Northern Greenland Sea Research Articles

Eddies in the Arctic Ocean Revealed from MODIS Optical Imagery

Here we investigate properties of ocean eddies in the key Arctic region of the northern Greenland Sea and the Fram Strait using visible and infrared Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua data acquired from April to September in 2007 and 2018–2020. We infer eddy properties using visual identification and automated processing of their signatures in sea surface temperature (SST) and chlorophyll-a (chl-a) maps, and their gradients. Altogether, 450 (721) eddies were identified in SST (chl-a) data. Their radii span from 2 to 40 km (mean value 12 km). Most eddies are elliptical with a mean aspect ratio (eccentricity) of their axes equal 0.77 (0.64). Cyclones are smaller than anticyclones and prevail in both data sources. Cyclones tend to be more prevalent over shallow shelves, and anticyclones over deep water regions. Peak eddy activity is registered in June, while chl-a data also possess a second peak in April. In SST, the highest eddy probability is found along the East Greenland Current in the Nordbukta region at 76–78°N and along the West Spitsbergen Current at 78–80°N. In chl-a, most of them are observed in the central Fram Strait. The overall number of eddies with a positive chl-a anomaly, dominated by cyclones, is larger (62%) than that with a negative one (~38%). The number of eddies with positive and negative SST anomalies is nearly equal. Eddy translation velocities are 0.9–9.6 km/day (mean value 4.2 km/day). Despite frequent cloud and ice cover, MODIS data is a rich source of information on eddy generation hot-spots, their spatial properties, dynamics and associated SST and chl-a anomalies in the Arctic Ocean.

Surface heat transfer changes over Arctic land and sea connected to Arctic warming

AbstractTurbulent heat transfer from surface to atmosphere is important in the atmospheric energy balance over the Arctic, thereby impacting the remarkable warming that occurs in this region. The impact of heat transfer over the Arctic land and sea (approximate Arctic Ocean) on Arctic warming could be distinct, owing to different heat sources and physical processes on the interfaces. This study sought to analyse the heat transfer and changes over the Arctic land and sea and their contributions to those over the entire Arctic, and discuss the connection between the heat transfer changes and Arctic warming based on ERA5 reanalysis. The Arctic surface releases heat to the atmosphere, with an annual average of 4.5 × 1014 W, which is mainly contributed by the Arctic land in summer and sea in winter. From 1979 to 2018, the amount of heat release was found to markedly increase, with an annual trend of 0.94 ± 0.31 W/(m2 decade). A faster increase occurred in summer and was contributed by both the Arctic land and sea. In contrast, a slower increase occurred in winter and was only contributed by the sea. The heat transfer changed heterogeneously over the Arctic land and sea, with three major changes: Northern Russia Increase over northern Russia in summer, Northern Barents Increase over the northern Barents Sea in winter, and Nordic Sea Decrease over the Nordic Sea and northern Greenland Sea in winter. Notably, the heat transfer change over the Arctic land and sea could be connected to Arctic warming, especially the three major changes, based on distinct feedback processes, which could impact future Arctic warming. Overall, the Arctic land and sea play distinct roles in the heat transfer change and connection to Arctic warming, with the land being more important in summer and the sea being more important in winter.

The multi-year comparisons of chlorophyll and sea ice in Greenland Sea and Barents Sea and their relationships with the North Atlantic Oscillation

The Arctic Ocean (AO) has experienced very significant warming in recent decades with clear impacts on the extent and depth of sea ice cover. Sea ice serves as a primary habitat and plays an important role in the AO marine food web. The surface distributions of chlorophyll_a (CHL), sea ice concentration (ICE), sea surface temperature (SST) and North Atlantic Oscillation (NAO) are analyzed in the study region (20°W-50°E, 70°N-80°N) over the decade (2003–2014). This region spans the Barents Sea (BS), Norwegian (NS) and Greenland Sea (GS). In general, the peak of spatial averaged CHL in the BS was about 60% higher than the GS. Due to elevated SST in the southern BS, CHL was much higher especially in 2010 and 2013. In 2011, there was a strong meridional gradient in CHL decreasing from south to north of the BS, and also a strong zonal gradient from the southern GS to the southern BS. The northern GS had higher CHL than the southern GS due to the increased ice melting and nutrient-enriched runoff from east Greenland glaciers to the northern and western coastal regions of the GS. Seasonal peaks of spatially averaged CHL occurred in April or May and were about two weeks earlier in the BS than the GS. Higher ice melt in the northern BS was the main reason for CHL blooms especially in 2010 and 2011. Earlier and more extensive ice melting and a persistent negative NAO index causing atmospheric circulation patterns that favoured ice loss were the possible drivers of enhanced phytoplankton blooms in 2010. A previous negative winter NAO is thought to be linked to an increase in ICE in the following spring. NAO is mostly negative during spring in the GS. Sea ice melt was positively correlated with CHL in the northern sector of the study region.

