Beaufort Gyre Research Articles

Application of A.M. Obukhov’s theory of correlation of vectors for scientific research and engineering calculations of ice drift in the Arctic Ocean.

The theory of vectors correlation of Obukhov was used to describe an ice drift in the Arctic Ocean. The main features are defined by an area. The tracks tortuosity in Beaufort Gyre and Transarctic current varies threefold: 1.5 and 4.5. Year to year and seasonal variations indicate the position of the Beaufort Gyre and the speed of the Transarctic drift. The average speed does not exceed 10 cm/s. The drift increases in summer, and in winter it intensifies in Fram Strait. In Fram Strait the velocity month–over–month increases from June to December–March from 2.5 to 6.5 cm/s. Overall correlation drift and wind is 0.95 and 0.85 for ice fields and icebergs. The non-wind drift reproduces the cyclonic circulation between Frantz Josef Land and Novaya Zemlya. The wind drift specifies the coefficients and angles of drift evasion from the wind. The coefficient for total drift is 0.14–0.18, fore wind drift 0.10–0.15, and the angles are 15–30° and 10–15° in the Barents Sea. In the Fram Strait the drift and wind directions are stable, the trend considers the modulus and direction. The drift trend has the direction towards the south. It decreases to 5 % of variance in June and increases to 15% from December to March.

A Synthesis of the Upper Arctic Ocean Circulation During 2000–2019: Understanding the Roles of Wind Forcing and Sea Ice Decline

Major changes have occurred in the Arctic Ocean during 2000–2019, including the unprecedented spin-up of the Beaufort Gyre and the emergence of Arctic Atlantification in the eastern Eurasian Basin. We explored the main drivers for these changes by synthesizing numerical simulations and observations in this paper. The Arctic atmospheric circulation was unusual in some years in this period, with strongly negative wind curl over the Canada Basin. However, the wind-driven spin-up of the Beaufort Gyre would have been much weaker had it not been for Arctic sea ice decline. The sea ice decline not only fed the ocean with meltwater, but also made other freshwater components more available to the Beaufort Gyre through mediating the ocean surface stress. This dynamical effect of shifting surface freshwater from the Eurasian Basin towards the Amerasian Basin also resulted in the Arctic Atlantification in the eastern Eurasian Basin, which is characterized by halocline salinification and the uplift of the boundary between the halocline and the Atlantic Water layer. Contemporarily, the sea ice decline caused a strong warming trend in the Atlantic Water layer. The Empirical Orthogonal Function (EOF) analysis of Arctic annual sea surface height for this period reveals that the first two modes of the upper ocean circulation have active centers associated with the Arctic Oscillation and Beaufort High variability, respectively. In the presence of sea ice decline the first two EOFs can better distinguish the ocean variability driven by the two atmospheric circulation modes. Therefore, the major changes in the Arctic Ocean in the past two decades are indicators of climate change as is the sea ice retreat. Our synthesis could help assess how the Arctic Ocean might change in future warming climate.

Increasing Multiyear Sea Ice Loss in the Beaufort Sea: A New Export Pathway for the Diminishing Multiyear Ice Cover of the Arctic Ocean

AbstractHistorically, multiyear sea ice (MYI) covered a majority of the Arctic and circulated through the Beaufort Gyre for years. However, increased ice melt in the Beaufort Sea during the early 2000s was proposed to have severed this circulation. Constructing a regional MYI budget from 1997 to 2021 reveals that MYI import into the Beaufort Sea has increased year‐round, yet less MYI now survives through summer and is transported onwards in the Gyre. Annual average MYI loss quadrupled over the study period and increased from ∼7% to ∼33% of annual Fram Strait MYI export, while the peak in 2018 (385,000 km2) was similar in magnitude to Fram Strait MYI export. The ice‐albedo feedback coupled with the transition toward younger thinner MYI is responsible for the increased MYI loss. MYI transport through the Beaufort Gyre has not been severed, but it has been reduced so severely to prevent it from being redistributed throughout the Arctic Ocean.

