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We study hysteresis properties of the Atlantic Meridional Overturning Circulation (AMOC) under a slowly‐varying North Atlantic (20°N–50°N) freshwater flux forcing in state‐of‐the‐art global climate model (GCM), the Community Earth System Model. Results are presented of a full hysteresis simulation (4,400 model years) and show that there is a hysteresis width of about 0.4 Sv. This demonstrates that an AMOC collapse and recovery do not only occur in conceptual and idealized climate models, but also in a state‐of‐the‐art GCM. The AMOC recovery is about a factor six faster than the AMOC collapse and this asymmetry is due to the major effect of the North Atlantic sea‐ice distribution on the AMOC recovery. The results have implications for projections of possible future AMOC behavior and for explaining relatively rapid climate transitions in the geological past. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC) is considered to be a tipping element in the climate system. We here simulate AMOC tipping events by slowly varying the North Atlantic freshwater forcing in a state‐of‐the‐art global climate model. The AMOC collapses when this forcing is sufficiently large and, when reversing this forcing, the recovery occurs at much smaller values of the forcing than the collapse, giving rise to hysteresis behavior. The AMOC changes are much faster during its recovery than during its collapse and this asymmetry is caused by the effect of the North Atlantic sea‐ice distribution on the AMOC recovery. The results demonstrate that AMOC tipping does not only occur in simplified climate models, but in a large hierarchy of climate models. Key Points: A wide Atlantic Meridional Overturning Circulation (AMOC) hysteresis is found in a state‐of‐the‐art global climate modelThe AMOC recovery is about six times faster than the AMOC collapseThe North Atlantic sea‐ice cover plays a dominant role in the hysteresis asymmetry [ABSTRACT FROM AUTHOR]

The Antarctic Marginal Ice Zone (MIZ) accounts for 15% of the Southern Ocean's primary production (PP), but limited data has hindered understanding of its variability and connection to carbon export. Using a combination of gliders, biogeochemical Argo floats and satellite observations in the northeast Weddell Sea, we show that years with more sea‐ice formation over winter are followed by more intense phytoplankton blooms (∼15% greater daily PP) and export to 100 m (∼50% higher daily carbon export) the following summer. However, the carbon export beyond the deepest winter mixed layer did not vary in proportion to PP, suggesting different drivers of carbon export at depth compared to surface waters. Furthermore, across the entire MIZ, the response of blooms to sea‐ice volume was spatially variable, indicating the need to consider spatial heterogeneity in the response of the biological carbon pump to future sea‐ice changes. Plain Language Summary: Algae in the ocean surface take up carbon dioxide from the atmosphere through photosynthesis and transfer it to the deep ocean when they die and sink. This process is key to maintaining a habitable planet and is known as the biological carbon pump (BCP). The seasonally ice‐covered ocean around Antarctica is one of the most active areas for algal growth, but also a region of rapid climate change. Because of the difficulty in taking measurements in this remote region, the physical and biological processes that control the growth and sinking of algae and its response to changing sea‐ice remain uncertain. In this study, we use a combination of satellites and autonomous robots to elucidate the role of sea‐ice variability on the BCP. We find that sea‐ice impacts algal growth by its influence on both the light and nutrient conditions needed for photosynthesis. Predicting the amount of algae that subsequently sinks to depth as carbon flux, although influenced by sea‐ice conditions, is more complex and linked to the greater marine ecosystem. Evidence suggests that the species of algae, zooplankton grazing, and the rate at which dead algae breaks down and sinks are important and should be a focus point for further research. Key Points: High‐resolution in‐situ observations are used to characterize multi‐year phytoplankton bloom phenology and amplitude in the Antarctic Marginal Ice ZoneYears with greater sea‐ice volume drive deeper mixing that tend to support higher magnitude blooms in the northeast Weddell SeaCarbon export efficiency is affected by bloom magnitude, community composition and water column stratification [ABSTRACT FROM AUTHOR]

This study investigates the interdecadal variation of the intensity of the interannual variability (IIV) of surface air temperature (SAT) over mid‐ and high‐latitude Asia (MHLA) in winter months (December–February) during 1980–2015 and the role of the Arctic sea‐ice. Results show only the IIV of February SAT is significantly enhanced around the late‐1990s, which partly results from enhanced IIV of the frequency of extreme warm and cold events (FEWE and FECE). The influence of the sea‐ice in the Barents Sea from December to February on the February SAT, FEWE and FECE over MHLA is remarkably enhanced around the late‐1990s. During 1998–2015, the sea‐ice anomalies contribute to the enhanced IIV of February SAT over MHLA by modulating the monthly and synoptic tripole patterns over northern Eurasia, the positive phase of which exhibits an anomalous low over Eurasia north of 60°N and anomalous highs over southern Europe and MHLA. On the monthly average, the increased sea‐ice induces Rossby waves and then a positive phase of the monthly tripole pattern, inducing weakened East Asian trough and Siberian high, enhanced westerlies over 50°–70°N, and resultant decreased blocking high frequency over Eurasia north of 60°N. These anomalies favor increased February SAT over MHLA. On synoptic timescales, the increased blocking over the north side of the Ural Mountains favors more (fewer) extreme positive (negative) phases of the synoptic tripole pattern and hence increased FEWE (decreased FECE) over MHLA, further strengthening the increased SAT. Numerical experiments basically confirm the mechanisms above. Key Points: Intensity of interannual variability of February surface air temperature over mid‐ and high‐latitude Asia reinforces after 1998Intensity of interannual variability of frequency of February extreme warm and cold events over the region is enhanced after 1998Sea‐ice anomalies in the Barents Sea play an important role via modulating monthly and synoptic circulation over northern Eurasia [ABSTRACT FROM AUTHOR]

