic instability by increasing the horizontal density gradient. On a seasonal time scale, variability of the eddy momentum and density fluxes modulate the barotropic and baroclinic instability, respectively. From observation-based datasets, we also found that the downstream eddy kinetic energy is highly correlated with the uISIW transports, suggesting that the amount of uISIW affects the eddy formation. Using a very high-resolution numerical model, our study provides new insight into the variability and mechanisms of eddy formation along the west Greenland coast. Significance Statement Vigorous eddy activity exists off the west Greenland coast. The eddies flux buoyancy to the interior Labrador Sea and thus weaken the convection, which feeds the lower limb of the Atlantic meridional overturning circulation. Given uncertainties in the eddy formation mechanisms, by using an ocean model with very high resolution that resolves those eddies, we show the factors that control the production and variability of the eddy formation off the western coast of Greenland. The eddy formation generally strengthens over the years 2008–18, which is a result of the intrusion of intermediate water on the continental slope and a stronger boundary current. The eddy formation shows a seasonal cycle—it is generally the strongest in winter and weakest in summer, which is modulated by the seasonal variability of eddy momentum and density fluxes.

Distinguishing Physical and Biological Controls on the Carbon Dynamics in a High‐Arctic Outlet Strait

AbstractThe water mass assembly of Nares Strait is variable, owing to fluctuating wind forcings over the central Arctic Basin, and irregular northward flows from the West Greenland Current (WGC) in Baffin Bay. Here we characterize the physico‐chemical properties of the water masses entering Nares Strait in August 2014. We employ an extended optimum multi‐parameter (OMP) analysis to estimate the mixing fractions of predefined source water masses, and to distinguish the role of physical and biological processes in governing the distribution of dissolved inorganic carbon (DIC) in Nares Strait. We show the first documented evidence of Siberian shelf waters arriving in Nares Strait, along with a diluted upper halocline layer of partial Pacific‐origin. These mixed‐origin water masses appear to play an important role in driving a modest phytoplankton bloom in Kane Basin, leading to decreased surface pCO2 concentrations in Nares Strait. Although inorganic nitrogen was already limited near the surface in northern Nares Strait, the rather shallow upper halocline layer and the shoaling bathymetry in Kane Basin facilitated upwelling of nutrients to the surface. Our observations suggest that the positioning of the Transpolar Drift, and hence the balance of Atlantic and Pacific water delivered to Nares Strait, may play an important role in regional biological productivity and carbon uptake from the atmosphere. We also observed water masses from the WGC transported as far north as Kane Basin, contributing to relatively high pCO2 and low pH in the intermediate and deep water column of southern Nares Strait and northern Baffin Bay.

The Changing Behavior of the West Greenland Current System in a Very High‐Resolution Model

AbstractAnalyzing a high‐resolution (1/60°) numerical model over 2008–2018, the inter‐annual variability of the West Greenland Coastal Current (WGCC) on the shelf and West Greenland Current (WGC) at the shelf break is presented. Both currents flow from Cape Farewell and extend to Davis Strait, with their speeds and transports corresponding well with observations. The trend over the study period of the WGCC and WGC strength near southwest Greenland are opposite, with the former declining while the latter strengthened, both by a speed change above 0.1 m/s. Both currents are predominantly buoyancy forced, but wind forcing becomes more dominant approaching Davis Strait. The main exchanges from the two currents with the interior of the Labrador Sea occur between Cape Desolation and Fylla Bank. The net volume, freshwater, heat transport decreases between these two sections are 1.4 Sv, 13 mSv, 36.7 TW, respectively. The freshwater transport of the WGC itself does not drop in between these sections, receiving freshwater from the WGCC to compensate for the losses to the basin interior. Thus, we see significant freshwater (83.1 mSv) and heat transports (70.7 TW) of the WGC remaining at Fylla Bank that reach the northern basin instead of being fluxed into the interior of the Labrador Sea. This suggests that the exchange between the boundary current system and the interior is more limited than previously thought, and most of the Greenland and Arctic melt reaches the northern Labrador Sea. Our results highlight the importance of resolving the WGCC and shelf processes.