Spinning ice floes reveal intensification of mesoscale eddies in the western Arctic Ocean

Under-ice eddies are prevalent in the major circulation system in the western Arctic Ocean, the Beaufort Gyre. Theoretical studies hypothesize that the eddy-driven overturning and the ice-ocean drag are crucial mechanisms of the gyre equilibration in response to atmospheric winds. However, due to severe weather conditions and limitations of remote sensing instruments, there are only sparse eddy observations in the ice-covered Arctic Ocean. Hence, the evolution of the under-ice eddy field, its impact on the gyre variability, and their mutual response to the ongoing Arctic warming remain uncertain. Here, we infer the characteristics of the under-ice eddy field by establishing its tight connection to the angular velocities of isolated spinning sea ice floes in marginal ice zones. Using over two decades of satellite observations of marginal ice zones in the western Arctic Ocean, we identified and tracked thousands of floes and used idealized eddy modeling to infer the interannual evolution of the eddy energetics underneath the ice. We find that the eddy field is strongly correlated to the strength of the Beaufort Gyre on interannual timescales, which provides the major observational evidence consistent with the hypothesis of the gyre equilibration by eddies. The inferred trends over the past two decades signify that the gyre and its eddy field have been intensifying as the sea ice cover has been declining. Our results imply that with continuing sea ice decline, the eddy field and the Beaufort Gyre will keep intensifying and leading to enhanced transport of freshwater and biogeochemical tracers.

Tight association between microbial eukaryote and giant virus communities in the Arctic Ocean

AbstractViruses are important regulatory factors of the marine microbial community including microeukaryotes. However, little is known about their role in the northern Chukchi Sea in the Arctic basin, which has oligotrophic conditions in summer. To clarify the link between microbial eukaryotic communities and viruses as well as environmental conditions, we investigated the community structures of microeukaryotes (from 3 to144 μm and from 0.23 μm size bio‐particles collected from seawater) and Imitervirales (from 0.23 μm size bio‐particles collected from seawater), a dominant group of viruses infecting marine microeukaryotes. To the best of our knowledge, no study has investigated both Imitervirales and eukaryotic communities in the Arctic Ocean. Surface water samples were collected at 21 ocean stations located in the northeastern Chukchi Sea and an adjacent area outside the Beaufort Gyre (Adjacent Sea), and at two melt ponds on sea ice in the summer of 2018. At the ocean stations, nutrient concentrations were low in most of the locations, except the shelf in the adjacent sea. The community variations were significantly correlated between eukaryotes and Imitervirales, even within the northeastern Chukchi Sea characterized by relatively homogeneous environmental conditions. The association of the eukaryotic community with the viral community was stronger than that with geographical and physicochemical environmental factors. These results suggest that Imitervirales actively infect their hosts even in the cold and oligotrophic seawater in the Arctic Ocean.

The Alaskan Arctic regime shift since 2017: A harbinger of years to come?

Recent fingerprints of Alaskan Arctic climate change include new weather patterns whose impacts propagate through the Alaskan marine ecosystem. Multiple lines of observational evidence draw attention to the last five years (2017–2021) as a remarkable period of change. Bering Sea winter and spring ice coverage was remarkably low in 2018 and 2019 associated with a shifted Aleutian Low (AL) into the western Bering Sea, and ridging of the overlying polar jet stream (PJS) that supported southerly winds and reduced ice growth. The climatological Beaufort High and associated Beaufort Gyre saw multiple winter-long collapses (2017 and 2020), the only two such events of the modern reanalysis era. In contrast, from 2012 to 2016 the AL and PJS were located southeastward across the Gulf of Alaska, which supported sea ice growth. The recent collocation of the exposed ocean surface and atmospheric anomalies supports the emergence of a regional atmospheric circulation response to sea-ice loss. We propose that the regional system shift after 2017 foreshadows the beginning of a period with an increasing frequency of major sea-ice loss events and associated impacts – floods, delayed spring blooms, and marine food chain disruptions – which is sooner than projected by climate models.