Arctic black carbon during PAMARCMiP 2018 and previous aircraft experiments in spring

Abstract. Vertical profiles of the mass concentration of black carbon (BC) were measured at altitudes up to 5 km during the PAMARCMiP (Polar Airborne Measurements and Arctic Regional Climate Model simulation Project) aircraft-based field experiment conducted around the northern Greenland Sea (Fram Strait) during March and April 2018 from operation base Station Nord (81.6∘ N, 16.7∘ W). Median BC mass concentrations in individual altitude ranges were 7–18 ng m−3 at standard temperature and pressure at altitudes below 4.5 km. These concentrations were systematically lower than previous observations in the Arctic in spring, conducted by ARCTAS-A in 2008 and NETCARE in 2015, and similar to those observed during HIPPO3 in 2010. Column amounts of BC for altitudes below 5 km in the Arctic (>66.5∘ N; COLBC), observed during the ARCTAS-A and NETCARE experiments, were higher by factors of 4.2 and 2.7, respectively, than those of the PAMARCMiP experiment. These differences could not be explained solely by the different locations of the experiments. The year-to-year variation of COLBC values generally corresponded to that of biomass burning activities in northern midlatitudes over western and eastern Eurasia. Furthermore, numerical model simulations estimated the year-to-year variation of contributions from anthropogenic sources to be smaller than 30 %–40 %. These results suggest that the year-to-year variation of biomass burning activities likely affected BC amounts in the Arctic troposphere in spring, at least in the years examined in this study. The year-to-year variations in BC mass concentrations were also observed at the surface at high Arctic sites Ny-Ålesund and Utqiaġvik (formerly known as Barrow, the location of Barrow Atmospheric Baseline Observatory), although their magnitudes were slightly lower than those in COLBC. Numerical model simulations in general successfully reproduced the observed COLBC values for PAMARCMiP and HIPPO3 (within a factor of 2), whereas they markedly underestimated the values for ARCTAS-A and NETCARE by factors of 3.7–5.8 and 3.3–5.0, respectively. Because anthropogenic contributions account for nearly all of the COLBC (82 %–98 %) in PAMARCMiP and HIPPO3, the good agreement between the observations and calculations for these two experiments suggests that anthropogenic contributions were generally well reproduced. However, the significant underestimations of COLBC for ARCTAS-A and NETCARE suggest that biomass burning contributions were underestimated. In this study, we also investigated plumes with enhanced BC mass concentrations, which were affected by biomass burning emissions, observed at 5 km altitude. Interestingly, the mass-averaged diameter of BC (core) and the shell-to-core diameter ratio of BC-containing particles in the plumes were generally not very different from those in other air samples, which were considered to be mostly aged anthropogenic BC. These observations provide a useful basis to evaluate numerical model simulations of the BC radiative effect in the Arctic region in spring.

NE Greenland Composite Tectono-Sedimentary Element, northern Greenland Sea and Fram Strait

Abstract The NE Greenland margin measures more than 300 km from coast to the continent–ocean transition at its widest part and is in an early state of exploration. It is expected to hold significant petroleum reserves due to a high concentration of large structures and prolific source-rock intervals anticipated in the subsurface. The margin is composed of five tectonic units: (1) the Koldewey Platform; (2) the Danmarkshavn Basin; (3) the Danmarkshavn Ridge; (4) the Thetis Basin; and (5) the Wandel Sea Basin. These units form the NE Greenland Rifted Margin Composite Tectono-Sedimentary Element. Rifting commenced at some time during the Devonian or Carboniferous and recurred during latest Jurassic–Cretaceous time. Major tilted fault blocks and other rift-related structures formed during latest Jurassic–Cretaceous rifting. The structures are typically cored by Triassic–Middle Jurassic sand-prone sequences. Oil-prone Upper Jurassic–lowermost Cretaceous marine source rock caps the structures or is located immediately downdip. A large concentration of structures exists in the eastern Danmarkshavn Basin, the western Thetis Basin and along the edges of the Danmarkshavn Ridge. Other major structures exist in the least data-covered parts of the margin furthest north.