Correctly representing the snow on sea‐ice has great potential to improve cryosphere‐atmosphere coupling in forecasting and monitoring (e.g., reanalysis) applications, via improved modeling of surface temperature, albedo and emissivity. This can also enhance the all‐weather all‐surface coupled data assimilation for atmospheric satellite radiances. Using wintertime observations from two Arctic field campaigns, SHEBA and N‐ICE2015, and satellite data, we explore the merits of different approaches to represent the snow over sea‐ice in a set of 5‐day coupled forecasts. Results show that representing the snow insulation effects is essential for capturing the wintertime surface temperature variability over sea‐ice and its response to changes in the atmospheric forcing. Modeling the snow over sea‐ice improves the representation of strong cooling events, reduces surface temperature biases in clear‐sky conditions and improves the simulation of surface‐based temperature inversions. In clear‐sky conditions, when using a multi‐layer snow scheme the root‐mean‐squared error in the surface temperature is reduced by about 60% for both N‐ICE2015 and SHEBA. This study also highlights the role of compensating errors in different components of the surface energy budget in the Arctic boundary layer. During warm air intrusions, errors in the surface temperature increase when cloud phase and cloud radiative processes are misrepresented in the model, inducing large errors in the net radiative energy at the surface. This work indicates that numerical weather prediction systems can fully benefit from a better representation of snow over sea‐ice, for example, with multi‐layer snow schemes, combined with improvements to other boundary layer processes including mixed phase clouds. Plain Language Summary: Correctly representing the snow on sea‐ice in coupled numerical weather prediction models has great potential to improve weather forecast and climate monitoring applications, such as climate reanalyses, which are usually produced using such systems. In this study two different methodologies to account for the effect of the snowpack accumulating over the sea‐ice in 5‐day global forecasts are compared. All experiments are performed with the ECMWF Integrated Forecast System. The presence of snow over sea‐ice enables an improved representation of extreme low temperatures occurring in the Arctic, and so reducing surface temperature errors in clear‐sky conditions. This study also investigates feedbacks between errors in processes controlling the surface temperature evolution in the wintertime Arctic boundary layer, and how more realistic representations of the snowpack accumulating on sea‐ice are affected by these. This work indicates that numerical weather prediction systems can fully benefit from a better representation of snow over sea‐ice, for example, with a multi‐layer snow scheme, combined with further improvements to other boundary layer processes. Key Points: Accounting for the snow over sea‐ice enables strong cooling events and extreme low temperatures in the Arctic to be better representedAccounting for the snow over sea‐ice improves the representation of Arctic winter states in the ECMWF Integrated Forecasting SystemCompensating errors between cloud and surface processes are a key factor affecting the near‐surface forecast biases in the Arctic [ABSTRACT FROM AUTHOR]

SWIM on board CFOSAT is the first spaceborne, low incidence, rotating scatterometer, aiming at measuring in near‐real time ocean waves spectra. With five off‐nadir beams at incidences between 2° and 10° plus one nadir beam, it covers the Earth in 13 days, including polar regions thanks to its polar orbit. This work aims at exploiting SWIM data over ice regions with two objectives. An off‐nadir data‐based sea‐ice flag is here proposed that allows, first, to eliminate sea‐ice polluted echoes for improving the wave spectrum retrieval, and second, to open perspectives for application of sea‐ice monitoring with near‐nadir Ku‐band active sensors. To this end, the signature of both open water and sea‐ice radar backscatter is parameterized into Geophysical Model Functions. Then, comparisons with observed profiles through a Bayesian scheme provide a probability of sea‐ice presence. After comparison with both model (ECMWF‐IFS) and radiometer (SSMI) derived reference data sets, the proposed flag is found to be ready for operational use. At latitudes greater than 40° in absolute value, the proposed flagging algorithm exhibits accuracies of approximately 98% for all beams compared to SSMI data. Beam to beam performances are characterized and show potential for the characterization of sea‐ice at Ku‐band. Key Points: For the first time, sea‐ice is detected globally from Ku‐band off‐nadir up to 11° incidence thanks to the SWIM/CFOSAT data setsFully analytical Geophysical Model Functions are derived for the Normalized Radar Cross‐Section over open water and sea‐iceA comparison of this new sea‐ice flag with SSMI ice concentration shows a high accuracy of our algorithm [ABSTRACT FROM AUTHOR]

Biological communities in the Arctic are changing through the climate‐driven encroachment of subarctic species. This "Atlantification" extends to keystone Calanoid copepods, as the small‐bodied Calanus finmarchicus increases in abundance in areas where it overlaps with larger Arctic congeners. The environmental factors that are facilitating this shift, whether related to optimal conditions in temperature or seasonality, remain unclear. Assessing these drivers at an Arctic‐wide scale is necessary to predict future ecosystem change and impacts. Here we have compiled range‐wide occurrences of C. finmarchicus and a suite of seasonal biophysical climatologies to build a boreo‐Arctic ecological niche model. The data set was divided into two eras, 1955–1984 and 1985–2017, and an optimized MaxEnt model was used to predict the seasonal distribution of the abiotic niche of C. finmarchicus in both eras. Comparing outputs between eras reveals an increase in habitat suitability at the Arctic range edge. Large and significant increases in suitability are predicted in the regions of the Greenland, Labrador, and Southern Barents Seas that have experienced reduced sea‐ice cover. With the exception of the Barents Sea, these areas also show a seasonal shift in the timing of peak habitat suitability toward an earlier season. Our findings suggest that the Atlantification of Arctic zooplankton communities is accompanied by climate‐driven phenology changes. Although seasonality is a critical constraint to the establishment of C. finmarchicus at Arctic latitudes, earlier sea‐ice retreat and associated productivity is making these environments increasingly favorable for this subarctic species. [ABSTRACT FROM AUTHOR]