A new state-dependent parameterization for the free drift of sea ice

Abstract. Free-drift estimates of sea ice motion are necessary to produce a seamless observational record combining buoy and satellite-derived sea ice motion vectors. We develop a new parameterization for the free drift of sea ice based on wind forcing, wind turning angle, sea ice state variables (thickness and concentration), and estimates of the ocean currents. Given the fact that the spatial distribution of the wind–ice–ocean transfer coefficient has a similar structure to that of the spatial distribution of sea ice thickness, we take the standard free-drift equation and introduce a wind–ice–ocean transfer coefficient that scales linearly with ice thickness. Results show a mean bias error of −0.5 cm s−1 (low-speed bias) and a root-mean-square error of 5.1 cm s−1, considering daily buoy drift data as truth. This represents a 35 % reduction of the error on drift speed compared to the free-drift estimates used in the Polar Pathfinder dataset (Tschudi et al., 2019b). The thickness-dependent transfer coefficient provides an improved seasonality and long-term trend of the sea ice drift speed, with a minimum (maximum) drift speed in May (October), compared to July (January) for the constant transfer coefficient parameterizations which simply follow the peak in mean surface wind stresses. Over the 1979–2019 period, the trend in sea ice drift in this new model is +0.45 cm s−1 per decade compared with +0.39 cm s−1 per decade from the buoy observations, whereas there is essentially no trend in a free-drift parameterization with a constant transfer coefficient (−0.09 cm s−1 per decade) or the Polar Pathfinder free-drift input data (−0.01 cm s−1 per decade). The optimal wind turning angle obtained from a least-squares fitting is 25∘, resulting in a mean error and a root-mean-square error of +3 and 42∘ on the direction of the drift, respectively. The ocean current estimates obtained from the minimization procedure resolve key large-scale features such as the Beaufort Gyre and Transpolar Drift Stream and are in good agreement with ocean state estimates from the ECCO, GLORYS, and PIOMAS ice–ocean reanalyses, as well as geostrophic currents from dynamical ocean topography, with a root-mean-square difference of 2.4, 2.9, 2.6, and 3.8 cm s−1, respectively. Finally, a repeat of the analysis on two sub-sections of the time series (pre- and post-2000) clearly shows the acceleration of the Beaufort Gyre (particularly along the Alaskan coastline) and an expansion of the gyre in the post-2000s, concurrent with a thinning of the sea ice cover and the observed acceleration of the ice drift speed and ocean currents. This new dataset is publicly available for complementing merged observation-based sea ice drift datasets that include satellite and buoy drift records.

Increasing Winter Ocean‐to‐Ice Heat Flux in the Beaufort Gyre Region, Arctic Ocean Over 2006–2018

AbstractOcean‐to‐ice heat flux (OHF) is important in regulating the variability of sea ice mass balance. Using surface drifting buoy observations, we show that during winter in the Arctic Ocean's Beaufort Gyre region, OHF increased from 0.76 ± 0.05 W/m2 over 2006–2012 to 1.63 ± 0.08 W/m2 over 2013–2018. We find that this is a result of thinner and less‐compact sea ice that promotes enhanced winter ice growth, stronger ocean vertical convection, and subsurface heat entrainment. In contrast, Ekman upwelling declined over the study period, suggesting it had a secondary contribution to OHF changes. The enhanced ice growth creates a cooler, saltier, and deeper ocean surface mixed layer. In addition, the enhanced vertical temperature gradient near the mixed layer base in later years favors stronger entrainment of subsurface heat. OHF and its increase during 2006–2018 were not geographically uniform, with hot spots found in an upwelling region where ice was most seasonally variable.

Planktonic Tintinnid Community Structure Variations in Different Water Masses of the Arctic Basin

