Summer Sea Ice Melt Research Articles

Phytoplankton responses to increasing Arctic river discharge under the present and future climate simulations

In recent decades, the unprecedented rate of Arctic warming has accelerated the high-latitude landmass hydrological cycle, leading to increased river discharge into the Arctic Ocean. This study elucidates the role of Arctic river discharge, which was the large model uncertainty in the Coupled Model Intercomparison Project 6, for the phytoplankton responses in present-day and future climate simulations by adding fresh water into the model. In the present-day climate simulation, additional river discharge decreases the spring phytoplankton biomass. Freshening of Arctic seawater facilitates freezing, increasing sea ice concentration in spring and eventually decreasing phytoplankton due to less availability of light. On the other hand, in the summer, phytoplankton increases due to the surplus of surface nitrate and the increase in the vertical mixing induced by the reduced summer sea ice melting water. In the future climate, the plankton response to the additional freshwater input is similar to the present-day climate. Nevertheless, the major phytoplankton responses are shifted from the Eurasian Basin to the Canada Basin and the East-Siberian Sea, mainly due to the marginal sea ice zone shift from the Barents-Kara Sea to the East Siberian-Chukchi Sea in the future.

Asymmetry variations in Arctic summer onset and ending: Role of sea-ice melting

Previous studies found that in Arctic regions with severe sea ice melting, summer ending (SE) variations are significantly larger than summer onset (SO) variations in the past few decades. Based on short-term observations, researchers preliminarily suggested that radiation variations caused by an earlier melting onset could be the possible reason for asymmetric Arctic SO/ending variations (AASV). Based on observations and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis I dataset, here we quantitatively characterize AASV by calculating the difference between the 11 year sliding trend of Arctic SO and SE. The results show that AASV positively correlates with sea ice melting in summer. The increased summer sea ice melting increases the area to absorb short-wave radiation in summer and then release more long-wave radiation to heat the lower atmosphere and delay the peak time of long-wave radiation releasing. The variations in radiation lead to a significant delay of the Arctic SE, with no significant variations in SO. We introduce CMIP6 historical and future simulations of 15 models to verify further the relationship between AASV and summer sea ice melting. Historical run reproduces the observed asymmetry, and future simulations under various warming levels show that AASV will vanish with disappeared melting variations or be strengthened with increased melting. The latter could delay freeze-up and further exacerbate the following years’ melting, which will enhance AASV. Furthermore, AASV will delay the onset and peak time of Arctic amplification.

An Optimal Atmospheric Circulation Mode in the Arctic Favoring Strong Summertime Sea Ice Melting and Ice–Albedo Feedback

Abstract The rapid decline of summer Arctic sea ice over the past few decades has been driven by a combination of increasing greenhouse gases and internal variability of the climate system. However, uncertainties remain regarding spatial and temporal characteristics of the optimal internal atmospheric mode that most favors summer sea ice melting on low-frequency time scales. To pinpoint this mode, we conduct a suite of simulations in which atmospheric circulation is constrained by nudging tropospheric Arctic (60°–90°N) winds within the Community Earth System Model, version 1 (CESM1), to those from reanalysis. Each reanalysis year is repeated for over 10 model years using fixed greenhouse gas concentrations and the same initial conditions. Composites show the strongest September sea ice losses are closely preceded by a common June–August (JJA) barotropic anticyclonic circulation in the Arctic favoring shortwave absorption at the surface. Successive years of strong wind-driven melting also enhance declines in Arctic sea ice through enhancement of the ice–albedo feedback, reaching a quasi-equilibrium response after repeated wind forcing for over 5–6 years, as the effectiveness of the wind-driven ice–albedo feedback becomes saturated. Strong melting favored by a similar wind pattern as observations is detected in a long preindustrial simulation and 400-yr paleoclimate reanalysis, suggesting that a summer barotropic anticyclonic wind pattern represents the optimal internal atmospheric mode maximizing sea ice melting in both the model and natural world over a range of time scales. Considering strong contributions of this mode to changes in Arctic climate, a better understanding of its origin and maintenance is vital to improving future projections of Arctic sea ice.

