Denmark Strait Research Articles

Holocene environmental evolution of the SE Greenland Shelf North and South of the Denmark Strait: Irminger and East Greenland current interactions

Holocene climatic and paleoceanographic development of the SE Greenland Shelf is studied from cores MD99-2317 and MD99-2322, at sites north and south of the Denmark Strait, respectively. Lithofacies, IRD counts, calcium carbonate percentages, benthic and planktic foraminiferal assemblages and oxygen isotope analyses, and summer SSTs reveal significant climate variations in the Holocene driven by declining solar insolation and its interaction with waning continental ice sheets, and changing atmospheric pressure patterns. Large changes in the East Greenland and Irminger Currents and the Greenland Ice Sheet are manifested as a 4-part division of the Holocene. An early Holocene cold interval dominated by melting of the Greenland Ice Sheet and Polar Front retreat extends from 11.8 to 9.5 cal kyr BP. A cold interval from 9.5 to 8.1 cal kyr BP involved episodic cooling of the Irminger Current resulting from the last phases of Laurentide Ice Sheet deglaciation and delayed the Holocene optimum off East Greenland by 3 kyr relative to peak summer solar insolation, which likely helped to limit the early Holocene melting of the Greenland Ice Sheet. The period 8.1–3.5 cal kyr BP represents a climatic optimum interval of maximum Greenland Ice Sheet retreat and strong Irminger Current inflow to the Denmark Strait. Between 6.8 and 3.5 cal kyr BP, the Irminger Current penetrated further North into the Nordic Seas than has been observed in recent decades. This signal is consistent with diminished northerly winds, a weaker Greenland High and contracted subpolar gyre. By 5 cal kyr BP, periods of increased Polar Water and decreasing salinity in the Irminger Current suggest a transition toward expansion of the subpolar gyre and increased Polar Water in the EGC. The Neoglacial interval from 3.5 to 0.2 cal kyr BP was cold and variable with increased freshwater forcing from the Arctic Ocean, advance of the Greenland Ice Sheet and southward advance of the Polar Front. Enhanced northerly winds and a strengthened Greenland High are consistent with thicker and more extensive Polar Water and greatly diminished northward advection of Irminger Current in the Denmark Strait.

Transport of the North Atlantic Deep Western Boundary Current about 39°N, 70°W: 2004–2008

Begun in spring 2004, a sustained measurement program – Line W – is returning high-resolution observations of the North Atlantic's Deep Western Boundary Current (DWBC) southeast of New England. The study focuses on the cold limb of the Atlantic Meridional Overturning Circulation near the boundary between the subpolar and subtropical gyres. The field study consists of a 6-element, continental-slope-spanning moored array on a line underlying an altimeter satellite ground track, and periodic reoccupations of a full-depth hydrographic section along the line extending from the continental shelf towards Bermuda. Here, data from the first 4 years of the array (May 10, 2004–April 9, 2008) are analyzed along with 9 realizations of the section. The array, a mix of Moored Profiler and discrete, fixed-depth instrument moorings, returned temperature, salinity and horizontal velocity data with various temporal and depth resolutions. After averaging to filter inertial, tidal and other high-frequency motions, the combined moored data set was binned to the lowest common temporal resolution of 5-days (the nominal burst sample interval of the Moored Profilers) and interpolated to 2-dbar vertical resolution. Temperature, salinity, dissolved oxygen, tracer chemical concentrations and direct velocity data were acquired on the hydrographic cruises. The present work focuses on the 4-year-mean and time-varying meridional transport in 4 layers bounded by neutral density surfaces: Upper and Classical Labrador Sea Waters, Iceland-Scotland Overflow Waters and Denmark Strait Overflow Waters. The 5-d, 4-layer-summed meridional transport estimates range between −3.5 and −79.9 Sv with a record mean average transport of −25.1 Sv and standard deviation of 12.5 Sv. Bias adjustment to account for the finite width of the moored array increases the 4-layer mean transport estimate to −28.7 Sv. At time scales longer than about 1 month, the variations in equatorward DWBC transport appear correlated with meridional position of the Gulf Stream North Wall with stronger transport observed when the Stream is displaced south.

