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1.
  • Arunraj, Kondetharayil Soman, et al. (author)
  • Linking Coherent Anticyclonic Eddies in the Iceland Basin to Decadal Oceanic Variability in the Subpolar North Atlantic
  • 2022
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 127:5
  • Journal article (peer-reviewed)abstract
    • The Iceland Basin in the eastern Subpolar North Atlantic is an eddy-rich region characterized by intense anticyclonic eddy activity. Our study present the variability of coherent Anticyclonic Eddies (AEs) generated in this region, using satellite altimetry and two ocean eddy tracking algorithms. The yearly count of AEs in the Iceland Basin reveals a decadal variability similar to that of ocean heat content change in the eastern subpolar gyre. Periods with higher number of AEs coincide with periods of increased ocean heat content, and vice versa. However, both algorithms agree that more than 50% of the detected AEs are confined to the central Iceland Basin. The annual number of AEs also tracks zonal shifts of the subpolar front, a variable that can explain about 53 (77)% of the interannual (decadal) variability of AEs in the Iceland Basin. Finally, a Lagrangian approach is used to demonstrate that the amount of subtropical versus subpolar water masses reaching the Iceland Basin appears to influence, via baroclinic instability, the generation of AEs.
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2.
  • Berglund, Sara, et al. (author)
  • The Water Mass Transformation in the Upper Limb of the Overturning Circulation in the Southern Hemisphere
  • 2021
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 126:8
  • Journal article (peer-reviewed)abstract
    • The warming and salinification of the northwards flowing water masses from the Southern Ocean to the tropics are studied with Lagrangian trajectories simulated using fields from an Earth System Model. The trajectories are used to trace the geographical distribution of the water mass transformation and connect it with the pathways of the upper limb of the overturning circulation in the Southern Hemisphere. In the Antarctic Circumpolar Current water gains heat just below the mixed layer, mainly when the layer is thin during Austral spring and summer. This gain is therefore suggested to be a consequence of heat flux from the atmosphere and mixing processes at the base of the mixed layer. In the Southern Hemispheric subtropical gyres on the other hand, a large warming and salinification of the northwards flowing water results from internal mixing with other warmer and more saline water masses. Close to the Antarctic shelf waters are getting fresher as a result of ice melting, whereas further north, in the Antarctic Circumpolar current, waters are getting more saline as a result of evaporation. Our results show that it is not only the heat and freshwater fluxes through the sea surface that control the heat and salt changes of the upper limb of the overturning circulation in the Southern Hemisphere. In fact, internal mixing accounts for 25% of the heat change, and 22% of the salinity change.
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3.
  • Blomquist, B. W., et al. (author)
  • Wind Speed and Sea State Dependencies of Air-Sea Gas Transfer : Results From the High Wind Speed Gas Exchange Study (HiWinGS)
  • 2017
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 122:10, s. 8034-8062
  • Journal article (peer-reviewed)abstract
    • A variety of physical mechanisms are jointly responsible for facilitating air-sea gas transfer through turbulent processes at the atmosphere-ocean interface. The nature and relative importance of these mechanisms evolves with increasing wind speed. Theoretical and modeling approaches are advancing, but the limited quantity of observational data at high wind speeds hinders the assessment of these efforts. The HiWinGS project successfully measured gas transfer coefficients (k(660)) with coincident wave statistics under conditions with hourly mean wind speeds up to 24 m s(-1) and significant wave heights to 8 m. Measurements of k(660) for carbon dioxide (CO2) and dimethylsulfide (DMS) show an increasing trend with respect to 10 m neutral wind speed (U-10N), following a power law relationship of the form: k660CO2 approximate to U10N1.68 and k660dms approximate to U10N1.33. Among seven high wind speed events, CO2 transfer responded to the intensity of wave breaking, which depended on both wind speed and sea state in a complex manner, with k660CO2 increasing as the wind sea approaches full development. A similar response is not observed for DMS. These results confirm the importance of breaking waves and bubble injection mechanisms in facilitating CO2 transfer. A modified version of the Coupled Ocean-Atmosphere Response Experiment Gas transfer algorithm (COAREG ver. 3.5), incorporating a sea state-dependent calculation of bubble-mediated transfer, successfully reproduces the mean trend in observed k(660) with wind speed for both gases. Significant suppression of gas transfer by large waves was not observed during HiWinGS, in contrast to results from two prior field programs.