Possible impacts of anomalous Arctic sea ice melting on summer atmosphere

AbstractArctic sea ice melting and its possible impacts on climate variability and extreme weather events in the mid‐latitudes have been of wide concern over the past two decades. Compared with the impacts of sea ice melting on winter, its impacts on summer atmospheric circulation variability and extreme weather events have received less attention, and it is not clear whether the rapid sea ice melting can affect summer temperature and precipitation in the mid‐latitudes. Through observational analyses and simulation experiments forced by observed Arctic sea ice concentration, this study investigates possible impacts of anomalous summer sea ice melting on air temperature, frequency of heatwaves, precipitation, and East Asian summer monsoon variability. It is found that the enhanced summer sea ice melting has occurred since 2007 and dynamically corresponded to the positive phase of the summer Arctic dipole anomaly. Simulation experiments showed that the ensemble means of anomalous summer sea ice melting cases significantly increase surface air temperatures (SATs) and frequencies of heatwaves in the mid‐ and high‐latitudes of both Asia and North America, and decreases SATs in Europe and parts of the Asian continent. Anomalous summer sea ice melting increases summer precipitation in the circumpolar region, mid‐latitudes of East Asia, and the Northwestern Pacific and significantly decreases precipitation in mid‐ and low‐latitudes of North America. In addition, anomalous sea ice melting in simulations deepens the thermal depression in East Asia and strengthens East Asian summer monsoon and its variability. These model results suggest that anomalous sea ice melting may exacerbate impacts of global warming on summer atmosphere over Asia and North America though its impact may not be dominant due to atmospheric internal variability and impacts of other factors.

Enhanced Arctic Stratification in a Warming Scenario: Evidence From the Mid Pliocene Warm Period

AbstractGlobal warming is most pronounced in the Arctic as evident from the massive sea ice loss during the past few decades. The Mid‐Pliocene Warm Period (MPWP), 3.264 – 3.025 million years ago with similar CO2 levels, is the nearest analogue for understanding the impacts of future global warming. High‐resolution studies of relative nutrient utilization and productivity from the Atlantic‐Arctic Gateway (AAG) can provide insight into the nutrient availability governed by stratification strength during past warm climates. Here, we present relative nutrient utilization and productivity variability during the MPWP using sediments collected during the Ocean Drilling Program (ODP) Leg 151 from Fram Strait, AAG. We find that the relative nutrient utilization was high (low) implying stronger (weaker) stratification during warm (cold) periods during the MPWP. Stronger stratification inhibits the nutrient influx from intermediate water depths into the surface leading to higher utilization of available nutrients. It existed during warm periods likely due to enhanced summer sea ice melt and river discharge from the hinterland. As a consequence, the freshened surface layer could have stored more heat and accelerated the sea ice melt further implying that in the present‐day warm scenario, stronger stratification and upper layer freshening may lead to more sea ice melt in the Arctic Ocean.

Delay in Arctic Sea Ice Freeze-Up Linked to Early Summer Sea Ice Loss: Evidence from Satellite Observations

The past decades have witnessed a rapid loss of the Arctic sea ice and a significant lengthening of the melt season. The years with the lowest summertime sea ice minimum were found to be accompanied by the latest freeze-up onset on record. Here, a synthetic approach is taken to examine the connections between sea ice melt timing and summer sea ice evolution from the remote sensing perspective. A 40-year (1979–2018) satellite-based time-series analysis shows that the date of autumn sea ice freeze-up is significantly correlated with the sea ice extent in early summer (r = −0.90, p < 0.01), while the spring melt onset is not a promising predictor of summer sea ice evolution. The delay in Arctic sea ice freeze-up (0.61 days year−1) in the Arctic was accompanied by a decline in surface albedo (absolute change of −0.13% year−1), an increase in net short-wave radiation (0.21 W m−2 year−1), and an increase in skin temperature (0.08 °C year−1) in summer. Sea ice loss would be the key reason for the delay in autumn freeze-up, especially in the Laptev, East-Siberian, Chukchi and Beaufort Seas, where sea ice has significantly declined throughout the summer, and strong correlations were found between the freeze-up onset and the solar radiation budget since early summer. This study highlights a connection between the summer sea ice melting and the autumn refreezing process through the ice-albedo feedback based on multisource satellite-based observations.