Studies have indicated that marine heatwaves (MHWs) have had severe impacts on the marine ecosystem in the Pacific, Atlantic, and Indian Oceans, but there have been few studies focused on MHWs in the Arctic. On the other hand, amplified rapid warming in the Arctic region makes it a hotspot strategically and economically worldwide. In this study, we documented that the average intensity of MHWs in the Arctic was comparable with that in the other regions of the global oceans. The annual intensity, frequency, duration, and areal coverage of MHWs have increased significantly in recent decades. The increase of the annual duration is mainly owing to the postponed end time, thus the prolonged periods, of the MHW seasons. Our analysis indicates that the increasing trends of the annual intensity, frequency, duration, and areal coverage in the Arctic are closely associated with the increasing surface air temperature and decreasing sea‐ice concentration under the global warming environment. These features are robust across three different sea surface temperature (SST) products and using different MHW criteria. Plain Language Summary: Events of extremely warm waters in the oceans are known as marine heatwaves (MHWs). Past MHW research has used sea surface temperature (SST) to diagnose MHWs and has focused on the tropical and subtropical oceans. The questions we address here are: (a) Are there any MHW events in the Arctic region where SSTs are generally low? and (b) Are the MHWs weaker or stronger in the Arctic than in the other oceans? Our study indicates that: (a) MHWs do exist in the Arctic, (b) their strengths increase with time, and (c) they are stronger than those in the other oceans in the most recent decades. These MHWs may have a strong impact on the Arctic bio‐ecosystem due to their low heat tolerance since the seasonal variation of SST in the Arctic is small. Key Points: The average marine heatwaves (MHWs) in the Arctic are as strong as in the other ocean basins in 1982–2020The annual intensity are stronger in 2000–2020 than in 1982–2000, which is consistent with changes in air temperature, sea‐ice, and cloudThe increase of annual duration is largely associated with the postponed end time of the MHW seasons [ABSTRACT FROM AUTHOR]

This study suggests a possible mechanism of how the Arctic sea‐ice loss can influence the mid‐latitude climate in the Eurasian continent. It is shown that the low sea‐ice concentration over the Barents‐Kara‐Laptev Seas in autumn typically leads to cold Eurasian in winters. It is demonstrated that the Arctic‐to‐midlatitude connection depends on the state of late autumn atmospheric circulation. When the autumn sea‐ice reduction is accompanied by anticyclonic circulation over northern Eurasia, Eurasia becomes anomalously cold in the early winter. However, when cyclonic circulation is dominant, Eurasian cold anomalies appear in the late winter. This seasonally delayed response is further found to be related to the wind‐driven sea‐ice drift that causes warm anomalies over the Barents‐Kara Seas in the following winter. These observational results are confirmed by model simulations, indicating that the recent Eurasian cold winters could be linked to their forced response to the Arctic sea‐ice loss. Plain Language Summary: In recent decades, midlatitude continents frequently experienced severe winters despite increasing greenhouse gas concentrations. Several observational and modeling studies have suggested that the Arctic‐to‐midlatitude connection might have played a central role in the cold winters. However, it is still controversial whether the Arctic‐to‐midlatitude connection is indeed driven by the Arctic sea‐ice variability or is simply the result of internal atmospheric processes. Here we examine that the Autumn sea‐ice loss in the Kara‐Barents‐Laptev Seas can induce Eurasian cooling in the late winter, suggesting a several‐month time delay. We suggest that the sea ice‐wind coupling enables to make the time delay, which eventually induces the warm Arctic‐cold continent pattern in the late winter. Our results support that the recent Eurasian cold winter could be partly contributed by the forced response to the Arctic sea‐ice decline. Key Points: The regional autumn Arctic sea‐ice loss has a significant delayed impact on Eurasian cold wintersThe Arctic‐to‐midlatitude connection depends on the state of late autumn atmospheric circulation over northern EurasiaThe sea ice‐wind coupling plays a role in delivering Autumn sea‐ice signal to the late winter [ABSTRACT FROM AUTHOR]

Temperature and precipitation change more strongly in the Arctic than at lower latitudes, with central Arctic boreal winter precipitation projected to double in the 21st century in a high‐emission scenario. This enhanced hydrological sensitivity has been explained in terms of the Arctic moisture budget and attributed to either moisture advection or surface evaporation. Here, we show that boreal winter moisture availability is less sensitive to surface temperature change in the Arctic than at lower latitudes, questioning a central role of the moisture budget. Hydrological sensitivity, that is, precipitation change per unit temperature change, is similar in models with or without sea‐ice changes, suggesting a secondary role of surface flux changes. Instead, we propose that the Arctic's larger hydrological sensitivity is energetically driven. Increases in latent heat release from precipitation locally balance increased atmospheric radiative cooling in Arctic winter, consistent with process‐level understanding of radiatively driven cloud and precipitation formation. Plain Language Summary: As Earth's climate warms, globally averaged precipitation increases. Both warming and precipitation increase are larger in the Arctic than at lower latitudes. In the atmospheric energy budget, changes in precipitation correspond to changes in latent heat release and must be balanced by advection, surface fluxes, or radiation. This article shows that stronger Arctic than lower‐latitude precipitation increases are mostly driven by stronger radiative loss of energy to space. Previous research had suggested that increased evaporation following the retreat of sea ice was a key driver of Arctic precipitation increases. Understanding the causes of amplified Arctic precipitation change is important to build confidence in model projections and to focus on the most important processes in further research and model development. The link between hydrological and radiative changes also allows to reconcile these two perspectives on Arctic amplification that have so far often been discussed as opposing views. Key Points: Precipitation change is greater in the Arctic than anywhere else outside the deep Tropics in boreal winterLarger Arctic hydrological sensitivity is energetically driven and independent of sea‐ice changesArctic increases in atmospheric radiative cooling closely match increases in latent heat release from precipitation [ABSTRACT FROM AUTHOR]