Information on tintinnid community structure variations in different water masses in the Arctic Basin is scarce. During the summer of 2020, tintinnid diversity and vertical distribution were investigated in the Arctic Ocean. A total of 21 tintinnid species were found in five water masses and each water mass had a unique tintinnid community structure. In the Pacific Summer Water (PSW), Salpingella sp.1 occupied the top abundance proportion (61.8%) and originated from the North Pacific. In the Remnant Winter Water (RWW), Acanthostomella norvegica occupied the top abundance proportion (85.9%) and decreased northward. In the Mixed Layer Water, Pacific Winter Water, and Atlantic-origin Water, Ptychocylis urnula had the highest abundance proportion (67.1, 54.9, and 52.2%, respectively). The high abundance distribution area of Salpingella sp.1 and A. norvegica were separated by the boundary of the Beaufort Gyre and Transpolar Drift. The above species could be indicator species of each water masses. The highest abundance proportion of Salpingella sp.1 contributes 81.9% to the dominance of 12–16 μm lorica oral diameter in the PSW, which indicated that the preferred food items of tintinnid were also getting smaller. The occurrence of North Pacific tintinnid in the PSW might be due to the increasing Pacific Inflow Water. Further studies are needed to explore the lasting period of this species and whether it can establish a local population under rapid Arctic warming progress.

Assimilation of sea ice thickness derived from CryoSat-2 along-track freeboard measurements into the Met Office's Forecast Ocean Assimilation Model (FOAM)

Abstract. The feasibility of assimilating sea ice thickness (SIT) observations derived from CryoSat-2 along-track measurements of sea ice freeboard is successfully demonstrated using a 3D-Var assimilation scheme, NEMOVAR, within the Met Office's global, coupled ocean–sea-ice model, Forecast Ocean Assimilation Model (FOAM). The CryoSat-2 Arctic freeboard measurements are produced by the Centre for Polar Observation and Modelling (CPOM) and are converted to SIT within FOAM using modelled snow depth. This is the first time along-track observations of SIT have been used in this way, with other centres assimilating gridded and temporally averaged observations. The assimilation leads to improvements in the SIT analysis and forecast fields generated by FOAM, particularly in the Canadian Arctic. Arctic-wide observation-minus-background assimilation statistics for 2015–2017 show improvements of 0.75 m mean difference and 0.41 m root-mean-square difference (RMSD) in the freeze-up period and 0.46 m mean difference and 0.33 m RMSD in the ice break-up period. Validation of the SIT analysis against independent springtime in situ SIT observations from NASA Operation IceBridge (OIB) shows improvement in the SIT analysis of 0.61 m mean difference (0.42 m RMSD) compared to a control without SIT assimilation. Similar improvements are seen in the FOAM 5 d SIT forecast. Validation of the SIT assimilation with independent Beaufort Gyre Exploration Project (BGEP) sea ice draft observations does not show an improvement, since the assimilated CryoSat-2 observations compare similarly to the model without assimilation in this region. Comparison with airborne electromagnetic induction (Air-EM) combined measurements of SIT and snow depth shows poorer results for the assimilation compared to the control, despite covering similar locations to the OIB and BGEP datasets. This may be evidence of sampling uncertainty in the matchups with the Air-EM validation dataset, owing to the limited number of observations available over the time period of interest. This may also be evidence of noise in the SIT analysis or uncertainties in the modelled snow depth, in the assimilated SIT observations, or in the data used for validation. The SIT analysis could be improved by upgrading the observation uncertainties used in the assimilation. Despite the lack of CryoSat-2 SIT observations available for assimilation over the summer due to the detrimental effect of melt ponds on retrievals, it is shown that the model is able to retain improvements to the SIT field throughout the summer months due to prior, wintertime SIT assimilation. This also results in regional improvements to the July modelled sea ice concentration (SIC) of 5 % RMSD in the European sector, due to slower melt of the thicker sea ice.

Intercomparison of Salinity Products in the Beaufort Gyre and Arctic Ocean

Salinity is the primary determinant of the Arctic Ocean’s density structure. Freshwater accumulation and distribution in the Arctic Ocean have varied significantly in recent decades and certainly in the Beaufort Gyre (BG). In this study, we analyze salinity variations in the BG region between 2012 and 2017. We use in situ salinity observations from the Seasonal Ice Zone Reconnaissance Surveys (SIZRS), CTD casts from the Beaufort Gyre Exploration Project (BGP), and the EN4 data to validate and compare with satellite observations from Soil Moisture Active Passive (SMAP), Soil Moisture and Ocean Salinity (SMOS), and Aquarius Optimally Interpolated Sea Surface Salinity (OISSS), and Arctic Ocean models: ECCO, MIZMAS, HYCOM, ORAS5, and GLORYS12. Overall, satellite observations are restricted to ice-free regions in the BG area, and models tend to overestimate sea surface salinity (SSS). Freshwater Content (FWC), an important component of the BG, is computed for EN4 and most models. ORAS5 provides the strongest positive SSS correlation coefficient (0.612) and lowest bias to in situ observations compared to the other products. ORAS5 subsurface salinity and FWC compare well with the EN4 data. Discrepancies between models and SIZRS data are highest in GLORYS12 and ECCO. These comparisons identify dissimilarities between salinity products and extend challenges to observations applicable to other areas of the Arctic Ocean.