Exceptionally Warm and Prolonged Flow of Warm Deep Water Toward the Filchner‐Ronne Ice Shelf in 2017

AbstractThe Filchner‐Ronne Ice Shelf, fringing the southern Weddell Sea, is Antarctica's second largest ice shelf. At present, basal melt rates are low due to active dense water formation; however, model projections suggest a drastic increase in the future due to enhanced inflow of open‐ocean warm water. Mooring observations from 2014 to 2016 along the eastern flank of the Filchner Trough (76°S) revealed a distinct seasonal cycle with inflow if Warm Deep Water during summer and autumn. Here we present extended time series showing an exceptionally warm and long inflow in 2017, with maximum temperatures exceeding 0.5°C. Warm temperatures persisted throughout winter, associated with a fresh anomaly, which lead to a change in stratification over the shelf, favoring an earlier inflow in the following summer. We suggest that the fresh anomaly developed upstream after anomalous summer sea ice melting and contributed to a shoaling of the shelf break thermocline.

Loss of shelf

Ice Shelves An unusually warm and prolonged inflow of warm ocean water intruded beneath Antarctica's Filchner Ronne Ice Shelf in 2017. Ryan et al. describe a 4-year-long time series of ocean temperature measurements that documents the surge and its unusual persistence. They discuss the hydrographic properties of the event and possible forcing mechanisms, suggesting that it was caused by anomalous summer sea ice melting. Much of the thinning of ice shelves like the Filchner Ronne is caused by warm water intrusions. Similar intrusions beneath other ice shelves will affect the future stability of the Antarctic Ice Sheet, a pressing concern in our warming climate. Geophys. Res. Lett. 10.1029/2020GL088119 (2020).

Summer sea ice melt and wastewater are important local sources of microlitter to Svalbard waters

Human activities leave traces of marine litter around the globe. The Arctic is, despite its remoteness, emerging as an area of no exception to this environmental issue. Arctic sea ice has previously been found to constitute a temporal sink of microplastics, but the potential release and subsequent fate of microplastics in the marine environment are yet unknown. Furthermore, the relative importance of local sources of microplastics in the Arctic marine environment is under discussion. In this study, the concentration and distribution of anthropogenic microparticles (AMPs, <5 mm, including microplastics) have been investigated in marine waters and sea ice of Svalbard. Seawater samples throughout the water column and floating sea ice samples were collected along a transect originating in Rijpfjorden, reaching northwards to the sea ice-edge. Seawater samples were also collected along a transect extending westwards from head to mouth of Kongsfjorden. Samples were collected throughout the water column with stations positioned to enable detection of potential AMP emissions from the wastewater outlet in Ny-Ålesund. Along both transects, environmental parameters were measured to explore potential correlations with AMP distribution. High concentrations of AMPs were detected in sea ice (158 ± 155 AMPs L−1). Based on both AMP concentrations and characteristics, AMPs identified in seawater of the marginal ice zone are to a large extent likely released during the melting of sea ice. The release of AMPs during summer melting of sea ice was concomitantly taking place with the ice-edge bloom, suggesting increased bioavailability to Arctic marine biota. Concentrations of AMPs were up to an order of magnitude higher in Kongsfjorden (up to 48.0 AMPs L−1) than in Rijpfjorden (up to 7.4 AMPs L−1). The distribution and composition of AMPs in Kongsfjorden suggest the wastewater outlet in Ny-Ålesund to be a likely source. Our results emphasize the importance of local point- and diffuse sources of AMPs in the Arctic and stress the urgency of considering their associated environmental impact. Implementation of regulatory policy is of importance, particularly since human activities and environmental pressures are increasing in the Arctic.