We present a 500‐year history of naturally felled driftwood incursion to northern Svalbard, directly reflecting regional sea ice conditions and Arctic Ocean circulation. Provenance and age determinations by dendrochronology and wood anatomy provide insights into Arctic Ocean currents and climatic conditions at a fine spatial resolution, as crossdating with reference chronologies from the circum‐Arctic boreal forests enables determination of the watershed the driftwood originated from. Sample crossdating may result in a wide range of matches across the pan‐boreal region, which may be biased toward regions covered by the reference chronologies. Our study considers alternate approaches to selecting probable origin sites, by weighting scores via reference chronology span and visualizing results through spatiotemporal density plots, as opposed to more basic ranking systems. As our samples come from naturally felled trees (not logged or both), the relative proportions of different provenances are used to infer past ocean current dominance. Our record indicates centennial‐to decadal‐scale shifts in source regions for driftwood incursion to Svalbard, aligning with Late Holocene high variability and high frequency shifts in the Transpolar Drift and Beaufort Gyre strengths and associated fluctuating climate conditions. Driftwood occurrence and provenance also track the northward ice formation shift in peripheral Arctic seas in the past century. A distinct decrease in driftwood incursion during the last 30 years matches the observed decline in pan‐Arctic sea ice extent in recent decades. Our new approach successfully employs driftwood as a proxy for Arctic Ocean surface circulation and sea ice dynamics. Plain Language Summary: We present a 500‐year history of driftwood arriving to the shorelines of northern Svalbard. Driftwood in the Arctic results from dying trees entering the large rivers that drain the circum‐Arctic land masses, which upon flowing into the Arctic Ocean can become locked up in forming sea ice. This enables the wood to travel across the Arctic Ocean without sinking, making it an invaluable proxy for sea ice extent by recording variations in Arctic Ocean surface currents (and therefore sea ice drift) and ice cover. With comparison of tree ring width (TRW) measurements of these driftwood samples to TRW series from trees throughout the boreal forests, we can determine the region each sample came from. We can thus approximate its likely trajectory across the Arctic Ocean. Arctic sea ice is rapidly declining in extent and thickness, with impacts on local and global climatic and ecological conditions. Knowledge of past changes is needed to place this modern trend within a broader context to aid future predictions for Arctic sea ice. Our record matches the observational record of Arctic Ocean surface circulation patterns and climate conditions, supporting the use of driftwood as a proxy for Arctic Ocean surface current and sea ice dynamics. Key Points: We present a novel approach to utilizing naturally felled driftwood as a proxy for Arctic Ocean surface circulation and sea ice dynamicsA 500‐year record of driftwood incursion to Svalbard reflects centennial to decadal variability in surface circulation and sea ice extentA distinct decrease in driftwood from all provenances in the last 30 years matches the observed decline in pan‐Arctic sea ice extent [ABSTRACT FROM AUTHOR]

Antarctica and the surrounding Southern Ocean act as an important sink in the global mercury cycle; however, corresponding studies of atmospheric mercury species in these regions are still scarce. Here, we report large‐scale observations of atmospheric gaseous elemental mercury (GEM) and gaseous oxidized mercury (GOM) in the Antarctic marine boundary layer (MBL) taken during a summer cruise. There is large variability in the spatial distribution of GOM, which is likely attributable to the diverse land surface types, including coastal Antarctica, sea‐ice region, and oceanic region, along the cruise route. In coastal Antarctica, the highest GOM level (56.23 ± 47.74 pg·m−3) might be attributed to the significant in‐situ oxidation there. Significant in‐situ oxidation of GEM could also occur in the sea‐ice region, causing the significant increase of GOM (up to 87.01 pg·m−3), while the uptake of the high content of sea‐salt aerosols in sea‐ice regions might efficiently eliminate GOM in the air, resulting in generally lower content of GOM (13.10 ± 14.48 pg·m−3) in the sea‐ice region. In the oceanic region, the lowest level of GOM (3.95 ± 4.69 pg·m−3) is due to both the uptake of sea‐salt aerosols and the seasonal melting of first‐year sea‐ice. This study provides insight to understand the mechanisms of the atmospheric mercury cycle in various land surface types in the Antarctic MBL. Key Points: Large spatial variability in gaseous oxidized mercury over different land surface types was observed in the Southern OceanLow gaseous oxidized mercury content in the sea‐ice region would be to some extent attributed to its greater uptake by sea salt aerosolsThe seasonal melting of first‐year sea ice can cause a clear decrease in gaseous oxidized mercury levels [ABSTRACT FROM AUTHOR]

All coastal systems experience disturbances and many across the planet are under unprecedented threat from an intensification of a variety of stressors. The West Antarctic Peninsula is a hotspot of physical climate change and has experienced a dramatic loss of sea‐ice and glaciers in recent years. Among other things, sea‐ice immobilizes icebergs, reducing collisions between icebergs and the seabed, thus decreasing ice‐scouring. Ice disturbance drives patchiness in successional stages across seabed assemblages in Antarctica's shallows, making this an ideal system to understand the ecosystem resilience to increasing disturbance with climate change. We monitored a shallow benthic ecosystem before, during and after a 3‐year pulse of catastrophic ice‐scouring events and show that such systems can return, or bounce back, to previous states within 10 years. Our long‐term data series show that recovery can happen more rapidly than expected, when disturbances abate, even in highly sensitive cold, polar environments. [ABSTRACT FROM AUTHOR]

We explore the climate system response to abrupt CO2 forcing, spanning the range 1× to 8×CO2, with two state‐of‐the‐art coupled atmosphere‐ocean‐sea‐ice‐land models: the NASA Goddard Institute for Space Studies Model E2.1‐G (GISS‐E2.1‐G) and the Community Earth System Model (CESM‐LE). We find that the effective climate sensitivity is a non‐monotonic function of CO2 in both models, reaching a minimum at 3×CO2 for GISS‐E2.1‐G, and 4×CO2 for CESM‐LE. A similar non‐monotonic response is found in Northern Hemisphere surface temperature, sea‐ice, precipitation, the latitude of zero precipitation‐minus‐evaporation, and the strength of the Hadley cell. Interestingly, the Atlantic meridional overturning circulation collapses when non‐monotonicity appears and does not recover for larger CO2 forcings. Analyzing the climate response over the same CO2 range with slab‐ocean versions of the same models, we demonstrate that the climate system's non‐monotonic response is linked to ocean dynamics. Plain Language Summary: We perform runs with two different models using CO2 concentrations in the atmosphere higher (from 1× to 8×CO2) relative to pre‐industrial conditions, in order to explore how the effective climate sensitivity (ECSeff) and the entire climate system change with increasing CO2. We show that ECSeff is a non‐monotonic function of CO2, minimizing at 3×CO2 in one model and 4×CO2 in the other. A similar non‐monotonic response appears in precipitation, sea‐ice, the edge of the dry zone, and Hadley cell strength. Interestingly, the Atlantic Meridional Overturning Circulation, which brings warm water into the North Atlantic, also shuts down at the same forcings when ECSeff is minimum and does not recover for higher forcings. We further show that the non‐monotonic response of the climate system stems from changes in ocean dynamics. Key Points: We examine the response of the climate system to abrupt 1.5×, 2×, 3×, 4×, 5×, 6×, 7×, and 8×CO2 forcing with two different coupled modelsClimate sensitivity, sea‐ice extent, global precipitation and the atmospheric circulation respond non‐monotonically across this range of CO2The non‐monotonicity of the response is associated with changes in ocean dynamics, notably over the North Atlantic [ABSTRACT FROM AUTHOR]