Interannual variability of Arctic sea ice export into the East Greenland Current

Observations since the 1950s suggest that the Arctic climate system is changing in response to rising global air temperatures. These changes include an intensified hydrological cycle, Arctic sea ice decline, and increasing Greenland glacial melt. Here we use new δ18O data from the East Greenland Current system at Cape Farewell and Denmark Strait to determine the relative proportions of the freshwater components within the East Greenland Current and East Greenland Coastal Current. Through the comparison of these new data with historical studies, we gain insight into the changing Arctic freshwater balance. We detect three key shifts in the net freshwater component δ18O values, these are (1) a shift to lighter values in the late 1990s that possibly indicates an increased Greenland glacial melt or a reduced sea ice melt admixture and (2) a short‐term shift to a ∼10‰ heavier value in 2005 followed by (3) a shift back to the historic average value in 2008. The latter fluctuation reflects a short‐term dramatic rise and fall of sea ice meltwater addition into the East Greenland Current system. We infer that this anomalously large inclusion of sea ice meltwater resulted from a short‐term peak in Arctic sea ice export via Fram Strait. Our findings, therefore, suggest that the freshwater carried in the East Greenland Current system is susceptible to short‐term, high‐amplitude changes in the upstream freshwater balance, which may have important ramifications for the global thermohaline circulation through the suppression of deep water formation in the North Atlantic.

Foraminifer isotope study of the Pleistocene Labrador Sea, northwest North Atlantic (IODP Sites 1302/03 and 1305), with emphasis on paleoceanographical differences between its “inner” and “outer” basins

Cores raised during IODP Expedition 303 off southern Greenland (Eirik Ridge site 1305) and off the Labrador Coast (Orphan Knoll site 1302/1303) were analyzed to establish an isotope stratigraphy, respectively for the “inner” and “outer” basins of the Labrador Sea (LS). These isotopic data also provide information on the Atlantic Meridional Overturning Circulation (AMOC), notably with regard to the intensity of the Western Boundary Under Current (WBUC), which is tightly controlled by the production of Denmark Strait Overflow Water (DSOW), and the production of Labrador Sea Water (LSW) in the inner basin through winter cooling and convection. The upper 184m of sediment at Eirik Ridge spans marine isotope stages (MIS) 32 to 1. At this site, two distinct regimes are observed: prior to MIS 20, the isotopic record resembles that of the open North Atlantic records of the interval, whereas a more site-specific pattern is observed afterwards. This later pattern was characterized by i) high DSOW production rates and strong WBUC during interglacial stages, as indicated by sedimentation rates, ii) large amplitude δ18O-shifts from glacial stages to interglacial stages (>2.5‰) and iii) an overall range of δ18O-values significantly more positive than before. At Orphan Knoll, the 105m record spans approximately 800ka and provides direct information on linkages between the northeastern sector of the Laurentide Ice Sheet and the North Atlantic. At this site, a shift towards larger amplitude glacial/interglacial ranges of δ18O-values occurred after MIS 13, although isotopic records bear a typical North Atlantic signature, particularly during MIS 5, in contradiction to those of Eirik Ridge, where substages 5a to 5c are barely recognized. Closer examination of δ18O-records in planktic and benthic foraminifera demonstrates the presence of distinct deep-water masses in the inner vs. outer LS basins during MIS 11 and more particularly MIS 5e. Data confirm that the modern AMOC, with LSW formation, seems mostly exclusive to the present interglacial, and also suggest some specificity of each interglacial with respect to the production rate of DSOW and the AMOC, in general.