On the seesaw in interannual variability of upper ocean heat advection between the North Atlantic Subpolar Gyre and the Nordic Seas

In this study we investigate the interannual variability of the heat content of the upper North Atlantic and the main factors, which influence the observed variability: the ocean-atmosphere heat exchange and the ocean heat transport. The data from the combined in situ and satellite dataset ARMOR-3D, and from the ocean reanalyzes ORAS5 and SODA3 (two versions) show a similar decadal interannual variability of the heat content, as well as of oceanic heat transports the study regions, though the mean values may differ. The observed variations are linked to the North Atlantic Oscillation Index (NAOI). The current velocity of the North Atlantic, East Greenland and Labrador currents of the Subpolar Gyre increases with the NAOI, but the velocity of the Irminger and West Greenland currents decreases. This forms a seesaw of heat advection by the North Atlantic Current between the Subpolar Gyre and Nordic Seas. In the Subpolar gyre during the periods of high NAOI, this negative anomaly of the oceanic heat convergence adds to the intensified sea-surface heat release to the atmosphere, together effectively reducing the upper ocean heat content. The upper ocean heat content of the Norwegian Sea shows practically no link to the NAOI, in spite of a somewhat larger oceanic heat flux across its southern boundary linked to high NAOI.

Seasonal Cycle of the Coastal West Greenland Current System Between Cape Farewell and Cape Desolation From a Very High‐Resolution Numerical Model

AbstractThe shelf based West Greenland Current System carries both Arctic Water and Greenland meltwater around Cape Farewell, the southern tip of Greenland, where it may impact the deep water formation in the Labrador Sea. Nevertheless, this current system remains largely unknown due to limited observations. Using a very high‐resolution (1°/60°) numerical simulation integrated from 2005 to 2014, the annual cycle of the current structure in the vicinity of southern Greenland is analyzed to present the seasonal variability of the West Greenland Current System. A distinct shelf break West Greenland Coastal Current (WGCC) exists for about half the year between Cape Farewell and Cape Desolation, although it merges with the main jet of the West Greenland Current (WGC) in winter and separates in summer. The WGC has three components at Juliannehaab Bight: A shelf component, a shelf‐break component and a main slope jet. The shelf system is most energetic in fall, when the freshwater transport is also the greatest in a range of 25–40 mSv, with a broad transport minimum in spring in a range of 15–25 mSv. The seasonal cycle is strongest near Cape Farewell and weakest at Cape Desolation. The strength of the shelf system decreases as one moves north as water is lost offshore, with strongest transports of 0.61 Sv to the east of Cape Farewell and weakest of 0.41 Sv at Cape Desolation. This decrease in transport between Cape Farewell and Cape Desolation suggests substantial exchange between WGCC and WGC in this region.

Surface and subsurface Labrador Shelf water mass conditions during the last 6000 years

Abstract. The Labrador Sea is important for the modern global thermohaline circulation system through the formation of intermediate Labrador Sea Water (LSW) that has been hypothesized to stabilize the modern mode of North Atlantic deep-water circulation. The rate of LSW formation is controlled by the amount of winter heat loss to the atmosphere, the expanse of freshwater in the convection region and the inflow of saline waters from the Atlantic. The Labrador Sea, today, receives freshwater through the East and West Greenland currents (EGC, WGC) and the Labrador Current (LC). Several studies have suggested the WGC to be the main supplier of freshwater to the Labrador Sea, but the role of the southward flowing LC in Labrador Sea convection is still debated. At the same time, many paleoceanographic reconstructions from the Labrador Shelf focussed on late deglacial to early Holocene meltwater run-off from the Laurentide Ice Sheet (LIS), whereas little information exists about LC variability since the final melting of the LIS about 7000 years ago. In order to enable better assessment of the role of the LC in deep-water formation and its importance for Holocene climate variability in Atlantic Canada, this study presents high-resolution middle to late Holocene records of sea surface and bottom water temperatures, freshening, and sea ice cover on the Labrador Shelf during the last 6000 years. Our records reveal that the LC underwent three major oceanographic phases from the mid- to late Holocene. From 6.2 to 5.6 ka, the LC experienced a cold episode that was followed by warmer conditions between 5.6 and 2.1 ka, possibly associated with the late Holocene thermal maximum. While surface waters on the Labrador Shelf cooled gradually after 3 ka in response to the neoglaciation, Labrador Shelf subsurface or bottom waters show a shift to warmer temperatures after 2.1 ka. Although such an inverse stratification by cooling of surface and warming of subsurface waters on the Labrador Shelf would suggest a diminished convection during the last 2 millennia compared to the mid-Holocene, it remains difficult to assess whether hydrographic conditions in the LC have had a significant impact on Labrador Sea deep-water formation.