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4.
  • Broomé, Sara, et al. (author)
  • A Satellite-Based Lagrangian Perspective on Atlantic Water Fractionation Between Arctic Gateways
  • 2021
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 126:11
  • Journal article (peer-reviewed)abstract
    • Warm Atlantic Water reaches the Arctic Ocean via two gateways: the Barents Sea Opening (BSO) and Fram Strait. Here, we study the near-surface flow of the Atlantic Water in the Nordic Seas and its fractionation between these Arctic gateways, using simulated Lagrangian trajectories based on satellite altimetry for 1994–2018. Lagrangian particles are released in the eastern Nordic Seas, where Atlantic Water flows poleward in two current cores: an inner branch along the Norwegian Continental Slope and an outer sea ward branch. The trajectories toward Fram Strait and the BSO are, in an averaged sense, largely steered by the bottom topography, and on inter-annual timescales we find an anticorrelation in the number of particles that reach the two gateways. Most of the particles released in the inner branch enter the Barents Sea and most of the particles seeded in the outer branch reach Fram Strait. However, there is a significant cross-over of particles from the outer to the inner branch in the Lofoten Basin, and nearly half of the total number of particles entering the BSO originate in the outer branch. This cross-over is accomplished solely by the time-fluctuating part of the velocity field, and it becomes stronger when the eddy kinetic energy in the Lofoten Basin is anomalously high. Thus, the outer branch may, via processes in the Lofoten Basin, be important for Barents Sea climate variability.
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5.
  • Browny, Nicola Jane, et al. (author)
  • Arctic Ocean Freshwater Dynamics : Transient Response to Increasing River Runoff and Precipitation
  • 2019
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 124:7, s. 5205-5219
  • Journal article (peer-reviewed)abstract
    • Simulations from a coupled ice-ocean general circulation model are used to assess the effects on Arctic Ocean freshwater storage of changes in freshwater input through river runoff and precipitation. We employ the climate response function framework to examine responses of freshwater content to abrupt changes in freshwater input. To the lowest order, the response of ocean freshwater content is linear, with an adjustment time scale of approximately 10years, indicating that anomalies in Arctic Ocean freshwater export are proportional to anomalies in freshwater content. However, the details of the transient response of the ocean depend on the source of freshwater input. An increase in river runoff results in a fairly smooth response in freshwater storage consistent with an essentially linear relation between total freshwater content and discharge of excess freshwater through the main export straits. However, the response to a change in precipitation is subject to greater complexity, which can be explained by the localized formation and subsequent export of salinity anomalies which introduce additional response time scales. The results presented here suggest that future increases in Arctic Ocean freshwater input in the form of precipitation are more likely to be associated with variability in the storage and release of excess freshwater than are increases in freshwater input from river runoff. Plain Language Summary This paper shows that the Arctic Ocean adjusts to changes in freshwater input over time scales of about one decade. How much of the added freshwater is stored in the Arctic depends, however, on how the freshwater enters the ocean. If it arrives as additional river runoff, the response in Arctic freshwater storage is relatively smooth and predictable. If it falls, instead, as increased precipitation, the response is less easy to predict because it is complicated by interactions between the ocean and sea ice. This is important because the part of the freshwater that is not stored in the Arctic Ocean is exported to the North Atlantic, where it can affect the global ocean circulation.
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6.