During the past four decades significant decrease in Arctic sea ice and a dramatic ice mass loss of the Greenland Ice Sheet (GIS) has been coincident with global warming and an increase in atmospheric CO2. In Northeast Greenland significant mass loss from the outlet glaciers Nioghalvfjerdsbræ (79NG) and Zachariæ Isstrøm (ZI) and intensive seasonal breakup of the local Norske Øer Ice Barrier (NØIB) have also been observed since 2000. In order to better understand the processes driving these modern changes, studies of paleoclimate records are important and of major societal relevance. A multiproxy study including organic‐biogeochemical and micropaleontological proxies was carried out on a marine sediment core recovered directly in front of 79NG. Data from Core PS100/270 evidenced a strong inflow of warm recirculating Atlantic Water across the Northeast Greenland shelf from the early Holocene between ~10 and 7.5 ka. An overall high in phytoplankton productivity occurred within a stable sea ice margin regime, accompanied by 79NG retreat most probably triggered by peak solar insolation and changes in the local ocean circulation. Enhanced basal melt of the underside of 79NG at ~7.5 ka then led to the total disintegration of the ice shelf. The released freshwater would have driven water column stratification and promoted the formation of the local landfast ice barrier, which is shown by lowered biomarker values and foraminifera abundances toward the end of the early Holocene. Near perennial sea ice conditions with short summers and 79NG retreat to the inner fjord then prevailed from ~7.5 to ~0.8 ka. Key Points: Multiproxy record allowed reconstruction of NEG ice sheet retreat and sea ice history from the last deglaciation to the late HoloceneIncreased inflow of warm recirculating Atlantic Water linked to retreat and disintegration of the 79NG between 10 and 7.5 kaExtended to even perennial sea ice conditions were predominant on the inner NE Greenland shelf since 7.5 ka [ABSTRACT FROM AUTHOR]

The seasonal ice zone around both the Arctic and the Antarctic coasts is typically characterized by warm and salty waters underlying a cold and fresh layer that insulates sea‐ice floating at the surface from vertical heat fluxes. Here, we explore how a mesoscale eddy field rubbing against ice at the surface can, through Ekman‐induced vertical motion, bring warm waters up to the surface and partially melt the ice. We dub this the "Eddy‐Ice‐Pumping" (EIP) mechanism. When sea‐ice is relatively motionless, underlying mesoscale eddies experience a surface drag that generates Ekman upwelling in anticyclones and downwelling in cyclones. An eddy composite analysis of a Southern Ocean eddying channel model, capturing the interaction of the mesoscale with sea‐ice, shows that within the compact ice zone, the mixed layer depth (MLD) is shallow in anticyclones (∼20 m) due to sea‐ice melt and deep in cyclones (∼50–200m) due to brine rejection. "EIP" warms the core of anticyclones without significantly affecting the temperature of cyclones, producing a net upward vertical heat flux that reduces the mean sea‐ice thickness by 10% and shoals the MLD by 60% over the course of winter and spring. In the following months, the sea‐ice thickness recovers with an overshoot, due to strong negative feedbacks associated with atmospheric cooling and salt stratification. Consequently, the effect of "EIP" does not accumulate over the years, but modulates the seasonal cycle of ice within the compact ice zone. Plain Language Summary: Polar oceans typically have cold, fresh waters at the surface and warmer, saltier waters below that. When the atmospheric temperature is cold enough, the top layer of the ocean cools to the freezing point and sea‐ice forms at the surface. This growth can be impeded by warmer waters at the subsurface, whose heat can melt sea‐ice at its base. The effectiveness of this melting depends on the amount of heat transported from the deeper layers of the ocean up to the surface underneath the ice. This study explores a novel mechanism by which frictional interactions between ocean and sea‐ice can increase the amount of heat delivered to the surface. At small scales (30–100 km), when the ocean rubs against relatively stationary sea‐ice, it experiences a torque that drives vertical motions in the water column. This process brings warmer waters in contact with sea‐ice and can reduces its mean thickness by 10% over the course of winter and spring. In the following months, the ice thickness recovers due to restoring processes, such that this mechanism does not lead to accumulated melt over the years, but changes the seasonality of sea‐ice. Key Points: Sea‐ice drag on mesoscale ocean eddies can generate Ekman pumping and bring warm waters up to the surfaceIn a process model, this reduces the ice thickness by 10% and the mixed layer depth by up to 60% in winter and springSea‐ice formation (melt) in cyclones (anticyclones) produce deeper (shallower) mixed layer depths [ABSTRACT FROM AUTHOR]

Nitric oxide (NO) is widely recognized as an important transmitter molecule in biological systems, from animals to plants and microbes. However, the role of NO in marine photosynthetic microbes remains unclear and even less is known about the role of this metabolite in Antarctic sea‐ice diatoms. Using a combination of microsensors, microfluidic chambers, and artificial sea‐ice tanks, a basic mechanistic insight into NO's dynamics within the Antarctic sea‐ice diatom Fragilariopsis cylindrus was obtained. Results suggest that NO production in F. cylindrus is nitrite‐dependent via nitrate reductase. NO production was abolished upon exposure to light but could be induced in the light when normal photosynthetic electron flow was disrupted. The addition of exogenous NO to cellular suspensions of F. cylindrus negatively influenced growth, disrupted photosynthesis, and altered non‐photochemical dissipation mechanisms. NO production was also observed when cells were exposed to stressful salinity and temperature regimes. These results suggest that during periods of environmental stress, NO could be produced in F. cylindrus as a "stress signa" molecule. [ABSTRACT FROM AUTHOR]