A high-resolution elemental record of post-glacial lithic sedimentation in Upernavik Trough, western Greenland: History of ice-sheet dynamics and ocean circulation changes over the last 9100 years

A better understanding of the past dynamics of local sectors of the Greenland ice sheet (GIS) with regards to ocean circulation and climate changes can be developed from proxy records derived from marine sedimentary archives. Here we investigate the post 9.1 cal. Kyr BP history of the western sector of the GIS from the XRF core scanner-derived geochemistry of a sediment core retrieved from Upernavik Trough, Melville Bay. The elemental signature of material derived from Melville Bay glaciers, Upernavik Isstrom and Rink Isbrae can be inferred from the geology of the bedrocks drained by these major ice streams of the western GIS. Changes in abundance of Fe-rich basaltic material reflect changes in West Greenland Current (WGC) strength. Contributions from Melville Bay glaciers were dominant during the early part of the record as a result of the rapid retreat of this stretch of the NW GIS. Atmospheric warming and strengthened WGC between 7.5 and 5 cal. Kyrs BP promoted contributions from distal southern sources of lithic material to the sedimentation over Upernavik Trough. The Neoglacial shift in climate and ocean circulation led to a general reduction in the delivery of lithic material from western GIS glacier outlets glaciers. The particular geomorphology of the fjord hosting Rink Isbrae as well as the important depth of the Uummannaq Trough likely explain the high relative contribution of this glacier to the lithic sedimentation over Upernavik Trough during the later part of the Holocene despite a weakened WGC. • Sediment delivery from tidewater glaciers of W Greenland over the last 9100 years • XRF core-scanner records from a marine sediment core in Upernavik trough. • Response of various sectors of the W GIS to ocean circulation and climate changes

Holocene variability in sea ice and primary productivity in the northeastern Baffin Bay