  • Cai, Yongqing, et al. (author)
  • Impact of wave breaking on upper-ocean turbulence
  • 2017
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 122:2, s. 1513-1528
  • Journal article (peer-reviewed)abstract
    • Previous studies have demonstrated that surface wave breaking can impact upper-ocean turbulence through wave-breaking-induced turbulence kinetic energy (TKE) flux and momentum flux. Wave-breaking-induced momentum flux decays approximately exponentially with depth, and the decay exponent depends on both the wind speed and wave age. With increasing wave age, the decay speed of wave-breaking-induced momentum flux first decreases, reaching a minimum around a wave age of 16, and then increases. In this study, a wave-breaking-induced momentum flux parameterization was proposed based on wave age and wind-speed dependence. The new proposed parameterization was introduced into a one-dimensional (1-D) ocean model along with a wave-age-dependent wave-breaking-induced TKE flux parameterization. The simulation results showed that the wave-breaking impact on the ocean mainly affected the upper-ocean layer. Adding the wave-age impact to the wave-breaking-induced TKE flux and momentum flux improved the 1-D model performance concerning the sea temperature. Moreover, the wave-breaking-induced momentum flux had a larger impact on the simulation results than the wave-breaking-induced TKE flux.
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7.
  • Chafik, L., et al. (author)
  • On the flow of Atlantic water and temperature anomalies in the Nordic Seas toward the Arctic Ocean
  • 2015
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 120:12, s. 7897-7918
  • Journal article (peer-reviewed)abstract
    • The climatic conditions over the Arctic Ocean are strongly influenced by the inflow of warm Atlantic water conveyed by the Norwegian Atlantic Slope Current (NwASC). Based on sea surface height (SSH) data from altimetry, we develop a simple dynamical measure of the NwASC transport to diagnose its spatio-temporal variability. This supports a dynamical division of the NwASC into two flow regimes; the SvinOy Branch (SvB) in the southern Norwegian Sea, and the Fram Strait Branch (FSB) west of Spitsbergen. The SvB transport is well correlated with the SSH and atmospheric variability within the Nordic Seas, factors that also affect the inflow to the Barents Sea. In contrast, the FSB is influenced by regional atmospheric conditions around Svalbard and northern Barents Sea. Using a composite analysis, we further relate anomalous strong SvB flow events to temperature fluctuations along the core of Atlantic water. A warm composite anomaly is found to propagate northward, with a tendency to amplify enroute, after these events. A roughly 12 months delayed temperature signal is identified in the FSB. However, also in the Lofoten Basin interior a delayed temperature signal is found, which appears to originate from the NwASC. This study suggests that hydrographic anomalies both upstream from the North Atlantic, and locally generated in the Norwegian Sea, are important for the oceanic heat and salt transport that eventually enters into the Arctic.
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8.
  • Chafik, Léon, 1985-, et al. (author)
  • The Faroe‐Shetland Channel Jet : Structure, Variability, and Driving Mechanisms
  • 2023
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 128:4
  • Journal article (peer-reviewed)abstract
    • The Faroe-Bank Channel (FBC) is a key gateway through which dense overflow water of the Nordic Seas supplies the lower limb of the Atlantic Meridional Overturning Circulation. Most recently, it was discovered that a deep jet through the Faroe-Shetland Channel carries the bulk of this overflow water, but numerous questions regarding its structure, seasonality, and interannual variability as well as its linkage to atmospheric forcing remain poorly understood. A realistic high-resolution ocean reanalysis (GLORYS12; 1993–2018) is, therefore, employed to address these questions. We first confirm that the Faroe-Shetland Channel Jet is a permanent feature in GLORYS12 as well as in an ensemble of low-resolution reanalyses. On seasonal time scales, we find a strong transport covariability between this deep jet and the observed FBC overflow. On interannual time scales, the strength of this deep jet is governed by the wind-forced circulation in the Nordic Seas. Due to the largely barotropic structure of these flows, they have a signature detectable in satellite sea-surface heights. Further, we suggest that the structure of the deep jet is qualitatively consistent with a geostrophic dynamical model that accounts for along-isobath density variations. This study indicates that GLORYS12 is a promising product to study the dense water pathways and dynamics in the Nordic Seas.