Climate impacts of parameterized Nordic Sea overflows

A new overflow parameterization (OFP) of density‐driven flows through ocean ridges via narrow, unresolved channels has been developed and implemented in the ocean component of the Community Climate System Model version 4. It represents exchanges from the Nordic Seas and the Antarctic shelves, associated entrainment, and subsequent injection of overflow product waters into the abyssal basins. We investigate the effects of the parameterized Denmark Strait (DS) and Faroe Bank Channel (FBC) overflows on the ocean circulation, showing their impacts on the Atlantic Meridional Overturning Circulation and the North Atlantic climate. The OFP is based on the Marginal Sea Boundary Condition scheme of Price and Yang (1998), but there are significant differences that are described in detail. Two uncoupled (ocean‐only) and two fully coupled simulations are analyzed. Each pair consists of one case with the OFP and a control case without this parameterization. In both uncoupled and coupled experiments, the parameterized DS and FBC source volume transports are within the range of observed estimates. The entrainment volume transports remain lower than observational estimates, leading to lower than observed product volume transports. Due to low entrainment, the product and source water properties are too similar. The DS and FBC overflow temperature and salinity properties are in better agreement with observations in the uncoupled case than in the coupled simulation, likely reflecting surface flux differences. The most significant impact of the OFP is the improved North Atlantic Deep Water penetration depth, leading to a much better comparison with the observational data and significantly reducing the chronic, shallow penetration depth bias in level coordinate models. This improvement is due to the deeper penetration of the southward flowing Deep Western Boundary Current. In comparison with control experiments without the OFP, the abyssal ventilation rates increase in the North Atlantic. In the uncoupled simulation with the OFP, the warm bias of the control simulation in the deep North Atlantic is substantially reduced along with salinity bias reductions in the northern North Atlantic. There are similar but more modest bias reductions in the deep temperature and salinity distributions especially in the northern North Atlantic in the coupled OFP case. In coupled simulations, there are noticeable impacts of the OFP on climate. The sea surface temperatures (SSTs) are warmer by more than 5°C off the North American coast and by more than 1°C in the Nordic Sea with the OFP. The surface heat fluxes mostly act to diminish these SST changes. There are related changes in the sea level pressure, leading to about 15% weaker westerly wind stress in the northern North Atlantic. In response to the warmer Nordic Sea SSTs, there are reductions in the sea ice extent, improving comparisons with observations. Although the OFP cases improve many aspects of the simulations compared to observations, some significant biases remain, more in coupled than in uncoupled simulations.

On the low-frequency phase relation between the Denmark Strait and the Faroe-Bank Channel overflows

On the low-frequency phase relation between the Denmark Strait and the Faroe-Bank Channel overflows

Variable source regions of Denmark Strait and Faroe Bank Channel overflow waters

Variable source regions of Denmark Strait and Faroe Bank Channel overflow waters

Holocene variations in mineral and grain-size composition along the East Greenland glaciated margin (ca 67°–70°N): Local versus long-distance sediment transport

Quantitative X-Ray Diffraction (qXRD) analysis of the <2 mm sediment fraction from surface (sea floor) samples, and marine sediment cores that span the last 10–12 cal ka BP, are used to describe spatial and temporal variations in non-clay mineral compositions for an area between Kangerlussuaq Trough and Scoresby Sund (∼67°–70°N), East Greenland. Bedrock consists primarily of an early Tertiary alkaline complex with high weight% of pyroxene and plagioclase. Farther inland and to the north, the bedrock is dominantly felsic with a high fraction of quartz and potassium feldspars. Principal Component (PC) analysis of the non-clay sediment compositions indicates the importance of quartz and pyroxene as compositional end members, with an abrupt shift from quartz and k-feldspar dominated sediments north of Scoresby Sund to sediments rich in pyroxene and plagioclase feldspars offshore from the early Tertiary basaltic outcrop. Coarse (<2 mm or <1 mm) ice-rafted sediments are largely absent from the trough sediments between ∼8 and 5 cal ka BP, but then increase in the last 4 cal ka BP. Compositional unmixing of the sediments in Grivel Basin and Kangerlussuaq Trough indicate the dominance of local over long distance sediment sources, with pulses of sediment from tidewater glaciers in Kangerlussuaq and Nansen fjords reaching the inner shelf during the Neoglaciation. The change in IRD is more dramatic in the sediment grain-size proxies than in the quartz wt%. Forty to seventy percent of the variance in the quartz records from either side of Denmark Strait is explained by low frequency trends, but the data from the Grivel Basin, East Greenland, are distinctly different, with an approximate 2500 yr periodicity.