Arctic sea ice is a critical component of the climate system, known to influence ocean circulation, earth’s albedo, and ocean–atmosphere heat and gas exchange. Current developments in the use of IP25 (a sea ice proxy with 25 carbon atoms only synthesized by Arctic sea ice diatoms) have proven it to be a suitable proxy for paleo-sea ice reconstructions over hundreds of thousands to even millions of years. In the NE Baffin Bay, off NW Greenland, Melville Bugt is a climate-sensitive region characterized by strong seasonal sea ice variability and strong melt-water discharge from the Greenland Ice Sheet (GIS). Here, we present a centennial-scale resolution Holocene sea ice record, based on IP25 and open-water phytoplankton biomarkers (brassicasterol, dinosterol and HBI III) using core GeoB19927-3 (73° 35.26′ N, 58° 05.66′ W). Seasonal to ice-edge conditions near the core site are documented for most of the Holocene period with some significant variability. In the lower-most part, a cold interval characterized by extensive sea ice cover and very low local productivity is succeeded by an interval (~ 9.4–8.5 ka BP) with reduced sea ice cover, enhanced GIS spring melting, and strong influence of the West Greenland Current (WGC). From ~ 8.5 until ~ 7.8 ka BP, a cooling event is recorded by ice algae and phytoplankton biomarkers. They indicate an extended sea ice cover, possibly related to the opening of Nares Strait, which may have led to an increased influx of Polar Water into NE-Baffin Bay. The interval between ~ 7.8 and ~ 3.0 ka BP is characterized by generally reduced sea ice cover with millennial-scale variability of the (late winter/early spring) ice-edge limit, increased open-water conditions (polynya type), and a dominant WGC carrying warm waters at least as far as the Melville Bugt area. During the last ~ 3.0 ka BP, our biomarker records do not reflect the late Holocene ‘Neoglacial cooling’ observed elsewhere in the Northern Hemisphere, possibly due to the persistent influence of the WGC and interactions with the adjacent fjords. Peaks in HBI III at about ~ 2.1 and ~ 1.3 ka BP, interpreted as persistent ice-edge situations, might correlate with the Roman Warm Period (RWP) and Medieval Climate Anomaly (MCA), respectively, in-phase with the North Atlantic Oscillation (NAO) mode. When integrated with marine and terrestrial records from other circum-Baffin Bay areas (Disko Bay, the Canadian Arctic, the Labrador Sea), the Melville Bugt biomarker records point to close ties with high Arctic and Northern Hemispheric climate conditions, driven by solar and oceanic circulation forcings.

The Nature of Eddy Kinetic Energy in the Labrador Sea: Different Types of Mesoscale Eddies, Their Temporal Variability, and Impact on Deep Convection

AbstractOceanic eddies are an important component in preconditioning the central Labrador Sea (LS) for deep convection and in restratifying the convected water. This study investigates the different sources and impacts of eddy kinetic energy (EKE) and its temporal variability in the LS with the help of a 52-yr-long hindcast simulation of a 1/20° ocean model. Irminger Rings (IR) are generated in the West Greenland Current (WGC) between 60° and 62°N, mainly affect preconditioning, and limit the northward extent of the convection area. The IR exhibit a seasonal cycle and decadal variations linked to the WGC strength, varying with the circulation of the subpolar gyre. The mean and temporal variations of IR generation can be attributed to changes in deep ocean baroclinic and upper-ocean barotropic instabilities at comparable magnitudes. The main source of EKE and restratification in the central LS are convective eddies (CE). They are generated by baroclinic instabilities near the bottom of the mixed layer during and after convection. The CE have a middepth core and reflect the hydrographic properties of the convected water mass with a distinct minimum in potential vorticity. Their seasonal to decadal variability is tightly connected to the local atmospheric forcing and the associated air–sea heat fluxes. A third class of eddies in the LS are the boundary current eddies shed from the Labrador Current (LC). Since they are mostly confined to the vicinity of the LC, these eddies appear to exert only minor influence on preconditioning and restratification.

Current Intensity Trends in the Labrador and Irminger Seas Based on Satellite Altimetry Data

Sea level measurements from an absolute dynamic typography dataset for the period of 1993–2015 were used to study the variability in geostrophic circulation in the Labrador and Irminger seas. Analysis of current intensity trends calculated by geostrophic formulas has shown a weakening in cyclonic circulation observed in these two convection centers. The intensity trends are negative in all the main currents making up the Subpolar Gyre: the Labrador, East Greenland, West Greenland, and Irminger currents. On average, the negative intensity trends in the indicated currents are –0.3 cm s–1 yr–1, in individual areas reaching –0.5 cm s–1 yr–1, which corresponds to a decrease in current speed of 6.9–11.5 cm/s for the 23-year interval. Weakening of cyclonic circulation is likely predetermined by weakening convection processes and a decrease in the dome-shaped curvature of isopycnal surfaces generated by upwelling of weakly stratified water closer to the surface. This is confirmed by analysis of satellite data: in periods when deep convection weakens, the intensity of cyclonic circulation decreases at the margin of the Labrador and Irminger seas. Simultaneously with weakening currents, an increase in intensity is observed from the outer (with respect to the convection centers) margin of the considered currents, including in coastal zones.