Transports and Accumulations of Greenland Sea Intermediate Waters in the Norwegian Sea

AbstractThe Greenland Sea intermediate waters transported via the Jan Mayen Channel (JMCh) are a significant source for the Iceland‐Scotland Overflow Water. Based on hydrographic data collected by a Norway‐China survey in 2015 and Argo floats, we classify the water masses of the Greenland Sea outflow and then reveal their distributions in the Norwegian Sea. The Atlantic‐origin water produced by density increase during winter in the northern Greenland Sea was an important component of the intermediate waters exported to the Norwegian Sea, accounting for about 30% of the total volume in 2015. The hydrographic data revealed that the major passage of outflow from the Greenland Sea was located in the southern Mohn Ridge in 2015, rather than in the JMCh, as generally recognized. The transport of intermediate waters from the Greenland Sea via the southern Mohn Ridge in the summer of 2015 is estimated to be about 0.8–1.7 Sv. This transport pattern provides a perspective that there exists a multi‐passage system of outflow in the JMCh and Mohn Ridge, promoting a stable supply of intermediate waters to the Norwegian Sea.

Eddies in the North Greenland Sea and Fram Strait From Satellite Altimetry, SAR and High‐Resolution Model Data

AbstractIn this study, we investigate eddy dynamics in the northern Greenland Sea and the Fram Strait using AVISO altimetry, spaceborne synthetic aperture radar (SAR), and Finite Element Sea ice‐Ocean Model (FESOM) high‐resolution numerical model data. In the region, eddies are thought to play an important role in the redistribution of the warmer and saltier Atlantic Water between the Arctic Ocean and the areas of deep convection in the central Greenland Sea. We found that eddies detected in AVISO and in SAR form two complementary data sets of large mesoscale eddies (with typical radii of 30–50 km) and of small mesoscale/submesoscale eddies (with typical radii of 1–5 km and not exceeding 30 km), respectively. For large mesoscale eddies, the number of cyclones and anticyclones is approximately the same, while for submesoscale eddies, cyclones are strongly dominating. The limitations and possible biases in each of the data sets are discussed and cross‐validated against FESOM results. It is noted that the most energetic eddies are concentrated along the major currents and in the northern part of the region. Eddy translations follow the mean currents in their overall cyclonic circulation around the northern Greenland Sea. A convergence of the eddies toward the Nordbukta area is detected. On seasonal time scale, a higher number of more intense mesoscale eddies is observed during winter, associated with a quasi‐simultaneous intensification of the mean currents. Model results also show an increase in the number of small eddies in spring‐early summer attributed to the decay of large eddies, while in late autumn, the opposite tendency suggests eddy merger.

Relationship between the sharp decrease in dust storm frequency over East Asia and the abrupt loss of Arctic sea ice in the early 1980s

AbstractBased on dust storm frequency (DSF) data from the China Meteorological Administration, Arctic sea-ice concentration (SIC) data from the Hadley Centre, and atmospheric reanalysis data from the National Centers for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR), temporal variations and regime shifts of East Asian DSF and Arctic SIC during 1961–2015 are revealed, and the possible relationship between them is explored. The results show that East Asian DSF in spring is closely associated with the preceding winter SIC from the northern Greenland Sea to the Barents Sea (20° W–60° E, 74.5° N–78.5° N). In the past half-century, both East Asian DSF and Arctic SIC have shown significant declining trends, with consistent regime shifts in the early 1980s. Further statistical analyses indicate that the abrupt decrease of East Asian DSF in spring may be attributed to the concurrent sharp loss of Arctic SIC in the preceding winter. It is the loss of Arctic SIC that causes the atmospheric circulation anomalies downstream by stimulating a Rossby wave train, resulting in decelerated wind speed, dampened vertical wind shear and restrained synoptic-scale disturbances over the dust source region, eventually leading to the decline in East Asian DSF over decadal timescales.