Improvements on the upper-ocean simulation from 2007 to 2018 in the Canada Basin

The Beaufort Gyre reserves most of the freshwater in the Arctic Ocean. Observation shows that the freshwater content over 2016–2018 far exceeds the level over the plateau period 2008–2012. Modeling the vertical temperature and salinity structure and their changes in the Beaufort Gyre is a way to understand the process related to such step changes. We configured a pan-Arctic sea ice-ocean model with a southern boundary at ∼7°N in the Atlantic Ocean. The numerical simulation with sea surface salinity restoring, a latitude dependent horizontal diffusivity scaled by the squared buoyancy frequency, and a weak vertical diffusivity of 5 × 10−7 m2 s−1 in the Arctic Ocean better reproduces the sea ice extent, the Pacific summer halocline water, and the freshwater variations as observed. The sea surface salinity restoring mitigates the surface desalination in the North Atlantic Ocean, therefore a weaker ocean stratification prevents the excessive warming in the intermediate Arctic Ocean. It is also found that the weak vertical background diffusivity is the major factor in our model to preserve the vertical ocean structure in the Canada Basin and also the step change of freshwater recently. In addition to resolving eddy activities in high resolution models, the success in our low resolution model suggests that tuning the vertical diffusivity serves as another approach to simulate the increasing freshwater content in the recent decade.

Evolution of Arctic Ocean surface circulation from 1958 to 2017

Arctic Ocean surface circulation plays an important role in the global ocean circulation system. In recent years, its position and velocity have been changing constantly, attracting increasing attention. The cause of this change, however, is still a matter of disagreement. In this paper, we use empirical orthogonal function analysis (EOF) to analyze the spatial and temporal distributions of sea level pressure (SLP), sea surface height (SSH) and sea surface temperature (SST) in the Arctic Ocean. We use the wavelet transform to obtain the periodic variation law governing Arctic Ocean surface circulation as well as to explore the relationship between the Arctic Oscillation (AO) and Arctic Ocean surface circulation. Our results show that the SSH of the Arctic Ocean exhibit a periodic variation with AO, in particular the sudden change seen from 1987 to 1992 coinciding with the trend of the AO index for the same period. When the AO index is positive, the Beaufort Gyre (BG) weakens and the source of transpolar drift (TPD) moves eastward, to the East Siberian Sea; when the AO index is negative, the BG accelerates and expands and the source of TPD moves to the New Siberian Islands. As a result, the direction and amplitude of the circulation change.

A 10-year record of Arctic summer sea ice freeboard from CryoSat-2

Satellite observations of pan-Arctic sea ice thickness have so far been constrained to winter months. For radar altimeters, conventional methods cannot differentiate leads from meltwater ponds that accumulate at the ice surface in summer months, which is a critical step in the ice thickness calculation. Here, we use over 350 optical and synthetic aperture radar (SAR) images from the summer months to train a 1D convolution neural network for separating CryoSat-2 radar altimeter returns from sea ice floes and leads with an accuracy >80%. This enables us to generate the first pan-Arctic measurements of sea ice radar freeboard for May–September between 2011 and 2020. Results indicate that the freeboard distributions in May and September compare closely to those from a conventional ‘winter’ processor in April and October, respectively. The freeboards capture expected patterns of sea ice melt over the Arctic summer, matching well to ice draft observations from the Beaufort Gyre Exploration Program (BGEP) moorings. However, compared to airborne laser scanner freeboards from Operation IceBridge and airborne EM ice thickness surveys from the Alfred Wegener Institute (AWI) IceBird program, CryoSat-2 freeboards are underestimated by 0.02–0.2 m, and ice thickness is underestimated by 0.28–1.0 m, with the largest differences being over thicker multi-year sea ice. To create the first pan-Arctic summer sea ice thickness dataset we must address primary sources of uncertainty in the conversion from radar freeboard to ice thickness.