Mechanism for the Formation of Temperature Anomalies in the Upper Layer of the North Atlantic

ORA-S3 oceanological reanalysis data for 1959–2011 is applied to analyze the role different factors play in forming advective heat transfer anomalies on an interannual–decadal scale in the upper mixed layer of the North Atlantic. Regions are revealed in which horizontal heat advection anomalies are determined by variations in current intensity, temperature gradients, and their joint influence. It is demonstrated that the contribution of different mechanisms responsible for advective heat transfer anomalies in the upper mixed layer to the total anomalies of advective origin varies fundamentally from one current to another in the North Atlantic. In the Gulf Stream area (after it separates from the continental slope), horizontal heat advection anomalies in the upper mixed layer result mainly from fluctuations in current intensity, while in the Caribbean Current and the Gulf Stream area (until its separation), they result from variations in the horizontal temperature gradients in the upper mixed layer. In the Labrador Current, both of these mechanisms have the same sign and approximately the same absolute values. In the East Greenland Current, they compensate each other. The contribution of anomalies in horizontal temperature gradients transferred by anomalous currents to the formation of heat transfer anomalies in the upper layer of the North Atlantic are, on the whole, relatively small throughout the water area. The areas of the North Atlantic and West Greenland currents are exceptions.

Variability of the Labrador Sea Surface Eddy Kinetic Energy Observed by Altimeter From 1993 to 2012

AbstractA merged along track altimeter data set is used to study the variability of eddy kinetic energy (EKE) in the Labrador Sea from 1993 to 2012. The EKE near the west Greenland current (WGC) has strong interannual variability without long‐term trend from 1993 to 2012. The propagation direction of the Irminger Rings (IRs) originating from the WGC can be inferred from the EKE derived from altimeter, and the southward propagation of the IRs varies interannually. The central Labrador Sea EKE increases significantly from 1993 to 2012. The central Labrador Sea temperature difference between the end and the beginning of the winter convections is defined as restratification index to measure the restratification strengths. The relation between the central Labrador Sea EKE and the restratification index shows that the enhanced eddy activity originating from the west of the central Labrador Sea may cool the central Labrador Sea significantly. The interannual variability of the WGC EKE is likely to be driven by the large scale Subpolar Gyre (SPG) circulation variability and the North Atlantic Oscillation (NAO). The NAO also affects the central Labrador Sea EKE through its fingerprint in the local wind stress and surface heat flux. The NAO affects the WGC EKE by changing the SPG circulation strength, which will subsequently affect the WGC EKE through unknown physical processes.

Spring phytoplankton communities of the Labrador Sea (2005–2014): pigment signatures, photophysiology and elemental ratios