ArcCRUST: Arctic Crustal Thickness From 3‐D Gravity Inversion

AbstractThe ArcCRUST model consists of crustal thickness and estimated crustal thinning factors grids for the High Arctic and Circum‐Arctic regions (north of 67°N). This model is derived by using 3‐D forward and inverse gravity modeling. Updated sedimentary thickness grid, an oceanic lithosphere age model together with inferred microcontinent rifting ages, variable crystalline crust and sediment densities, and dynamic topography models constrain this inversion. We use published high‐quality 2‐D seismic crustal‐scale models to create a database of Depths to Seismic Moho (DSM) profiles. To check the quality of the ArcCRUST model, we have performed a statistical analysis of misfits between the ArcCRUST Moho depths and DSM values. Systematic analysis of the misfits within the Arctic sedimentary basins provides information about tectonic processes unaccounted by the assumed model of pure‐shear lithospheric extension. In particular, our model implies a less dense and/or thin mantle lithosphere underneath microcontinents in the deep Arctic Ocean where the ArcCRUST depth to Moho values exceed the DSM. A systematically larger gravity‐derived crustal thickness (~3 km) under the western and northern Greenland Sea points to a hotter upper mantle implied by the seismic tomography models in the North Atlantic.

Relative Contributions of Atmospheric Energy Transport and Sea Ice Loss to the Recent Warm Arctic Winter

Abstract The relative contributions of atmospheric energy transport (via heat and moisture advection) and sea ice decline to recent Arctic warming were investigated using high-resolution reanalysis data up to 2017. During the Arctic winter, a variation of downward longwave radiation (DLR) is fundamental in modulating Arctic surface temperature. In the warm Arctic winter, DLR and precipitable water (PW) are increasing over the entire Arctic; however, the major drivers for such increases differ regionally. In areas such as the northern Greenland Sea, increasing DLR and PW are caused mainly by convergence of atmospheric energy transport from lower latitudes. In regions of maximum sea ice retreat (e.g., northern Barents–Kara Seas), continued sea ice melting from previous seasons drive the DLR and PW increases, consistent with the positive ice–insulation feedback. Distinct local feedbacks between open water and ice-retreat regions were further compared. In open water regions, a reduced ocean–atmosphere temperature gradient caused by atmospheric warming suppresses surface turbulent heat flux (THF) release from the ocean to the atmosphere; thus, surface warming cannot accelerate. Conversely, in ice-retreat regions, sea ice reduction allows the relatively warm ocean to interact with the colder atmosphere via surface THF release. This increases temperature and humidity in the lower troposphere consistent with the positive ice–insulation feedback. The implication of this study is that Arctic warming will slow as the open water fraction increases. Therefore, given sustained greenhouse warming, the roles of atmospheric heat and moisture transport from lower latitudes are likely to become increasingly critical in the future Arctic climate.

Drastic decrease in high Arctic gulls—ivory Pagophila eburnea and Ross’s Rhodostethia rosea—density in the northern Greenland Sea and Fram Strait between 1988 and 2014

Our long-term study on top predator (seabirds and marine mammals) distribution in polar marine ecosystems aims at detecting possible temporal and spatial population changes, especially in the context of global changes in temperature and ice cover. Quantitative data on the seabird distribution were collected in the northern Greenland Sea and Fram Strait onboard the icebreaker RV Polarstern between 1988 and 2014, applying 30-min transect counts from the bridge, without width limitation (n = 7320). A drastic decrease in numbers by a factor of seven was detected for the ivory gull Pagophila eburnea from 2007 on. These data confirm the decrease of ivory gull previously detected in North Canada and North Greenland, leading to the conclusion that the species is “endangered” or “near threatened”. These changes are discussed in relation to the decreasing Arctic pack ice coverage leading to the opening of the Northwest and Northeast Passages in 2007, at the time the year with the lowest ice coverage ever recorded. On the other hand, the decrease was even stronger for Ross’s gull Rhodostethia rosea after 1993, apparently reflecting changes in migratory habits. The third high Arctic gull, Sabine’s gull Xema sabini, was only tallied in very low numbers, without any clear temporal evolution in numbers.