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9.
  • Chamizo, E., et al. (author)
  • The Potential of U-233/U-236 as a Water Mass Tracer in the Arctic Ocean
  • 2022
  • In: Journal of Geophysical Research - Oceans. - : Wiley-Blackwell Publishing, Inc.. - 2169-9275 .- 2169-9291. ; 127:3
  • Journal article (peer-reviewed)abstract
    • This study explores for the first time the possibilities that the U-233/U-236 atom ratio offers to distinguish waters of Atlantic or Pacific origin in the Arctic Ocean. Atlantic waters entering the Arctic Ocean often carry an isotopic signature dominantly originating from European reprocessing facilities with some smaller contribution from global fallout nuclides, whereas northern Pacific waters are labeled with nuclides released during the atmospheric nuclear testing period only. In the Arctic Ocean, U-233 originates from global fallout while U-236 carries both, a global fallout and a prominent nuclear reprocessing signal. Thus, the U-233/U-236 ratio provides a tool to identify water masses with distinct U sources. In this work, U-233 and U-236 were analyzed in samples from the GN01 GEOTRACES expedition to the western Arctic Ocean in 2015. The study of depth profiles and surface seawater samples shows that: (a) Pacific and Atlantic waters show enhanced signals of both radionuclides, which can be unraveled based on their U-233/U-236 signature; and (b) Deep and Bottom Waters show extremely low U-233 and U-236 concentrations close to or below analytical detection limits with isotopic ratios distinct from known anthropogenic U sources. The comparably high U-233/U-236 ratios are interpreted as a relative increase of naturally occurring U-233 and U-236 and thus for gradually reaching natural U-233/U-236 levels in the deep Arctic Ocean. Our results set the basis for future studies using the U-233/U-236 ratio to distinguish anthropogenic and pre-anthropogenic U in the Arctic Ocean and beyond.
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10.
  • Charette, M. A., et al. (author)
  • The Transpolar Drift as a Source of Riverine and Shelf-Derived Trace Elements to the Central Arctic Ocean
  • 2020
  • In: Journal of Geophysical Research-Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 125:5
  • Journal article (peer-reviewed)abstract
    • A major surface circulation feature of the Arctic Ocean is the Transpolar Drift (TPD), a current that transports river-influenced shelf water from the Laptev and East Siberian Seas toward the center of the basin and Fram Strait. In 2015, the international GEOTRACES program included a high-resolution pan-Arctic survey of carbon, nutrients, and a suite of trace elements and isotopes (TEIs). The cruises bisected the TPD at two locations in the central basin, which were defined by maxima in meteoric water and dissolved organic carbon concentrations that spanned 600 km horizontally and similar to 25-50 m vertically. Dissolved TEIs such as Fe, Co, Ni, Cu, Hg, Nd, and Th, which are generally particle-reactive but can be complexed by organic matter, were observed at concentrations much higher than expected for the open ocean setting. Other trace element concentrations such as Al, V, Ga, and Pb were lower than expected due to scavenging over the productive East Siberian and Laptev shelf seas. Using a combination of radionuclide tracers and ice drift modeling, the transport rate for the core of the TPD was estimated at 0.9 +/- 0.4 Sv (10(6) m(3)s(-1)). This rate was used to derive the mass flux for TEIs that were enriched in the TPD, revealing the importance of lateral transport in supplying materials beneath the ice to the central Arctic Ocean and potentially to the North Atlantic Ocean via Fram Strait. Continued intensification of the Arctic hydrologic cycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean. Plain Language Summary A major feature of the Arctic Ocean circulation is the Transpolar Drift (TPD), a surface current that carries ice and continental shelf-derived materials from Siberia across the North Pole to the North Atlantic Ocean. In 2015, an international team of oceanographers conducted a survey of trace elements in the Arctic Ocean, traversing the TPD. Near the North Pole, they observed much higher concentrations of trace elements in surface waters than in regions on either side of the current. These trace elements originated from land, and their journey across the Arctic Ocean is made possible by chemical reactions with dissolved organic matter that originates mainly in Arctic rivers. This study reveals the importance of rivers and shelf processes combined with strong ocean currents in supplying trace elements to the central Arctic Ocean and onward to the Atlantic. These trace element inputs are expected to increase as a result of permafrost thawing and increased river runoff in the Arctic, which is warming at a rate much faster than anywhere else on Earth. Since many of the trace elements are essential building blocks for ocean life, these processes could lead to significant changes in the marine ecosystems and fisheries of the Arctic Ocean.