Abstract. The Labrador Sea is an ideal region to study the biogeographical, physiological, and biogeochemical implications of phytoplankton community composition due to sharp transitions between distinct water masses across its shelves and central basin. We have investigated the multi-year (2005–2014) distributions of late spring and early summer (May to June) phytoplankton communities in the various hydrographic settings of the Labrador Sea. Our analysis is based on pigment markers (using CHEMTAX analysis), and photophysiological and biogeochemical characteristics associated with each phytoplankton community. Diatoms were the most abundant group, blooming first in shallow mixed layers of haline-stratified Arctic shelf waters. Along with diatoms, chlorophytes co-dominated at the western end of the section (particularly in the polar waters of the Labrador Current (LC)), whilst Phaeocystis co-dominated in the east (modified polar waters of the West Greenland Current (WGC)). Pre-bloom conditions occurred in deeper mixed layers of the central Labrador Sea in May, where a mixed assemblage of flagellates (dinoflagellates, prasinophytes, prymnesiophytes, particularly coccolithophores, and chrysophytes/pelagophytes) occurred in low-chlorophyll areas, succeeding to blooms of diatoms and dinoflagellates in thermally stratified Atlantic waters in June. Light-saturated photosynthetic rates and saturation irradiance levels were highest at stations where diatoms were the dominant phytoplankton group ( > 70 % of total chlorophyll a), as opposed to stations where flagellates were more abundant (from 40 up to 70 % of total chlorophyll a). Phytoplankton communities from the WGC (Phaeocystis and diatoms) had lower light-limited photosynthetic rates, with little evidence of photoinhibition, indicating greater tolerance to a high light environment. By contrast, communities from the central Labrador Sea (dinoflagellates and diatoms), which bloomed later in the season (June), appeared to be more sensitive to high light levels. Ratios of accessory pigments (AP) to total chlorophyll a (TChl a) varied according to phytoplankton community composition, with polar phytoplankton (cold-water related) having lower AP : TChl a. Polar waters (LC and WGC) also had higher and more variable particulate organic carbon (POC) to particulate organic nitrogen (PON) ratios, suggesting the influence of detritus from freshwater input, derived from riverine, glacial, and/or sea ice meltwater. Long-term observational shifts in phytoplankton communities were not assessed in this study due to the short temporal frame (May to June) of the data. Nevertheless, these results add to our current understanding of phytoplankton group distribution, as well as an evaluation of the biogeochemical role (in terms of C : N ratios) of spring phytoplankton communities in the Labrador Sea, which will assist our understanding of potential long-term responses of phytoplankton communities in high-latitude oceans to a changing climate.

Ice stream retreat following the LGM and onset of the west Greenland current in Uummannaq Trough, west Greenland

The deglacial history and oceanography of Uummannaq Trough, central West Greenland continental shelf, was investigated using foraminiferal, sedimentological, and bathymetric records together with a radiocarbon chronology, providing a timeline for the retreat of glacial ice after the Last Glacial Maximum (LGM). To map ice stream retreat, data were collected from cores from the outer (JR175-VC45 and JR175-VC43) and inner (JR175-VC42) Uummannaq Trough. A large ice stream, fed by confluent glaciers draining the interior of the Greenland Ice Sheet, extended across the outer shelf during the LGM and was in retreat by 15.0 cal kyr BP. Foraminiferal data indicate that the ‘warm’ West Greenland Current (WGC) was established prior to 14.0 cal kyr BP, which is the hitherto earliest record of Atlantic Water found on the West Greenland shelf. For each of the cores, foraminifera indicate that ice sheet retreat was followed quickly by incursion of the WGC, suggesting that the warm water may have enhanced ice retreat. Prior to the Younger Dryas cold event, the radiocarbon chronology indicates that the ice sheet retreated to the mid-shelf, where it subsequently stabilised and formed a large grounding-zone wedge (GZW). After the Younger Dryas, around 11.5 cal kyr BP, the ice retreated rapidly from the GZW and into the fjords.

Surface and sub-surface multi-proxy reconstruction of middle to late Holocene palaeoceanographic changes in Disko Bugt, West Greenland

We present new surface water proxy records of meltwater production (alkenone derived), relative sea surface temperature (diatom, alkenones) and sea ice (diatoms) changes from the Disko Bugt area off central West Greenland. We combine these new surface water reconstructions with published proxy records (benthic foraminifera - bottom water proxy; dinocyst assemblages – surface water proxy), along with atmospheric temperature from Greenland ice core and Greenland lake records. This multi-proxy approach allows us to reconstruct centennial scale middle to late Holocene palaeoenvironmental evolution of Disko Bugt and the Western Greenland coastal region with more detail than previously available.Combining surface and bottom water proxies identifies the coupling between ocean circulation (West Greenland Current conditions), the atmosphere and the Greenland Ice Sheet. Centennial to millennial scale changes in the wider North Atlantic region were accompanied by variations in the West Greenland Current (WGC). During periods of relatively warm WGC, increased surface air temperature over western Greenland led to ice sheet retreat and significant meltwater flux. In contrast, during periods of cold WGC, atmospheric cooling resulted in glacier advances.We also identify potential linkages between the palaeoceanography of the Disko Bugt region and key changes in the history of human occupation. Cooler oceanographic conditions at 3.5 ka BP support the view that the Saqqaq culture left Disko Bugt due to deteriorating climatic conditions. The cause of the disappearance of the Dorset culture is unclear, but the new data presented here indicate that it may be linked to a significant increase in meltwater flux, which caused cold and unstable coastal conditions at ca. 2 ka BP. The subsequent settlement of the Norse occurred at the same time as climatic amelioration during the Medieval Climate Anomaly and their disappearance may be related to harsher conditions at the beginning of the Little Ice Age.