Fate of Warm Pacific Water in the Arctic Basin

AbstractPacific Summer Water (PSW) plays a critical role in the ecosystem of the western Arctic Ocean, impacting sea‐ice melt and providing freshwater to the basin. Most of the water exits the Chukchi Sea shelf through Barrow Canyon, but the manner in which this occurs and the ultimate fate of the water remain uncertain. Using an extensive collection of historical hydrographic and velocity data, we demonstrate how the PSW outflow depends on different wind conditions, dictating whether the warm water progresses eastward or westward away from the canyon. The current carrying the water westward along the continental slope splits into different branches, influenced by the strength and extent of the Beaufort Gyre, while the eastward penetration of PSW along the shelfbreak is limited. Our results provide the first broad‐scale view of how PSW is transferred from the shelf to the basin, highlighting the role of winds, boundary currents, and eddy exchange.

A Driftwood‐Based Record of Arctic Sea Ice During the Last 500 Years From Northern Svalbard Reveals Sea Ice Dynamics in the Arctic Ocean and Arctic Peripheral Seas

AbstractWe 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.

Circulation Pathways and Exports of Arctic River Runoff Influenced by Atmospheric Circulation Regimes

River runoff supplies the Arctic Ocean with a large amount of freshwater and land-derived material, so it is important for both the physical and biogeochemical marine environment. In this study, we used wind perturbation simulations to elucidate the response of the circulation pathways and exports of Arctic river runoff to different atmospheric circulation regimes. Specifically, wind perturbations representing the negative and positive phases of the Arctic Oscillation and Beaufort High modes were imposed over the Arctic Ocean separately in different sensitivity experiments. In addition, some combinations of the two modes were also considered in sensitivity experiments. By comparing these experiments with a control simulation, we revealed the impact of different wind perturbations. The atmospheric circulation regimes influence the Arctic surface geostrophic currents through changing the halosteric height, which is associated with the changes in spatial distribution of surface freshwater. The circulation pathways of river runoff, and Pacific and Atlantic derived surface waters are mainly determined by the surface geostrophic currents. The positive (negative) Arctic Oscillation reduces (increases) freshwater storage and sea surface height in the Makarov and Eurasian basins, thus strengthening (weakening) the cyclonic circulation and weakening (strengthening) the anticyclonic circulation; Accordingly, the Eurasian runoff leaves the Siberian shelf at more eastern (western) locations, and has an enhanced export through the Fram Strait (Canadian Arctic Archipelago). The positive (negative) Beaufort High increases (reduces) freshwater storage and sea surface height in the Amerasian Basin, thus strengthening (weakening) the anticyclonic circulation; Accordingly, the Eurasian runoff export through the Fram Strait and the Mackenzie River runoff export through the Canadian Arctic Archipelago are reduced (increased). The positive Arctic Oscillation increases freshwater available to the Beaufort Gyre, which can be efficiently accumulated there in the presence of a positive Beaufort High forcing. The impact of the Beaufort High mode on the location of the Transpolar Drift Stream and runoff circulation pathways is stronger with a positive Arctic Oscillation than with a neutral Arctic Oscillation state. Our results also showed that Eurasian runoff can only have a relatively small contribution to freshwater accumulation in the Beaufort Gyre region.