Unusual phytoplankton bloom phenology in the northern Greenland Sea during 2010

Arctic marine ecosystems are disproportionately impacted by global warming. Sea ice plays an important role in the regional climate system and the loss of perennial sea ice has diverse ecological implications. Here we investigate the causes of an unusually early and strong phytoplankton bloom in the northern Greenland Sea (20°W–10°E, 75°N–80°N) during the 2010 season. In order to better understand the anomalous bloom in 2010, we examine the correlation between satellite-derived biomass and several possible environmental factors for the period 2003–2012. Results show that the timing of sea ice melt played an important role in promoting the growth of phytoplankton. Multivariate lagged regression analysis shows that phytoplankton biomass (CHL) is correlated with ice concentration (ICE) and ice melting, as well as sea surface temperature (SST) and photosynthetically active radiation (PAR). During 2010, the spring peak in biomass came much earlier and achieved a higher value than most other years in the satellite archive record, which was due to earlier and more extensive sea ice melt in that year. Relative lower SST and PAR in spring and early summer in year 2010 associated with a persistent negative North Atlantic Oscillation (NAO) index were possible drivers of the bloom. Wind direction changed from the southeast to southwest direction in spring, possibly transporting nutrient enriched melt runoff from glaciers on Greenland and other sources from the south to northern coastal regions.

Association between Arctic autumn sea ice concentration and early winter precipitation in China

Associations between autumn Arctic sea ice concentration (SIC) and early winter precipitation in China are studied using singular value decomposition analysis. The results show that a reduced SIC almost everywhere in the Arctic Ocean, except the northern Greenland Sea and Canadian Basin, are accompanied by dry conditions over central China, extending northeast from the Tibetan Plateau toward the Japan Sea, the Bohai Sea and the Yellow Sea, and wet conditions over South China and North China. Atmospheric circulation anomalies associated with SIC variability show two wave-train structures, which are persistent from autumn to winter, leading to the identified relationship between autumn Arctic SIC and early winter precipitation in China. Given that the decline in autumn SIC in the Arctic Ocean is expected to continue as the climate warms, this relationship provides a possible long-term outlook for early winter precipitation in China.

Water mass evolution of the Greenland Sea since late glacial times

Abstract. Four sediment cores from the central and northern Greenland Sea basin, a crucial area for the renewal of North Atlantic deep water, were analyzed for planktic foraminiferal fauna, planktic and benthic stable oxygen and carbon isotopes as well as ice-rafted debris to reconstruct the environmental variability in the last 23 kyr. During the Last Glacial Maximum, the Greenland Sea was dominated by cold and sea-ice bearing surface water masses. Meltwater discharges from the surrounding ice sheets affected the area during the deglaciation, influencing the water mass circulation. During the Younger Dryas interval the last major freshwater event occurred in the region. The onset of the Holocene interglacial was marked by an increase in the advection of Atlantic Water and a rise in sea surface temperatures (SST). Although the thermal maximum was not reached simultaneously across the basin, benthic isotope data indicate that the rate of overturning circulation reached a maximum in the central Greenland Sea around 7 ka. After 6–5 ka a SST cooling and increasing sea-ice cover is noted. Conditions during this so-called "Neoglacial" cooling, however, changed after 3 ka, probably due to enhanced sea-ice expansion, which limited the deep convection. As a result, a well stratified upper water column amplified the warming of the subsurface waters in the central Greenland Sea, which were fed by increased inflow of Atlantic Water from the eastern Nordic Seas. Our data reveal that the Holocene oceanographic conditions in the Greenland Sea did not develop uniformly. These variations were a response to a complex interplay between the Atlantic and Polar water masses, the rate of sea-ice formation and melting and its effect on vertical convection intensity during times of Northern Hemisphere insolation changes.