Variable source regions of Denmark Strait and Faroe Bank Channel overflow waters

The pathways of the water that overflows the Greenland–Scotland Ridge and their source regions are studied with an ocean circulation model. Depending on the size of wind stress curl around Iceland, different water mass properties are found in the less dense fraction of the overflows on both sides of Iceland. Although in both cases, this fraction contributes to only a small part of the overflow, their pathways and associated source regions characterize also changes in the pathways of the denser fractions which is confirmed by backward float experiments. In detail, large Denmark Strait overflow during high wind stress curl conditions is associated with a primary pathway along the East Greenland Current with one branch encircling the Greenland Sea that is supplied by regions further southward while the other branch enters through Fram Strait. During low forcing conditions, the source region shifts into the Iceland Sea associated with a reversal of a current north of Iceland. The Faroe Bank Channel overflow is primarily fed by sources in the Norwegian Sea with water flowing southward along the Norwegian shelf. A smaller contribution that is enhanced during high forcing follows along the Jan Mayen Ridge and provides water from the Iceland Sea and the Greenland Sea.

On the low-frequency phase relation between the Denmark Strait and the Faroe-Bank Channel overflows

Numerical simulations of the Atlantic and Arctic Oceans using an ocean-sea ice model are analysed with respect to interannual-to-decadal variability (1948–2008) of the dense water overflows east and west of Iceland. Besides the good agreement between observed and simulated Denmark Strait and Faroe-Bank Channel overflows, the model shows an antiphase relation between both transports at time scales of 3–10 yr. The analysis of wind stress curl over the Nordic Seas suggests that the phase relation is locally forced. The main mode of atmospheric variability in the Atlantic reinforces the dense overflow west of Iceland during positive phases of the North Atlantic Oscillation (NAO) and decreases it during negative phases. East of Iceland the overflow is decreased during positive phases of the NAO and increased during negative phases. After the mid-1990s, the antiphase relation is less clear due to a reduction of wind stress curl variance and a change in the production of dense water in the Nordic Seas. In that period, the Norwegian Sea experienced a reduction of its cyclonicity, leading to the export of a surplus of dense water across the Faroe-Shetland Channel. The forcing of the gyre variability is attributed to large-scale wind stress curl changes and to shifts of convection regimes linked to Arctic-Nordic Seas dense water exchange modifications at Fram Strait. Simulated sea surface height variability is highly correlated with the dense water volume in the Greenland and Norwegian Seas gyres, implying that altimetric measurements are good indicators of dense water content in the Nordic Seas and therefore provide a potential for monitoring the reservoirs and explain/anticipate changes in the overflows.

Interannual to multidecadal variability and predictability of North Atlantic circulation in a coupled earth system model with parametrized hydraulics

Several 1000 yr runs of the University of Victoria Earth System Climate Model (UVic ESCM) with a hydraulically controlled overflow in the Denmark Strait are used to analyse the effects of NAO-like variations of the wind stress localized in the subpolar North Atlantic. The focus is laid on improving the representation of the Atlantic meridional overturning circulation (AMOC), the sea surface temperatures in the Nordic Seas and the sea ice coverage without increasing the resolution of the global model. We show that by implementing hydraulic control in the Denmark Strait Overflow the AMOC can be enhanced at depths between 1000 and 3000 m by up to 7 Sverdrup (Sv) towards more realistic values. The stability of the Deep Western Boundary Current is considerably enhanced. The expansion of sea ice into the Nordic Seas in the standard run is pushed back from about 65°N to 75°N when hydraulic parametrization is switched on. In this case sea ice variations at 75°N and Northern Europe air temperatures exhibit a lag of 9 yr to variations in the wind stress curl.

High‐frequency fluctuations in Denmark Strait transport

Denmark Strait ocean current transport exhibits quasi‐regular fluctuations immediately south of the sill with periods of 2–4 days. The transport variability is similar to the mean transport itself. Using a circulation model we explore prospects to monitor the fluctuations. The model has realistic transport and shows water leaving Denmark Strait in equivalent‐barotropic cyclones that are nearly geostrophic and correlate with sea‐surface height (SSH). Existing satellite altimeter observations of SSH have adequate space/time sampling to reconstruct the transport fluctuations using a regression developed from the model results, but measurement error overwhelms the signal. From the model results, the pending Surface Water and Ocean Topography (SWOT) wide‐swath altimeter appears accurate enough, and with good‐enough coverage, to allow the transport fluctuations to be reconstructed. Bottom pressure recorders at the exit of the Denmark Strait can also reproduce the transport variability.