Arctic Summer Sea Ice Melt and Related Atmospheric Conditions in Coupled Regional Climate Model Simulations and Observations

AbstractObservations from 1979 to 2014 show a positive trend in the summer sea ice melt rate with an acceleration particularly in June and August. This is associated with atmospheric circulation changes such as a tendency toward a dipole pattern in the mean sea level pressure (SLP) trend with an increase over the Arctic Ocean and a decrease over Siberia. Consistent with previous studies, we here show the statistical relationship between the summer sea ice melt rate and SLP and that more than one SLP pattern is associated with anomalously high melt rates. Most high melt rates occur during high pressure over the Arctic Ocean accompanied by low pressure over Siberia, but a strong Beaufort High and advection of warm air associated with a cyclone located over the Taymyr Peninsula can also trigger anomalous high ice melt. We evaluate 10‐member ensemble simulations with the coupled atmosphere‐ice‐ocean Arctic regional climate model HIRHAM‐NAOSIM. The simulations have systematically low acceleration of sea ice melt rate in August, related to shortcomings in representing the strengthening pressure gradient from the Barents/Kara Sea toward Northern Greenland in recent decades. In general, the model shows the same classification of SLP patterns related to anomalous melt rates as the observations. However, the evolution of sea ice melt‐related cloud‐radiation feedback over the summer reveals contrary effects from low‐level clouds in the reanalysis and in the simulations.

Airborne Observations of Summer Thinning of Multiyear Sea Ice Originating From the Lincoln Sea.

To better understand recent changes of Arctic sea ice thickness and extent, it is important to distinguish between the contributions of winter growth and summer melt to the sea ice mass balance. In this study we present a Lagrangian approach to quantify summer sea ice melt in which multiyear ice (MYI) floes that were surveyed by airborne electromagnetic thickness sounding within Nares Strait during summer were backtracked, using satellite imagery, to a region in close proximity (3–20 km) to spring ice thickness surveys carried out in the Lincoln Sea. Typical modal total MYI thicknesses, including ~0.4‐m snow, ranged between 3.9 and 4.7 m in the Lincoln Sea during April. Ice‐only modal thicknesses were between 2.2 and 3.0 m in Nares Strait during August. Total thinning including snow and ice was 1.3 ± 0.1 m including 0.4 ± 0.09 m of snow melt and 0.9 ± 0.2 m of ice melt. This translates to a seasonal net heat input of 305 ± 69 MJ/m2 (262 ± 60 MJ/m2 for ice only) and seasonal net heat flux of 57 ± 13 W/m2 (45 ± 10 W/m2 for ice only), which is unlikely to be explained by solar radiation fluxes alone. Furthermore, our approach provides an improvement on traditional ice mass balance buoy estimates because it integrates melt over larger spatial scales, where melt can be highly variable due to differential melt experienced between melt ponds, bare ice, hummocks, and ridges.

Fingerprints of internal drivers of Arctic sea ice loss in observations and model simulations

The relative contribution and physical drivers of internal variability in recent Arctic sea ice loss remain open questions, leaving up for debate whether global climate models used for climate projection lack sufficient sensitivity in the Arctic to climate forcing. Here, through analysis of large ensembles of fully coupled climate model simulations with historical radiative forcing, we present an important internal mechanism arising from low-frequency Arctic atmospheric variability in models that can cause substantial summer sea ice melting in addition to that due to anthropogenic forcing. This simulated internal variability shows a strong similarity to the observed Arctic atmospheric change in the past 37 years. Through a fingerprint pattern matching method, we estimate that this internal variability contributes to about 40–50% of observed multi-decadal decline in Arctic sea ice. Our study also suggests that global climate models may not actually underestimate sea ice sensitivities in the Arctic, but have trouble fully replicating an observed linkage between the Arctic and lower latitudes in recent decades. Further improvements in simulating the observed Arctic–global linkage are thus necessary before the Arctic’s sensitivity to global warming in models can be quantified with confidence. Internal low-frequency variability in the Arctic atmosphere can explain about half the summer sea ice decline over the past decades, according to an analysis of large ensembles of fully coupled climate model simulations.