Freshwater in the Arctic Ocean 2010–2019

Abstract. The Arctic climate system is rapidly transitioning into a new regime with a reduction in the extent of sea ice, enhanced mixing in the ocean and atmosphere, and thus enhanced coupling within the ocean–ice–atmosphere system; these physical changes are leading to ecosystem changes in the Arctic Ocean. In this review paper, we assess one of the critically important aspects of this new regime, the variability of Arctic freshwater, which plays a fundamental role in the Arctic climate system by impacting ocean stratification and sea ice formation or melt. Liquid and solid freshwater exports also affect the global climate system, notably by impacting the global ocean overturning circulation. We assess how freshwater budgets have changed relative to the 2000–2010 period. We include discussions of processes such as poleward atmospheric moisture transport, runoff from the Greenland Ice Sheet and Arctic glaciers, the role of snow on sea ice, and vertical redistribution. Notably, sea ice cover has become more seasonal and more mobile; the mass loss of the Greenland Ice Sheet increased in the 2010s (particularly in the western, northern, and southern regions) and imported warm, salty Atlantic waters have shoaled. During 2000–2010, the Arctic Oscillation and moisture transport into the Arctic are in-phase and have a positive trend. This cyclonic atmospheric circulation pattern forces reduced freshwater content on the Atlantic–Eurasian side of the Arctic Ocean and freshwater gains in the Beaufort Gyre. We show that the trend in Arctic freshwater content in the 2010s has stabilized relative to the 2000s, potentially due to an increased compensation between a freshening of the Beaufort Gyre and a reduction in freshwater in the rest of the Arctic Ocean. However, large inter-model spread across the ocean reanalyses and uncertainty in the observations used in this study prevent a definitive conclusion about the degree of this compensation.

Change in Sediment Provenance on the Inner Slope of the Chukchi Rise and Their Paleoenvironmental Implications

The Arctic Ocean is one of the world’s most remarkable regions with respect to global climate change. The core ARA09C-St03 was analyzed for mineral composition and Nd isotope to determine the sediment provenance and reconstruct the paleoenvironment in the inner slope of the Chukchi Rise. Core ARA09C-St03 represents overall cycles of brown and gray color with three distinct dark brown layers and two pinkish-white layers and is divided into eight sedimentary units based on the lithological feature. The core has a continuous record of the late marine isotope stage (MIS) 5 to the Holocene and in particular provides a particularly high-resolution record from the Last Glacial Maximum (LGM). Sediment is derived mainly from the adjacent East Siberian Sea and the North American region, and changes in sediment provenance are controlled by climate-dependent particle size. During the glacial/stadial periods, sediments in Units 3, 5, and 8 were supplied by the East Siberian Sea via meltwater-derived suspension. The major ice-rafted debris (IRD) events in Units 2, 4, and 7, characterized by abundant dolomite and K/C ratio, were sourced from North America. The North America-derived materials reflect the initiation and disintegration of the Laurentide Ice Sheet and icebergs transported them across the open Arctic Ocean. The differences in provenance within these periods may be related to the scale of the Laurentide Ice Sheet. Interglacial sediments, including those from Units 1 and 6, are of mixed origin from Eurasia and the Canadian Archipelago and may have been transported by oceanic current and seasonal sea ice. These periods are likely associated with the negative Arctic Oscillation (AO) intensifying the Beaufort Gyre.

Persistent freshening of the Arctic Ocean and changes in the North Atlantic salinity caused by Arctic sea ice decline

Arctic sea ice has been declining over past several decades with the largest ice loss occurring in summer. This implies a strengthening of the sea ice seasonal cycle. Here, we examine global ocean salinity response to such changes of Arctic sea ice using simulations wherein we impose a radiative heat imbalance at the sea ice surface, inducing a sea ice decline comparable to the observed. The imposed perturbation leads to enhanced seasonal melting and a rapid retreat of Arctic sea ice within the first 5–10 years. We then observe a gradual freshening of the upper Arctic ocean that continues for about a century. The freshening is most pronounced within the central Arctic, including the Beaufort gyre, and is attributed to excess surface freshwater associated with the stronger seasonal sea ice melting, as well as a greater upper-ocean freshwater storage due to changes in ocean circulation. The freshening of the Nordic Seas can also occur via a distillation-like process in which denser saline waters with increased salinity are exported to the subtropical/tropical North Atlantic by meridional overturning circulation. Thus, enhanced seasonal sea ice melting in a warmer climate can lead to a persistent Arctic freshening with large impacts on the global salinity distribution.