Submesoscale flows in the ocean are energetic motions, O(1–10 km), that influence stratification and the distributions of properties, such as heat and carbon. They are believed to play an important role in sea‐ice‐impacted oceans by modulating air‐sea‐ice fluxes and sea‐ice extent. The intensity of these flows and their response to wind forcing are unobserved in the sea‐ice regions of the Southern Ocean. We present the first submesoscale‐resolving observations in the Antarctic marginal ice zone (MIZ) collected by surface and underwater autonomous vehicles, for >3 months in austral summer. We observe salinity‐dominated lateral density fronts occurring at sub‐kilometer scales. Surface winds are shown to modify the magnitude of the mixed‐layer density fronts, revealing strongly coupled atmosphere‐ocean processes. We posture that these wind‐front interactions occur as a continuous interplay between front slumping and vertical mixing, which leads to the dispersion of submesoscale fronts. Such processes are expected to be ubiquitous in the Southern Ocean MIZ. Plain Language Summary: Satellite radar imagery shows evidence of 1–10 km eddies and jets in the ocean adjacent to the sea‐ice edge around Antarctica. We use field observations of temperature, salinity, and wind speed from autonomous robotic platforms deployed in the sea‐ice zone for >3 months. These measurements provide estimates of the surface ocean density fronts which are controlled primarily by lateral variations in salinity. We show that, during high wind speeds, these surface fronts temporarily dissipate, indicating an atmosphere‐ocean coupling occurring at the submesoscale. The fronts strengthen again during low wind speed, which is thought to be because the stirring of the fresher surface layer by mesoscale eddies leads to the generation of the submesoscale fronts. Providing in situ observations of such features improves our understanding of the small‐scale ocean and climate processes at play, such as how heat and carbon may exchange between the atmosphere and the ocean. Key Points: Using combined surface and underwater robotic observations, we observe haline‐dominated submesoscale fronts in the Antarctic MIZEnhanced wind speeds reduce the magnitude of lateral submesoscale fronts within the surface mixed layerSubmesoscale wind‐front interactions cause a continuous interplay between front slumping and vertical mixing, arresting lateral shear [ABSTRACT FROM AUTHOR]

The Arctic Ocean (AO) and its associated marginal seas have recently experienced rapid climate and environmental changes, most notably sea‐ice area (SIA) loss and warming potentially impacting its uptake of carbon dioxide (CO2). We used the state‐of‐the‐art ECCO2‐Darwin coupled ocean‐biogeochemistry model to simulate the 2006–2013 period and investigate the impact of changing SIA on the CO2 uptake of the AO. We find that the mean annual CO2 sink of the AO is 153 ± 14 TgC a−1 and the CO2 sink decreased at a rate of 3.6 TgC a−1 even though SIA decreased by 8 × 104 km2 a−1 over the same period. Extreme SIA loss in 2007 resulted in a 185.4 TgC CO2 sink, an increase ∼20% over the 2006–2013 mean. In contrast, extreme SIA loss of 2012 resulted in a CO2 sink of the AO of only 146.3 TgC due to two main factors: (1) increased both wind speed and stratification in the Eastern Siberian Sea absorbing less CO2 and (2) decreased primary production and area of air‐sea gas exchange in the Chukchi and Nordic Seas. Our model captures a trend of decreasing CO2 sink in most of the Chukchi Sea during fall but does not show the changes in winter CO2 sink in the Nordic and Barents Seas as previous independent studies have suggested. Our results indicate that future AO‐atmosphere CO2 exchange will be determined by complex interplay of SIA and other environmental drivers. Plain Language Summary: The seasonal process of melting and formation of sea‐ice area (SIA) in the Arctic Ocean (AO) is a key factor to control the physical, chemical, and biological processes of the upper ocean. We used a numerical model to understand the potential consequences of two drastic events of SIA reduction in both 2007 and 2012. Our model simulations show that the biological and chemical responses of the AO were different when we compared 2007 versus 2012. In 2007, the severe reduction of SIA was followed by a large uptake of carbon dioxide (CO2, as also expected from theory) due to the increase in ice‐free area and to the enhanced photosynthetic activity. However, in 2012, the response was different due to several factors: (1) increased wind speed and decreased primary production in the Eastern Siberian Sea in the summer causing less CO2 uptake and (2) the increase in the SIA in the fall in the Chukchi sea and in the summer in the Nordic Seas (close to Greenland) inhibiting the air‐sea gas CO2 exchange. All these processes, when combined together, yielded a reduced CO2 sink of the AO in 2012 when compared to 2007. Key Points: Changes in the Arctic Ocean CO2 sink are mainly controlled by sea‐ice extent, but changes in wind speed and SST can contribute tooModel results suggest that the events of sea‐ice loss in 2007 and 2012 triggered two different responses of the Arctic Ocean CO2 sinkDuring the 2006‐2013 period in the Arctic Ocean, the simulated CO2 sink and sea‐ice area show opposite trends [ABSTRACT FROM AUTHOR]

Abstract: Freshwater export from the Arctic is critical in determining the density of water at sites of North Atlantic deep water formation, which in turn influences the global flux of oceanic heat and nutrients. We need geochemical tracers and high‐resolution observations to refine our freshwater budgets and constrain models for future change. The use of seawater barium concentrations in the Arctic Ocean as a freshwater tracer relies on the conservative behavior of barium in seawater; while this has been shown to be an unreliable assumption in Arctic summers, there are a lack of studies observing seasonal progressions. Here, we present barium concentrations from seawater and sea‐ice collected during the Norwegian Young Sea ICE expedition from boreal winter into summer. We use other tracers (salinity, oxygen isotopes, and alkalinity) to reconstruct freshwater inputs and calculate a barium “deficit” that can be attributed to nonconservative processes. We locate a deficit in winter when biological production is low, which we attribute to uptake by barite formation associated with old organic matter or by internal sea‐ice processes. We also find a significant barium deficit during the early spring bloom, consistent with uptake into organic‐matter associated microenvironments. However, in summer, there no strong barium deficit near the surface, despite high biological production and organic carbon standing stocks, perhaps reflecting phytoplankton assemblage changes, and/or rapid internal cycling. Our findings challenge the assumptions surrounding the use of barium as an Arctic freshwater tracer, and highlight the need to improve our understanding of barium in sea‐ice environments. [ABSTRACT FROM AUTHOR]