Norwegian Sea warm pulses during Dansgaard-Oeschger stadials: Zooming in on these anomalies over the 35–41 ka cal BP interval and their impacts on proximal European ice-sheet dynamics

Abstract The last glacial millennial climatic events (i.e. Dansgaard-Oeschger and Heinrich events) constitute outstanding case studies of coupled atmosphere-ocean-cryosphere interactions. Here, we investigate the evolution of sea-surface and subsurface conditions, in terms of temperature, salinity and sea ice cover, at very high-resolution (mean resolution between 55 and 155 years depending on proxies) during the 35–41 ka cal BP interval covering three Dansgaard-Oeschger cycles and including Heinrich event 4, in a new unpublished marine record, i.e. the MD99-2285 core (62.69°N; -3.57s°E). We use a large panel of complementary tools, which notably includes dinocyst-derived sea-ice cover duration quantifications. The high temporal resolution and multiproxy approach of this work allows us to identify the sequence of processes and to assess ocean-cryosphere interactions occurring during these periodic ice-sheet collapse events. Our results evidence a paradoxical hydrological scheme where (i) Greenland interstadials are marked by a homogeneous and cold upper water column, with intensive winter sea ice formation and summer sea ice melting, and (ii) Greenland and Heinrich stadials are characterized by a very warm and low saline surface layer with iceberg calving and reduced sea ice formation, separated by a strong halocline from a less warm and saltier subsurface layer. Our work also suggests that this stadial surface/subsurface warming started before massive iceberg release, in relation with warm Atlantic water advection. These findings thus support the theory that upper ocean warming might have triggered European ice-sheet destabilization. Besides, previous paleoceanographic studies conducted along the Atlantic inflow pathways close to the edge of European ice-sheets suggest that such a feature might have occurred in this whole area. Nonetheless, additional high resolution paleoreconstructions are required to confirm such a regional scheme.

Mechanism of an Abrupt Decrease in Sea-Ice Cover in the Pacific Sector of the Arctic during the Late 1980s

The recent decline in Arctic sea-ice cover (SIC) shows seasonal and regional characteristics. The retreat of summer sea ice has occurred mainly in the Pacific sector of the Arctic. In this study, using the moving t-test, we found an abrupt change event in the long-term sea-ice area in the Pacific sector in summer 1989. This event was linked to the phase shift of the Arctic Oscillation (AO) or the Northern Annular Mode (NAM). Corresponding with the AO/NAM phase shift from negative to positive, the area of the northern hemisphere stratospheric polar vortex decreased abruptly in winter 1988/89. Comparisons of two periods before (1979–1988) and after (1989–1993) the abrupt decrease in sea ice show that an anomalous winter sea level pressure (SLP) was induced by changes in the polar vortex leading to an anomalous cyclonic ice drift in the Pacific sector. The changes in SLP and wind field persisted into the following spring, resulting in a decrease in SIC and warming of the surface air temperature (SAT). The influence of the spring SLP and SAT on ice persisted into the following summer. Meanwhile, the increased summer net surface heat flux over the ocean and sea ice as a result of the decreased spring ice cover further contributed to the summer sea-ice melt.

Atmosphere and Ocean Impacts on Recent Western Arctic Summer Sea Ice Melt

AbstractSummer sea ice in the western Arctic, specifically in the Chukchi and Beaufort Seas, has been rapidly declining during the last several decades. Annual minima in six recent Septembers (2007–2012) have witnessed the lowest western Arctic sea ice extents of the modern satellite era. The western Arctic summer sea ice has become less extensive amidst a fundamental transition from once widely prevalent and thick multi‐year ice to a thinner and readily melted first‐year ice cover. Questions remain for the Arctic climate research community involving the combination of factors that may be forcing these negative sea ice trends, particularly as current understanding of these complex ice–climate interactions evolves. Interannual variations of atmospheric pressure systems such as the Beaufort Sea High help determine the overlying wind direction and strength as well as the severity of summer ice cover. Atmospheric circulation teleconnections are important in sea ice variability but the role of some, such as the Arctic Oscillation, may in fact have changed as the western Arctic transitioned to a thinner ice cover. Reductions in summer sea ice extent became noticeable from 1998 onward and can be partly attributed to oceanic warming in the northern Pacific across the Bering Strait into the western Arctic. Warming of the northern Atlantic has had dramatic impacts in the Eurasian Arctic but a viable physical mechanism by which it can produce western Arctic ice melt is not yet established. Arctic warming induced by forcing from greenhouse gases is estimated to rival the impact of atmospheric teleconnections and helps produce the “Arctic amplification” of air temperatures that enhances summer sea ice melt. Late summer open water may in turn be altering the atmospheric circulation, weather and climate of subsequent autumn and winter seasons throughout the middle latitudes.