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11.
  • Charette, M, et al. (author)
  • The Transpolar Drift as a Source of Riverine and Shelf‐Derived Trace Elements to the Central Arctic Ocean
  • 2020
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 125, s. 1-34
  • Journal article (peer-reviewed)abstract
    • A major surface circulation feature of the Arctic Ocean is the Transpolar Drift (TPD), a current that transports river‐influenced shelf water from the Laptev and East Siberian Seas toward the center of the basin and Fram Strait. In 2015, the international GEOTRACES program included a high‐resolution pan‐Arctic survey of carbon, nutrients, and a suite of trace elements and isotopes (TEIs). The cruises bisected the TPD at two locations in the central basin, which were defined by maxima in meteoric water and dissolved organic carbon concentrations that spanned 600 km horizontally and ~25–50 m vertically. Dissolved TEIs such as Fe, Co, Ni, Cu, Hg, Nd, and Th, which are generally particle‐reactive but can be complexed by organic matter, were observed at concentrations much higher than expected for the openocean setting. Other trace element concentrations such as Al, V, Ga, and Pb were lower than expected due to scavenging over the productive East Siberian and Laptev shelf seas. Using a combination of radionuclide tracers and ice drift modeling, the transport rate for the core of the TPD was estimated at 0.9 ± 0.4 Sv(106m3 s−1). This rate was used to derive the mass flux for TEIs that were enriched in the TPD, revealing the importance of lateral transport in supplying materials beneath the ice to the central Arctic Ocean and potentially to the North Atlantic Ocean via Fram Strait. Continued intensification of the Arctic hydrologicc ycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean.
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12.
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13.
  • Chen, Sheng, et al. (author)
  • On the First Observed Wave-Induced Stress Over the Global Ocean
  • 2020
  • In: Journal of Geophysical Research - Oceans. - : AMER GEOPHYSICAL UNION. - 2169-9275 .- 2169-9291. ; 125:12
  • Journal article (peer-reviewed)abstract
    • Despite many investigations/studies on the surface wave-induced stress, the global feature of the wave-induced stress has not been obtained previously as that requires a simultaneous observation of wave spectra and wind on a global scale. The China France Oceanography Satellite (CFOSAT) provided an opportunity for the first time to evaluate the global wave-induced stress and its contribution to the total wind stress. In this study, the global spatial distributions of wave-induced stress and its correlated index for August to November in 2019 are presented using the simultaneous ocean surface winds and wave spectra from the CFOSAT. The main results show that the wave-induced stress is fundamentally dependent on the wind and wave fields on a global scale and shows significant temporal and spatial variations. Further analyses indicate that there is an upward momentum flux under strong swells and low wind speeds (below similar to 5 m/s), and an anticorrelation between the dimensionless wave-induced stress and the proportion of swell energy to the total. Finally, the variations of the surface wave induced wind stress are clear asymmetric between northern and southern hemispheres in late summer but symmetric in late fall, which are closely associated with the seasonal changes in large-scale atmospheric circulation.