Seven ice mass balance instruments deployed near 83°N on different first-year and second-year ice floes, representing variable snow and ice conditions, documented the evolution of snow and ice conditions in the Arctic Ocean north of Svalbard in January-March 2015. Frequent profiles of temperature and thermal diffusivity proxy were recorded to distinguish changes in snow depth and ice thickness with 2 cm vertical resolution. Four instruments documented flooding and snow -ice formation. Flooding was clearly detectable in the simultaneous changes in thermal diffusivity proxy, increased temperature, and heat propagation through the underlying ice. Slush then progressively transformed into snow-ice. Flooding resulted from two different processes: (i) after storm-induced breakup of snow-loaded floes and (ii) after loss of buoyancy due to basal ice melt. In the case of breakup, when the ice was cold and not permeable, rapid flooding, probably due to lateral intrusion of seawater, led to slush and snow-ice layers at the ocean freezing temperature (−1.88°C). After the storm, the instruments documented basal sea-ice melt over warm Atlantic waters and ocean-to-ice heat flux peaked at up to 400 W m−2. The warm ice was then permeable and flooding was more gradual probably involving vertical intrusion of brines and led to colder slush and snow-ice (−3°C). The N-ICE2015 campaign provided the first documentation of significant flooding and snow-ice formation in the Arctic ice pack as the slush partially refroze. Snow-ice formation may become a more frequently observed process in a thinner ice Arctic. [ABSTRACT FROM AUTHOR]

In this study, a superoxide dismutase gene (PsSOD) from Pseudoalteromonas sp. ANT506 was cloned and over expressed in Escherichia coli. The PsSOD has an open reading frame of 582 bp with a putative product of 193 amino acid residue and an estimated molecular size of 21.4 kDa. Histagged PsSOD was subsequently purified 12.6-fold by Ni-affinity chromatography and the yield of 22.9%. The characterization of the purified rPsSOD exhibited maximum activity at 30 °C and pH 8.0. The enzyme exhibited 13.9% activity at 0 °C and had high-thermo lability at higher than 50 °C. rPsSOD exhibited well capability to 2.5M NaCl (62.4%). These results indicated that rPsSOD exhibited special catalytic properties. [ABSTRACT FROM AUTHOR]

Using lessons from idealised predictability experiments, we discuss some issues and perspectives on the design of operational seasonal to inter-annual Arctic sea-ice prediction systems.Wefirst review the opportunities to use a hierarchy of different types of experiment to learn about the predictability of Arctic climate.We also examine key issues for ensemble system design, such as measuring skill, the role of ensemble size and generation of ensemble members. When assessing the potential skill of a set of prediction experiments, using more than onemetric is essential as different choices can significantly alter conclusions about the presence or lack of skill. We find that increasing both the number of hindcasts and ensemble size is important for reliably assessing the correlation and expected error in forecasts. For other metrics, such as dispersion, increasing ensemble size is most important. Probabilistic measures of skill can also provide useful information about the reliability of forecasts. In addition, various methods for generating the different ensemble members are tested. The range of techniques can produce surprisingly different ensemble spread characteristics. The lessons learnt should help inform the design of future operational prediction systems. [ABSTRACT FROM AUTHOR]

A quantitative assessment of observed and projected environmental changes in the Southern Ocean ( SO) with a potential impact on the marine ecosystem shows: (i) large proportions of the SO are and will be affected by one or more climate change processes; areas projected to be affected in the future are larger than areas that are already under environmental stress, (ii) areas affected by changes in sea-ice in the past and likely in the future are much larger than areas affected by ocean warming. The smallest areas (<1% area of the SO) are affected by glacier retreat and warming in the deeper euphotic layer. In the future, decrease in the sea-ice is expected to be widespread. Changes in iceberg impact resulting from further collapse of ice-shelves can potentially affect large parts of shelf and ephemerally in the off-shore regions. However, aragonite undersaturation (acidification) might become one of the biggest problems for the Antarctic marine ecosystem by affecting almost the entire SO. Direct and indirect impacts of various environmental changes to the three major habitats, sea-ice, pelagic and benthos and their biota are complex. The areas affected by environmental stressors range from 33% of the SO for a single stressor, 11% for two and 2% for three, to <1% for four and five overlapping factors. In the future, areas expected to be affected by 2 and 3 overlapping factors are equally large, including potential iceberg changes, and together cover almost 86% of the SO ecosystem. [ABSTRACT FROM AUTHOR]

We report the analyses of a dataset spanning 39 years of near-annual fishing for Dissostichus mawsoni in McMurdo Sound, Antarctica, 1972-2011. Data on total length, condition and catch per unit effort ( CPUE) were derived from the > 5500 fish caught, the large majority of which were measured, tagged and released. Contrary to expectation, the length frequency of the McMurdo Sound catch was dominated by fish in the upper two-thirds of the overall distribution exhibited in the industrial catch for the Ross Sea shelf. Fish length and condition increased from the early 1970s to the early 1990s and then decreased. Fish length positively correlated with Ross Sea ice extent in early spring, a relationship possibly caused by more ice encouraging larger fish to move farther south over the shelf and into the study area. Fish condition positively correlated with the amount of open water in the Ross Sea during the previous summer (Feb), perhaps reflecting greater availability of prey with the higher productivity that more open water brings. Decreasing fish size corresponds to the onset of the fishery, which targets the large individuals. CPUE was constant through 2001 and then decreased dramatically. We hypothesize that this decrease is related to the industrial fishery, which began in the 1996-97 austral summer, and concentrates effort over the ice-free Ross Sea continental slope. As a result of limited prey choices and close coupling among mesopredators of the region, Antarctic toothfish included, the fishery appears to be dramatically altering the trophic structure of the Ross Sea. [ABSTRACT FROM AUTHOR]