Interaction of an anticyclonic eddy with sea ice in the western Arctic Ocean: an eddy-resolving model study

The dramatic decline of summer sea ice extent and thickness has been witnessed in the western Arctic Ocean in recent decades, which has motivated scientists to search for possible factors driving the sea ice variability. An eddy-resolving, ice-ocean coupled model covering the entire Arctic Ocean is implemented, with focus on the western Arctic Ocean. Special attention is paid to the summer Alaskan coastal current (ACC), which has a high temperature (up to 5°C or more) in the upper layer due to the solar radiation over the open water at the lower latitude. Downstream of the ACC after Barrow Point, a surface-intensified anticyclonic eddy is frequently generated and propagate towards the Canada Basin during the summer season when sea ice has retreated away from the coast. Such an eddy has a warm core, and its source is high-temperature ACC water. A typical warm-core eddy is traced. It is trapped just below summer sea ice melt water and has a thickness about 60 m. Temperature in the eddy core reaches 2–3°C, and most water inside the eddy has a temperature over 1°C. With a definition of the eddy boundary, an eddy heat is calculated, which can melt 1 600 km2 of 1 m thick sea ice under extreme conditions.

Changes in the freshwater composition of the upper ocean west of the Antarctic Peninsula during the first decade of the 21st century

In recent decades, the west Antarctic Peninsula (WAP) has warmed more rapidly than anywhere else in the Southern Hemisphere. Associated with this, there has been a marked shortening of the sea ice season, a retreat of the majority of glaciers, and an increase in precipitation. Each of these changes in the freshwater system has the potential to exert significant influence on the ecosystem, via processes such as stabilisation of the upper water column, and supply of micronutrients to the mixed layer. Here we use a time series of hydrographic and stable oxygen isotope ( δ 18O) measurements collected at a near-coastal site in Marguerite Bay to quantify the prevalence of meteoric freshwater (glacial melt plus precipitation) separately from sea ice melt. During 2002–2009, meteoric water dominated, with summer water column inventories of order 4–6 m. Summer sea ice melt inventories were lower, ranging from −1 to 0.5 m (where a negative value indicates net sea ice formation from this water). In the near-surface layers, we find highest meteoric water prevalence in February 2006 (∼6%) and lowest in October 2007 (∼1%), whilst sea ice melt is highest in February 2005 (2%) and lowest in July 2002 (−2%). The ranges in both meteoric water and sea ice melt are significantly larger than derived previously using a subset of the data, reflecting the strong interannual variability present. The largest single determinant of the near-surface freshwater percentages is found to be changes in mixed layer depth. Notably deep layers occurred in the winters of 2003, 2007 and 2008, due to northerly winds associated with El Niño/Southern Oscillation and the Southern Annular Mode. These led to greatly reduced sea ice cover in northern Marguerite Bay, and allowed persistent air-sea heat fluxes and stronger rates of sea ice production, which is a key factor in controlling mixed layer depth. We also discuss the possible role of interannual changes in wind-induced mixing in this context. As climate change at the WAP continues, we expect further changes in each of the components of the freshwater budget, and also changes in the vertical redistribution of this freshwater by oceanographic processes. Our ongoing δ 18O monitoring will help track these changes, and elucidate their consequences for the operation of the marine ecosystem.

Paleoenvironment changes in the NW Okhotsk Sea for the last 18 kyr determined with micropaleontological, geochemical, and lithological data