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14.
  • de Boer, Agatha M., et al. (author)
  • Interconnectivity Between Volume Transports Through Arctic Straits
  • 2018
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 123:12, s. 8714-8729
  • Journal article (peer-reviewed)abstract
    • Arctic heat and freshwater budgets are highly sensitive to volume transports through the Arctic-Subarctic straits. Here we study the interconnectivity of volume transports through Arctic straits in three models; two coupled global climate models, one with a third-degree horizontal ocean resolution (High Resolution Global Environmental Model version 1.1 [HiGEM1.1]) and one with a twelfth-degree horizontal ocean resolution (Hadley Centre Global Environment Model 3 [HadGEM3]), and one ocean-only model with an idealized polar basin (tenth-degree horizontal resolution). The two global climate models indicate that there is a strong anticorrelation between the Bering Strait throughflow and the transport through the Nordic Seas, a second strong anticorrelation between the transport through the Canadian Arctic Archipelago and the Nordic Seas transport, and a third strong anticorrelation is found between the Fram Strait and the Barents Sea throughflows. We find that part of the strait correlations is due to the strait transports being coincidentally driven by large-scale atmospheric forcing patterns. However, there is also a role for fast wave adjustments of some straits flows to perturbations in other straits since atmospheric forcing of individual strait flows alone cannot lead to near mass balance fortuitously every year. Idealized experiments with an ocean model (Nucleus for European Modelling of the Ocean version 3.6) that investigate such causal strait relations suggest that perturbations in the Bering Strait are compensated preferentially in the Fram Strait due to the narrowness of the western Arctic shelf and the deeper depth of the Fram Strait. Plain Language Summary The Arctic is one of the most fragile places on the Earth, facing double the rate of warming as the rest of the globe. This warming is partly due to melting of sea ice because open water reflects less sunlight than ice. One of the major controls on Arctic sea ice concentration is the heat flowing into the Arctic through its straits. However, due to the harsh conditions in the Arctic, there are limited long-term observations of the currents flowing through these straits. Here we turn to climate models to investigate these Arctic straits flows and in particular focus on how flows into and out of the Arctic balance each other. We find that in some instances specific pairs of strait flows are simultaneously affected by large-scale atmospheric. In other instances, the inflow through one strait flows out through another distant strait because of the way the ocean floor guides the currents. Traditionally, the flows through Arctic straits are studied in relation to local forces such as wind and sea level. Our work suggests value in a more holistic approach; one that also accounts for flow changes in a strait as a response to flow changes in other straits.
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15.
  • de Boer, Agatha M., et al. (author)
  • The control of the Southern Hemisphere Westerlies on the position of the Subtropical Front
  • 2013
  • In: Journal of Geophysical Research - Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 118:10, s. 5669-5675
  • Journal article (peer-reviewed)abstract
    • In recent years the latitudinal position of the Subtropical Front (STF) has emerged as a key parameter in the global climate. A poleward positioned front is thought to allow a greater salt flux from the Indian to the Atlantic Ocean and so drive a stronger Atlantic Meridional Overturning Circulation. Here the common view that the STF aligns with the zero wind stress curl (WSC) is challenged. Based on the STF climatologies of Orsi et al. (1995), Belkin and Gordon (1996), Graham and De Boer (2013), and on satellite scatterometry winds, we find that the zero WSC contour lies on average ∼10°, ∼8°, and ∼5° poleward of the front for the three climatologies, respectively. The circulation in the region between the Subtropical Gyres and the zero WSC contour is not forced by the WSC but rather by the strong bottom pressure torque that is a result of the interaction of the Antarctic Circumpolar Current with the ocean floor topography. The actual control of the position of the STF is crucially dependent on whether the front is regarded as simply a surface water mass boundary or a dynamical front. For the Agulhas Leakage problem, the southern boundary of the so-called Super Gyre may be the most relevant property but this cannot easily be identified in observations.