Effect of ice melting on bacterial carbon fluxes channelled by viruses and protists in the Arctic Ocean

During the last few years, extensive sea ice melting in the Arctic due to climate change has been detected, which could potentially modify the organic carbon fluxes in these waters. In this study, the effect of sea ice melting on bacterial carbon channelling by phages and protists has been evaluated in the northern Greenland Sea and Arctic Ocean. Grazing on bacteria by protists was evaluated using the FLB disappearance method. Lysis of bacteria due to viral infections was measured using the virus reduction approach. Losses of bacterial production caused by protists (PMMBP) dominated losses caused by viruses (VMMBP) throughout the study. Lysogenic viral production was detected in 7 out of 21 measurements and constituted from 33.9 to 100.0% of the total viral produc- tion. Significantly higher PMMBP and lower VMMBP were detected in waters affected by ice melting compared with unaffected waters. Consequently, significantly more bacterial carbon was channelled to the higher trophic levels in affected waters (13.05 ± 5.98 lgC l -1 day -1 ) than in unaffected waters (8.91 ± 8.33 lgC l -1 day -1 ). Viruses channelled 2.63 ± 2.45 lgC l -1 day -1 in affected waters and 4.27 ± 5.54 lgC l -1 day -1 in unaffected waters. We conclude that sea ice melting in the Arctic could modify the carbon flow through the microbial food web. This process may be especially important in the case of massive sea ice melting due to climate change.

Decadal variability of circulation in the Arctic Ocean retrieved from climatological data by a variational method

An inverse 3D finite‐element ocean circulation model has been designed and used to study variability of the Arctic Ocean circulation in the last 4 decades. We obtained stationary model solutions with the temperature and salinity fields close to the ones given by the Environmental Working Group (EWG) [1998] atlas. Transports at the open boundaries, wind forcing and hydrographic fields are treated as unknowns, which are varied to minimize a quadratic cost function subject to model constraints. The inverse problem is solved for 10 gridded hydrographic data sets that were obtained as winter and summer averages of EWG data over each of the 4 decades (1950s to 1980s) and over the whole period of observations (1948–1993) documented in the atlas. The results show that Arctic circulation in the last 4 decades has undergone significant changes, which manifest themselves in (1) 10% reduction of the ventilation rate in the Atlantic sector of the Arctic Ocean; (2) substantial decrease of the advective heat and freshwater import at the lateral boundaries; (3) spinning down of the cyclonic gyre in the northern Greenland sea, which is partly driven by deep convection; and (4) 3400 km3 increase of the net fresh water storage, with 75% taking place in the Atlantic sector of the Arctic Ocean. Most of these changes are similar to the ones observed on seasonal transition from winter to summer climatologies, and indicate that the Arctic Ocean is experiencing a shift toward a warmer state.

Absolute transport estimates of total and individual water masses in the northern Greenland Sea derived from hydrographic and acoustic Doppler current profiler measurements

Conductivity‐temperature‐depth transects across the Greenland Sea and Fram Strait provide geostrophic shear velocity profiles, which were absolutely referenced using the simultaneous measurements from a vessel‐mounted acoustic Doppler current profiler. The measurements are performed mainly in 1997, with supplements in 1998, 1995, and 1994. The observations confirm that all currents within the area have a strong barotropic component so that calculations based on the geostrophic method underestimate the transports considerably. Particulars for the transports of mass and heat are given for all individual and total water masses in the area. Meanders and eddies on horizontal scales between 20 and 70 km affect the total transport in the West Spitsbergen Current significantly. The Atlantic Water transports range between 2 and 7 Sv, mainly dominated by the cross‐sectional area occupied by this water mass. The total northward heat transport across 75°N is estimated as 54.7 TW and across 79°40′N as 42 TW in September 1997.

Carnivorous feeding and respiration of the Arctic under-ice amphipod Gammarus wilkitzkii

The dominant Arctic under-ice amphipod Gammarus wilkitzkii consumes a wide range of food items. The carnivorous feeding activity and energy budget of this large species were studied using three different approaches. Maximum potential ingestion rates Imax estimated from an allometric function taken from the literature and based on body mass were 2.1±0.4% of body carbon day–1. Based on respiration measurements, the specific ingestion rates required to meet metabolic demands were lower (1.4±0.4% of body carbon day–1). Feeding experiments, in which co-occurring pelagic calanoid (Calanus hyperboreus) or sympagic harpacticoid (Halectinosoma sp.) copepods were offered as prey, yielded actual ingestion rates of 8.0±5.6% of body carbon day–1 and 0.1±0.1% of body carbon day–1, respectively. These results indicate that predatory feeding on pelagic copepods may constitute an important food source for G. wilkitzkii. Abundances of G. wilkitzkii at the ice underside (median: 1.6 ind. m–2), Calanus spp. in the upper metre below the ice (2.6 ind. m–3), and Halectinosoma sp. in the lowermost 2–3 cm of the ice (393.5 ind. m–2) were determined from several multi-year pack-ice floes in the northern Greenland Sea and Fram Strait. Potential predation impact of G. wilkitzkii was estimated by combining information on ingestion rates with population densities. It was very high on Calanus spp. in the under-ice water layer (61.5% of the under-ice standing stock day–1), but comparatively low on Halectinosoma sp. in the bottom of the ice (3.8% of standing stock day–1). The observation of G. wilkitzkii preying on pelagic copepods in the under-ice water layer represents a hitherto unknown but obviously significant process and a new direction in the cryo-pelagic coupling in the Arctic marine ecosystem.