Coupling of palaeoceanographic shifts and changes in marine reservoir ages off North Iceland through the last millennium

Our study of spatial and temporal marine reservoir age changes on the North Icelandic shelf is based on two high-resolution 1000-year sedimentary records from IMAGES core sites MD99-2273 and MD99-2275, located at 650 m and 470 water depths, respectively. The sedimentary and fossil record is extremely sensitive to past oceanographic and climatic changes in this region. It has been demonstrated that the position of the Polar Front across the shelf has been very dynamic through the last millennium, and our results show that deviations in the marine reservoir ages can be used as a palaeoceanographic tool for tracing changes in the water masses. The presence of historically and terrestrially dated tephra markers from Icelandic volcanoes makes it possible to construct a reliable tephrochronological age-depth model for each of the marine sediment cores. A comparison of these age models with age models based on calibrated AMS radiocarbon age determinations of marine molluscs displays major deviations through the last 1000 years. The discrepancy between the two types of age models shows that there was an average reservoir age of about 650 years at MD99-2273 and 590 years at site MD99-2275, with considerable changes through time. This is suggested to be related to a generally high, but fluctuating, inflow of apparently old Polar waters to the area. A comparison with the distribution of two benthic foraminiferal indicator species in the cores, one for Polar and Arctic water masses ( Islandiella norcrossi) and one for Atlantic Water ( Cassidulina neoteretis), as well as with different proxies for sea-ice cover in the area, shows that there was an increase in the penetration of cold Polar and Arctic water masses after AD 1300, i.e. during the Little Ice Age, at both sites, resulting in an increase in the reservoir ages. Relatively high reservoir ages throughout the last millennium at the deepest core site (MD99-2273) can be attributed to a continuously high Polar Water influence at the sea floor. The location of this site to the west of the Kolbeinsey Ridge, which has acted as a submarine barrier throughout the Holocene, may have contributed to a relatively high influence of cold Denmark Strait overflow water in this area.

The oceanographic work of the Institute of Sea Fisheries in Greenland Waters, 1952-2008

Summary In 1952 marked the onset of the Institute of Sea Fisheries’ Greenland research when fishery scientists of the Institute joined German trawlers fishing for cod in West Greenland waters. Later on research was done from chartered trawlers; so-called ‘scouting trawler’ cruises. Starting in 1955, the Institute of Sea Fisheries also used research vessels to obtain information on demersal fish stocks and environmental parameters off Greenland. The months September to November were chosen for this survey, since at this time of the year the waters off Greenland, especially off East Greenland are least frozen. The most common oceanographic instruments during the 1950s, 1960s and the first half of the 1970s were Nansen Bottles and Mechanical Bathythermograph. A CTD system was used from 1978 onwards, and then changing to the Seabird 911+ system from the end-1990s. Oceanographic measurements were obtained at a set of international oceanographic standard stations initiated by the International Commission for the Northwest Atlantic Fisheries (ICNAF), and of national standard stations off East Greenland. Long-term climatic information on water column temperature and salinity from Fyllas Bank autumn data, and on deep water profiles in the eastern Labrador Sea, is available from measurements made by the Institute of Sea Fisheries. Recently, an oceanographic section across the Davis Strait Sill was installed which reveals information on the flux of water masses between Greenland and Canada during early-November. The complete set of ICNAF/NAFO oceanographic sections off West Greenland, and of national sections off East Greenland, obtained during October and November 1984, is available through the internet http://www.klima-bml.de/wh067/. The sections show the distribution of major water masses of the North Atlantic from surface layer to the deep water layers of the Labrador Sea including the Denmark Strait Overflow Water. It was found that Overflow Water entering the Labrador Sea at its eastern slope had higher oxygen values than the Overflow Water leaving the Labrador Sea after completing its cyclonic path along the Labrador basin. The long-term time series from NAFO Standard Stations off West Greenland, and the series from the new hydrographic section across Davis Strait passage, show that the observed warming of the North Atlantic Subpolar Gyre has had a tremendous impact on West Greenland Shelf and off-slope waters. While sea-surface temperature warming and sub-surface ocean temperature warming seem to have peaked during 2003 (the year with warmest mean annual air temperatures at Nuuk since 1950), the long-term trend observed in the temperature time series of the upper 300 m of the ocean off southwest Greenland suggests further warming may be in store. As part of international co-operation of the Institute, a Russian-German oceanographic project resulted in a primary literature publication on ‘Greenland cod (Gadus morhua): modeling recruitment variation during the second half of the 20th century’. At the end of this paper, a table of research cruises of the Institute of Sea Fisheries to Greenland waters (1955–2008) is given. The basic idea of the present paper is to document the historical development of the oceanographic work of the Institute of Sea Fisheries in Greenland Waters. In this sense, the paper is not a research document which presents recent oceanographic research results.