Lithological, geochemical, and micropaleontological data indicate that the Late Glacial of the northwestern Okhotsk Sea (OS) is characterised by severe climatic and environmental conditions with mainly perennial sea ice coverage and low productivity accompanied by weak deep-water ventilation and a temperate formation of the upper Sea of Okhotsk Intermediate Water (SOIW). The age model of the studied core sediments was constructed by AMS 14C dating. The most severe environmental conditions occurred during the period 15.8–14.8 kyr, synchronous with cold Heinrich event 1. Insignificant regional environmental amelioration accompanied by an increase of productivity and ice weakening during summer occurred almost simultaneously with the Bølling–Allerøed (BA) warming. The obtained results distinguished both the Bølling and Allerøed warmings as having different environmental conditions. Oxygen content in the surface sediment was low, as seen from the production of the benthic foraminifera (BF) species. During 12.6–11.1 kyr, synchronous with the Younger Dryas (YD) cold event, the regional environment conditions were cold, but not as severe as the glacial ones. Some climatic warming since the Preboreal has stimulated sea ice melting and surface amelioration during the summer season, which in turn led to a productivity rise and changes in the water column and bottom environment. Some increase in the surface water stratification and the intensified oceanic diatom and surface radiolarian production is parallel with the development of a mesopelagic regime of productivity. The surface sediment condition favours BF abundance and domination by BF species tolerant to oxygen deficiencies. During the Boreal period more stable surface conditions were accompanied by continuously high productivity and an intensifying of its mesopelagic regime.Significant regional climate warming since the Atlantic (9 kyr ago) strongly intensified the summer sea ice melting in the OS, and this created considerable surface environment amelioration with the preferential transport of bacteria and phytodetritus into the SOIW. Further considerable warming of the regional climate from 6 kyr ago contributed to slight sea ice changes, surface water warming, and the enhancement of its stratification; all typical for most of the OS. Along with a high nutrient supply from the Amur River, the NW OS experienced a strong diatom production increase with the maximum amount occurring during the last 3.6 kyr. This changed the productivity type and organic matter export into the water column while increasing the feeding of the “productive” Plagoniidae spp . group and decreasing the microbial biomass supply into the upper SOIW. Some sea surface water cooling or saltier conditions at the beginning of the Subatlantic (2.4–1.8 kyr) was followed by its warming or freshening 1.5–1.0 kyr ago, which likely correlated with the Medieval Warm Period. In turn, that probably led to strong surface water stratification, productivity deterioration and considerable changes in the overall NW OS environment. The established sequence of the northwestern OS environmental changes during the Late Glacial–Holocene is related to the Northern Hemispheric climate changes and was likely forced by atmospheric teleconnection in line with the polar circulation index variability.

Rapid changes in surface water carbonate chemistry during Antarctic sea ice melt

The effect of sea ice melt on the carbonate chemistry of surface waters in the Weddell–Scotia Confluence, Southern Ocean, was investigated during January 2008. Contrasting concentrations of dissolved inorganic carbon (DIC), total alkalinity (TA) and the fugacity of carbon dioxide (fCO2) were observed in and around the receding sea ice edge. The precipitation of carbonate minerals such as ikaite (CaCO3·6H2O) in sea ice brine has the net effect of decreasing DIC and TA and increasing the fCO2 in the brine. Deficits in DIC up to 12 ± 3 ¼mol kg-1 in the marginal ice zone (MIZ) were consistent with the release of DIC-poor brines to surface waters during sea ice melt. Biological utilization of carbon was the dominant processes and accounted for 41 ± 1 ¼mol kg-1 of the summer DIC deficit. The data suggest that the combined effects of biological carbon uptake and the precipitation of carbonates created substantial undersaturation in fCO2 of 95 ìatm in the MIZ during summer sea ice melt. Further work is required to improve the understanding of ikaite chemistry in Antarctic sea ice and its importance for the sea ice carbon pump.

A numerical study of ice-drift divergence by cyclonic wind with a Lagrangian ice model

In polar regions low-pressure systems drive sea ice divergence, which can accelerate summer sea ice melt through energy absorption at resulting open water areas. This paper examines the mechanisms that cause the ice divergence and its seasonal change with a Lagrangian ice model. We focus on the effects of initial ice concentration, ice strength and ocean stratification. A series of idealized simulations (initially at 5 km resolution) are carried out with a Rankine combined vortex as external wind forcing. We have found a characteristic length scale r *1 in free drift, based on the influence of the Coriolis term. The results show that ice concentration decreases most greatly within the range of r *1. In addition, ice divergence becomes small in the inner region for high concentrations (i.e. over 0.95), due to inward internal force blocking divergent deformations. The effects of ocean stratification on ice-drift divergence are also examined. Numerical results show that as the density stratification increases divergent Ekman flows beneath ice further promote the ice-drift divergence and lead to more reduction in the ice concentration through thinning of surface Ekman layer.