An inverse modeling study in Fram Strait. Part I: dynamics and circulation

In order to reconstruct the circulation in the northern Greenland Sea, between 77°N and 81°N, and the exchanges with the Arctic Ocean through Fram Strait, a variational inverse model is applied to the density field observed in summer 1984 during the MIZEX 84 experiment. An estimate of the three-dimensional large-scale pressure field is obtained in which the solution is decomposed into a limited number of vertical modes and the mode amplitudes are described by piece-wise polynomials on a finite-element grid. The solution should be consistent with a frictional depth-integrated vorticity balance and with the density data. The global model parameters are tuned to ensure agreement between the retrieved geostrophic velocity and independent currentmeter data. In a companion paper ( Schlichtholz and Houssais, 1999b), the same method, but without dynamical constraint, is applied to the same hydrographic dataset to perform a detailed water mass analysis and to estimate individual water mass transports. A comprehensive picture of the summer geostrophic circulation in Fram Strait is obtained in which northward recirculations in the East Greenland Current (EGC) and various recirculations from the West Spitsbergen Current (WSC) to the EGC are identified. It is suggested that the branch of the WSC following the upper western slope of the Yermak Plateau turns westward beyond 81°N and recirculates southward along the lower slope, then merging with a westward recirculating branch south of 79°N. At 79°N, a southward net transport of 6.5 Sv is found in the EGC which, combined with a northward net transport of only 1.5 Sv in the WSC, results in a fairly large outflow of 5 Sv from the Arctic Ocean to the Greenland Sea. The inverse solutions show that, in summer, the local induction of vorticity by the wind stress curl or by meridional advection of planetary vorticity should be small, so that, in the EGC and in the WSC, the vorticity balance is mainly achieved between the bottom pressure torque and dissipation of vorticity through bottom friction. A substantial barotropic flow associated with along-slope potential energy gradients is indeed identified on both sides of the strait.

129I in archived seawater samples

Anthropogenic 129I (t1/2=15.7My) discharged by the nuclear fuel reprocessing facilities at Sellafield (UK) and La Hague (France) is a promising tracer of physical and biogeochemical processes in the North Atlantic and Arctic Oceans. To improve understanding of its releases and dispersal, 129I was measured in archived seawater samples that had been collected as part of previous tracer studies, thus allowing direct comparison of 129I with other anthropogenic radionuclides. A sample collected in the eastern subtropical North Atlantic in 1969 was selected in order to directly measure the impact of weapons-fallout 129I in the oceans. The measured 129I/127I ratio of the sample is 0.53±0.08×10-10 (0.53±0.08IU: 1IU≡129I/127I=10-10), compared to the pre-anthropogenic ratio of ∼10-12 (0.01 IU). The ratio of 129I to 137Cs is 2.0±0.6 (atom ratio), 1.6±0.6 when corrected for 7yr of radioactive decay from the bomb-input peak. This observed ratio is ten times higher than predicted from fission yields, possibly reflecting the greater volatility of iodine relative to cesium. 129I/137Cs ratios in Scottish and Norwegian Coastal waters, sampled in 1976 and 1978, are in good agreement with predictions based on the available release data. Reprocessing 129I is clearly seen in the northern Greenland Sea (>6 IU) and in Denmark Straits Overflow Water (1.4 IU) in samples collected during the TTO/NAS program in 1981. The strength of the tracer signal in the overflow water – the ratio of the concentration in the overflow core to its minimum value in the station profile – is approximately four times higher for 129I than for the other tracers measured (137Cs, 90Sr, and 3H).