Entrainment in the Denmark Strait overflow plume by meso-scale eddies

Abstract. The entrainment of buoyant ambient water into the overflow plume of Denmark Strait and the associated downstream warming of the plume are estimated using time series of currents and temperature from moored instrumentation and classical hydrographic data. Warming rates are highest (0.4–0.5 K/100 km) within the first 200 km of the sill, and decrease to 0.05–0.1 K/100 km further downstream. Stirring by mesoscale eddies causes lateral heat fluxes that explain the 0.1 K/100 km warming, but in the first 200 km from the sill also vertical diapycnal fluxes, probably caused by breaking internal waves, must contribute to the entrainment.

Interannual to multidecadal variability and predictability of North Atlantic circulation in a coupled earth system model with parametrized hydraulics

Several 1000 yr runs of the University of Victoria Earth System Climate Model (UVic ESCM) with a hydraulically controlled overflow in the Denmark Strait are used to analyse the effects of NAO-like variations of the wind stress localized in the subpolar North Atlantic. The focus is laid on improving the representation of the Atlantic meridional overturning circulation (AMOC), the sea surface temperatures in the Nordic Seas and the sea ice coverage without increasing the resolution of the global model. We show that by implementing hydraulic control in the Denmark Strait Overflow the AMOC can be enhanced at depths between 1000 and 3000m by up to 7 Sverdrup (Sv) towards more realistic values. The stability of the Deep Western Boundary Current is considerably enhanced. The expansion of sea ice into the Nordic Seas in the standard run is pushed back from about 65◦N to 75◦N when hydraulic parametrization is switched on. In this case sea ice variations at 75◦N and Northern Europe air temperatures exhibit a lag of 9 yr to variations in the wind stress curl.

On the low-frequency phase relation between the Denmark Strait and the Faroe-Bank Channel overflows

Numerical simulations of the Atlantic and Arctic Oceans using an ocean-sea ice model are analysed with respect to interannual-to-decadal variability (1948–2008) of the dense water overflows east and west of Iceland. Besides the good agreement between observed and simulated Denmark Strait and Faroe-Bank Channel overflows, the model shows an antiphase relation between both transports at time scales of 3–10 yr. The analysis of wind stress curl over the Nordic Seas suggests that the phase relation is locally forced. The main mode of atmospheric variability in the Atlantic reinforces the dense overflow west of Iceland during positive phases of the North Atlantic Oscillation (NAO) and decreases it during negative phases. East of Iceland the overflow is decreased during positive phases of the NAO and increased during negative phases. After the mid-1990s, the antiphase relation is less clear due to a reduction of wind stress curl variance and a change in the production of dense water in the Nordic Seas. In that period, the Norwegian Sea experienced a reduction of its cyclonicity, leading to the export of a surplus of dense water across the Faroe-Shetland Channel. The forcing of the gyre variability is attributed to large-scale wind stress curl changes and to shifts of convection regimes linked to Arctic-Nordic Seas dense water exchange modifications at Fram Strait. Simulated sea surface height variability is highly correlated with the dense water volume in the Greenland and Norwegian Seas gyres, implying that altimetric measurements are good indicators of dense water content in the Nordic Seas and therefore provide a potential for monitoring the reservoirs and explain/anticipate changes in the overflows.