1. Polar bear Ursus maritimus population dynamics under conditions of climate change has become a controversial topic. A survey of expert opinion based on modelled sea-ice data was performed in order to quantify the trends and variance surrounding possible impacts of climate change on polar bear populations. 2. Polar bears have become an iconic species in the communication of climate change. Negative impacts of climatic warming on polar bears have been suggested, but cannot be fully quantified as no Arctic-wide models yet exist to analyse the relationship between polar bear population dynamics and climate change. 3. Ten polar bear experts participated in an expert opinion survey in early 2007, quantifying the trends and variance surrounding possible impacts of climate change on polar bear populations. The experts were provided with maps and time-series of sea-ice extent and duration to 2050, simulated under a mid-range emissions scenario. Expert projections of future polar bear habitat range and population size across the Arctic, and for population size in five regions, were obtained. Experts were asked to define ‘best conservation practice’, and to re-evaluate the total Arctic population projection if this best practice was implemented. 4. Most experts project a substantial decline in polar bear range and population size across the Arctic and in population size across each region. Expert best estimates for total Arctic polar bear population size lie from no change to a 70% decrease by 2050 relative to today; with half the experts projecting at least a 30% decrease. The median best estimates show the Barents Sea, Hudson Bay and the Chukchi Sea populations experiencing the greatest population decline under this scenario. There is much uncertainty both within and between expert responses, especially in little-researched regions such as the Chukchi Sea. 5. Synthesis and applications. Based on projected changes in sea-ice extent, experts suggest that polar bear populations will undergo significant declines by 2050, even implementing best management practices, under the scenario of climatic warming outlined here. The expert survey approach could be applied to a wide range of species for which there is a lack of available data and considerable uncertainty surrounding all aspects of the problem that prevent quantification with more formal modelling approaches. [ABSTRACT FROM AUTHOR]

1. We compared the reproduction of a marine diving bird, Brunnich's guillemot ( Uria lomvia), breeding at two Arctic colonies close to the northern and southern limits of the species’ range in the Canadian Arctic. 2. At both colonies, timing of breeding for Brunnich's guillemots was positively correlated with summer ice cover, which was determined by winter and spring temperatures. Spring temperatures also modified the effects of ice conditions on timing of breeding. 3. At Coats Island, northern Hudson Bay, in low Arctic waters, the date of egg-laying has advanced since 1981, simultaneous with a decrease in summer ice cover in surrounding waters. Lower ice cover in this region is correlated with lower chick growth rates and lower adult body mass, suggesting that reduction in summer ice extent is having a negative effect on reproduction. 4. Conversely, at Prince Leopold Island, in the High Arctic, there has been no trend in summer ice cover and no detectable change in timing of breeding. Reproduction there is less successful in years of late ice than in years of early ice break-up. 5. Current trends suggest that continued warming should benefit birds breeding on the northern limit of the species range, while adversely affecting reproduction for those on the southern margin. The probable result will be an eventual northward displacement of the population. Although this type of effect has been widely predicted, this study is among the first to demonstrate a potential causal mechanism. [ABSTRACT FROM AUTHOR]

Extreme environmental conditions have been thought to limit algal growth in the upper sea-ice. In McMurdo Sound, Antarctica, chrysophyte statocysts (stomatocysts) and dinoflagellate hypnozygotes (resting cysts) over winter in first- and second-year land-fast sea-ice exposed to temperatures of -20° C or lower. In early November, when temperature in the upper ice are <-8° C and brine salinities are >126 psu, dinoflagellate cysts activate and shortly thereafter excyst. During early November, chrysophyte statocysts also begin to excyst. Net daily primary production occurs in the sea-ice brine at temperatures as low as -7.1° C, at brine salinities as high as 129 psu, and at average photon flux densities as low as 5 μmol photons·m[SUP2].s[SUP-1]. Dinoflagellate densities were >10° vegetative cells·L[SUP-1] of ice while temperatures in the upper ice were between -6.8 and -5.8° C and brine salinities were ∼100 psu. Chrysophyte densities reached >10[SUP6].L[SUP-1] of ice by early December. High densities of physiologically active cryo- and halotolerant algae can occur in the upper land-fast sea-ice under extreme conditions of temperature and salinity. [ABSTRACT FROM AUTHOR]

Changes in Arctic sea ice have been proposed to affect midlatitude winter atmospheric circulation, often based on observed coincident variability. However, causality of this covariability remains unclear. Here, we address this issue using atmospheric model experiments prescribed with observed sea surface temperature variations and either constant or time‐varying sea ice variability. We show that the observed relationship between late‐autumn Barents‐Kara sea ice and the winter North Atlantic Oscillation can be reproduced by simulated atmospheric internal variability but is not simulated as a forced response to sea ice. Observations and models suggest reduced sea ice is linked to a weaker Aleutian Low. We show that simulated Aleutian Low variability is correlated with observed sea ice variability even in simulations with fixed sea ice, implying that this relationship is not incidental. Instead, we suggest that covariability between sea ice and the Aleutian Low originates from tropical sea surface temperature and rainfall variations and their teleconnections to the extratropics. Plain Language Summary: Recent dramatic changes in Arctic sea ice due to climate change have been linked to changes in weather patterns across the Northern Hemisphere. Many studies have proposed such links, but correlation does not necessarily imply causality. Here, we explore the causality of this link using atmospheric models run with observed sea surface temperature variations and either constant or time‐varying sea ice. We find that changes in weather patterns over the Atlantic that are correlated with sea ice variations are not caused by changes in sea ice. Instead, the correlation appears to be an incidental occurrence due to internal atmospheric variability. Additionally, we find that changes in weather patterns over the North Pacific, which are also correlated with sea ice variations, are reproduced in model experiments with no knowledge of these sea ice variations. In this case, the correlation appears to arise due to a third factor: rainfall variations over the tropical Pacific Ocean, which can affect midlatitude weather irrespective of sea ice changes. Key Points: Atmospheric model experiments aid causal interpretation of links between Arctic sea ice and midlatitude winter atmospheric circulationObserved links between autumn Barents‐Kara sea ice and the winter North Atlantic Oscillation is largely explained by internal variabilityObserved links between autumn Barents‐Kara sea ice and the winter Aleutian Low appears to originate from tropical SST and rainfall changes [ABSTRACT FROM AUTHOR]