Variable influx of West Greenland Current water into the Labrador Current through the last 7200 years: a multiproxy record from Trinity Bay (NE Newfoundland)

A multiproxy study of marine sediment gravity core AI07-06G from Trinity Bay, Newfoundland, recorded changes in the strength of the Labrador Current (LC) during the Holocene. From ca. 7.2–5.7 cal kyr BP, Trinity Bay’s seafloor was influenced by cooled Atlantic water derived from the West Greenland Current (WGC) Davis Strait branch, merging into the relatively cold LC. This Atlantic water influence gradually decreased after ca. 5.7 cal kyr BP, reaching a minimum at ca. 4.9 cal kyr BP. In contrast, surface temperatures were relatively low due to cold surface water dominated by sea ice and meltwater carried south by the LC. Icebergs from outlet glaciers around Baffin Bay were abundant in the LC prior to ca. 5.5 cal kyr BP. From ca. 4.9–2.9 cal kyr BP, bottom waters became slightly colder and salinity decreased, as increased mixing of the water column brought less saline surface waters towards the seafloor. This may be explained by a weaker North Atlantic subpolar gyre, transporting less Atlantic water from the WGC to the (outer) LC. Arctic meltwater transport was reduced as glacial melting decreased at the end of the Holocene Thermal Optimum. At ca. 2.9 cal kyr BP, bottom waters returned to colder, more stable conditions, indicating a slight decrease in bottom-water ventilation. After ca. 2.1 cal kyr BP, surface water temperatures dropped and sea ice flux increased. The seafloor of Trinity Bay saw warmer conditions, consistent with a stronger subpolar gyre and increased influx of Atlantic-sourced water.

Diachronous evolution of sea surface conditions in the Labrador Sea and Baffin Bay since the last deglaciation

Assessing changes in sea surface conditions due to the effects of past freshwater outflow through Baffin Bay and Davis Strait to the Labrador Sea, hereafter referred to as the Baffin Bay corridor, is relevant in understanding the variability in Labrador Sea Water (LSW) formation. Here, regional changes in oceanographic circulation and sea surface conditions are reconstructed based on organic-walled dinoflagellate cyst (dinocyst) assemblages from four cores collected from deep, central sites of the Baffin Bay corridor. All cores exhibit a major shift in dinocyst assemblages since the late glacial period. This shift consists of a change from a polar–subpolar heterotrophic species assemblage tolerating cold and near permanent ice-covered conditions, to assemblages characterized by a higher diversity and the occurrence of phototrophic taxa associated with mild conditions. Sea surface reconstructions from the modern analogue technique display a shift from harsh, quasi-perennial ice cover to warmer summer sea surface temperatures and a seasonal sea ice. South of the Davis Strait sill, this regime shift occurred at ca. 11.9 cal ka BP due to the influence of North Atlantic waters. Baffin Bay, however, remained densely sea ice covered until about 7.4 cal ka BP, when these warmer waters penetrated into Baffin Bay and mixed with the West Greenland Current (WGC). This mixing was facilitated by the retreat of the Greenland and Laurentide Ice Sheet (LIS) margins. A major change in Labrador Sea surface conditions occurred nearly at about the same time (~7.6 cal ka BP) when the strong stratification of surface waters weakened because of the reduction in meltwater supplies from the LIS that allowed winter convection and the inception of LSW formation. All these new records demonstrate large amplitude fluctuations in sea surface conditions tightly controlled by the relative strengths and shifts of the warmer WGC and colder Baffin Island Current.