Variable source regions of Denmark Strait and Faroe Bank Channel overflow waters

The pathways of the water that overflows the Greenland—Scotland Ridge and their source regions are studied with an ocean circulation model. Depending on the size of wind stress curl around Iceland, different water mass properties are found in the less dense fraction of the overflows on both sides of Iceland. Although in both cases, this fraction contributes to only a small part of the overflow, their pathways and associated source regions characterize also changes in the pathways of the denser fractions which is confirmed by backward float experiments. In detail, large Denmark Strait overflow during high wind stress curl conditions is associated with a primary pathway along the East Greenland Current with one branch encircling the Greenland Sea that is supplied by regions further southward while the other branch enters through Fram Strait. During low forcing conditions, the source region shifts into the Iceland Sea associated with a reversal of a current north of Iceland. The Faroe Bank Channel overflow is primarily fed by sources in the Norwegian Sea with water flowing southward along the Norwegian shelf. A smaller contribution that is enhanced during high forcing follows along the Jan Mayen Ridge and provides water from the Iceland Sea and the Greenland Sea.

Cessation and partial reversal of deep water freshening in the northern North Atlantic: observation-based estimates and attribution

Recent decadal salinity changes in the Greenland-Scotland overflow-derived deep waters are quantified using CTD data from repeated hydrographic sections in the Irminger Sea. The Denmark Strait OverflowWater salinity record shows the absence of any net change over the 1980s–2000s; changes in the Iceland—Scotland Overflow Water (ISOW) and in the deep water column (σ0 > 27.82), enclosing both overflows, show a distinct freshening reversal in the early 2000s. The observed freshening reversal is a lagged consequence of the persistent ISOW salinification that occurred upstream, in the Iceland Basin, after 1996 in response to salinification of the northeast Atlantic waters entrained into the overflow. The entrainment salinity increase is explained by the earlier documented North Atlantic Oscillation (NAO)-induced contraction of the subpolar gyre and corresponding northwestward advance of subtropical waters that followed the NAO decline in the mid-1990s and continued through the mid-2000s. Remarkably, the ISOW freshening reversal is not associated with changes in the overflow water salinity. This suggests that changes in the NAO-dependent relative contributions of subpolar and subtropical waters to the entrainment south of the Iceland—Scotland Ridge may dominate over changes in the Nordic Seas freshwater balance with respect to their effect on the ISOW salinity.

Serial slaughter: the development of the north Norwegian sealing fleet: 1859–1909

ABSTRACTThis paper describes the growth and development of the sealing industry of northern Norway from 1859 to 1909 and is based largely on hunting returns and shipping records of the Tromsø fleet. Arctic hunting operations displayed remarkable diversity across the years and the ascendancy of sealing only emerged towards the end of the 19th century. The paper shows how the fleet increased in size despite the lack of capital in northern Norway for investment in new vessels and how hunting grew in significance as a commercial operation following the introduction of modern hunting techniques and better weapons. The fleet increased almost eightfold, from 6 vessels in 1859 to 46 in 1909 while the harvest of seals increased from less than 1500 to over 30000 animals annually. The geographical range of the hunting grounds expanded correspondingly from a limited area around Jan Mayen and the west coast of Spitsbergen to a huge area which included the western ice (north and south of Jan Mayen), the northern ice (Svalbard), the eastern ice (Kola Peninsula to Novaya Zemlya, the White Sea), Zemlya Frantsa-Isoifa [Franz Joseph Land], the Denmark Strait and northeast Greenland. The species composition of the harvest underwent a remarkable series of transitions, one species being replaced by another as local stocks became successively depleted. Thus, it was dominated numerically first by Svalbard reindeer and walrus, then beluga whales and then cod before finally consisting largely of polar bears, bottlenose whales and seals. Owners and skippers responded to reductions in numbers by searching for new hunting grounds and, in doing so, sailed further north, then east and then west than ever before, coincidentally making a series of historical voyages of discovery. By the end of these five decades sloops had largely been replaced by ketch rigged diesel sealers, these being an assortment of new, salvaged and second hand foreign ships.