Lateral Heat Exchange after the Labrador Sea Deep Convection in 2008

Abstract The mechanisms through which convected water restratifies in the Labrador Sea are still under debate. The Labrador Sea restratification after deep convection in the 2007/08 winter is studied with an eddy-resolving numerical model. The modeled mixed layer depth during wintertime resembles the Argo observed mixed layer very well, and the lateral heat flux during the subsequent restratification is in line with observations. The Irminger rings (IRs) are reproduced with fresher caps above the 300-m depths, and they are identified and tracked automatically. The model underestimates both the number of IRs in the convection area and the heat they carry. The underestimation is most likely caused by the errors in the direction of the west Greenland currents in the model, which causes more IRs propagating westward, and only the IRs originating south of 61.5°N are able to propagate southward, yet with speed much slower than observed speed. The model still observed three eddies propagating into the convection area during the restratification phase in 2008, and their thermal contribution ranges from 1% to 4% if the estimation is made at the time when they enter the convection area. If all newly generated eddies are considered, then the ensemble-mean contributions by the IRs become 5.3%. The more detailed and direct heat flux by IRs is difficult to derive because of the strong fluctuation of the identified eddy radius. Nevertheless, the modeled lateral heat flux is largely composed of the boundary current eddies and convective eddies, thus it is possible for the model to maintain an acceptable thermal balance.

Model simulated volume fluxes through the Canadian Arctic Archipelago and Davis Strait: Linking monthly variations to forcing in different seasons

The solution of a 10 year simulation of the Arctic Ocean, produced using a 6 km resolution coupled ocean and sea-ice model, is analyzed to understand the variability, control, and forcing mechanisms of the volume fluxes through the Canadian Arctic Archipelago (CAA) and Davis Strait (DS). The analysis focuses on variability at monthly time scales. Analysis confirms the “control” of volume fluxes through the CAA, proposed in previous studies, by (1) variations of sea surface height (SSH) in the “upstream” regions and the relationship of this control to alongshore wind in the Beaufort Sea and (2) by SSH in the “downstream” region in Baffin Bay that may be related to wind stress in Baffin Bay and the northern Labrador Sea. The effectiveness of these control and forcing mechanisms vary for fluxes through different sections and for different seasons. Variation of the southward flux through DS is directly influenced by fluxes through Nares Strait (NS) and Barrow Strait (BS) in summer, fall, and winter. In spring, variations of the southward and northward fluxes through DS are closely related to each other and correspond to changes in the SSH along pathways of the Irminger Current, and the East and West Greenland Currents.

Establishment of modern circulation pattern at c. 6000 cal a BP in Disko Bugt, central West Greenland: opening of the Vaigat Strait

ABSTRACTVariations in the Holocene circulation of the West Greenland Current (WGC) in the Disko Bugt region have been reconstructed from a suite of sediment cores. Palaeoceanographic proxies include magnetic susceptibility (MS) and X‐ray fluorescence titanium counts, which document a major shift in circulation at c. 6000 cal a BP. Before this date, sediments in southern Disko Bugt were characterized by high terrigeneous and basaltic input, suggesting widespread influence of meltwater plumes. Our data show that the WGC re‐circulated in the southern Disko Bugt area because a potential northern pathway, the narrow Vaigat Strait, was blocked by icebergs that calved from marine outlet glaciers in eastern Disko Bugt. Sediments in southern Disko Bugt deposited after c. 6000 cal a BP have significantly lower terrestrial and basaltic sediment input, which coincides with minimum Holocene ice sheet extent. The reduced meltwater and iceberg discharge to the embayment caused the Vaigat Strait to become free of blocking icebergs and terrigenous input was partly diverted to the outer shelf. Thus, the modern circulation pattern of the WGC was established in the Disko Bugt region through the opening of the Vaigat Strait c. 6000 cal a BP.