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16.
  • Dotto, Tiago S., et al. (author)
  • Control of the Oceanic Heat Content of the Getz-Dotson Trough, Antarctica, by the Amundsen Sea Low
  • 2020
  • In: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291. ; 125:8
  • Journal article (peer-reviewed)abstract
    • The changing supply of warm Circumpolar Deep Water (CDW) to the West Antarctic continental shelf is responsible for the basal melting and thinning of the West Antarctic ice shelves that has occurred in recent decades. Here we assess the variability in CDW supply, and its drivers, from a multiyear mooring deployed in, and a regional ocean model spanning, the Getz-Dotson Trough, Amundsen Sea. Between 2010 to 2015, the CDW within the trough underwent a pronounced cooling and freshening, associated with changes in thermohaline properties on isopycnals. Variability in the rate of CDW inflow is controlled by local wind forcing of a shelf break undercurrent, which determines the hydrographic properties of inflowing CDW via tilting of density surfaces above the continental slope. Local wind is coupled to the Amundsen Sea Low (ASL) low-pressure system, which is modulated by large-scale climatic modes via atmospheric teleconnections. For the period analyzed, a deeper ASL was associated with westward wind anomaly at the shelf break. Changes in the sea surface slope decelerated the shelf break undercurrent, resulting in less heat accessing the continental shelf and, consequently, a cooling of the Getz-Dotson Trough. Therefore, the present work suggests that the fate of the West Antarctic ice shelves is closely tied to the future evolution of the ASL.
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17.
  • du Plessis, M., et al. (author)
  • Submesoscale processes promote seasonal restratification in the Subantarctic Ocean
  • 2017
  • In: Journal of Geophysical Research-Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 122:4, s. 2960-2975
  • Journal article (peer-reviewed)abstract
    • Traditionally, the mechanism driving the seasonal restratification of the Southern Ocean mixed layer (ML) is thought to be the onset of springtime warming. Recent developments in numerical modeling and North Atlantic observations have shown that submesoscale ML eddies (MLE) can drive a restratifying flux to shoal the deep winter ML prior to solar heating at high latitudes. The impact of submesoscale processes on the intraseasonal variability of the Subantarctic ML is still relatively unknown. We compare 5 months of glider data in the Subantarctic Zone to simulations of a 1-D mixing model to show that the magnitude of restratification of the ML cannot be explained by heat, freshwater, and momentum fluxes alone. During early spring, we estimate that periodic increases in the vertical buoyancy flux by MLEs caused small increases in stratification, despite predominantly down-front winds that promote the destruction of stratification. The timing of seasonal restratification was consistent between 1-D model estimates and the observations. However, during up-front winds, the strength of springtime stratification increased over twofold compared to the 1-D model, with a rapid shoaling of the MLD from >200 m to <100 m within a few days. The ML stratification is further modified under a negative Ekman buoyancy flux during down-front winds, resulting in the destruction of ML stratification and deepening of the MLD. These results propose the importance of submesoscale buoyancy fluxes enhancing seasonal restratification and mixing of the Subantarctic ML.
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18.
  • du Plessis, Marcel, 1990, et al. (author)
  • The Daily-Resolved Southern Ocean Mixed Layer: Regional Contrasts Assessed Using Glider Observations
  • 2022
  • In: Journal of Geophysical Research-Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 127:4
  • Journal article (peer-reviewed)abstract
    • Water mass transformation in the Southern Ocean is vital for driving the large-scale overturning circulation, which transports heat from the surface to the ocean interior. Using profiling gliders, this study investigates the role of summertime buoyancy forcing and wind-driven processes on the intraseasonal (1-10 days) mixed layer thermohaline variability in three Southern Ocean regions southwest of Africa important for water mass transformation-the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), and Marginal Ice Zone (MIZ). At intraseasonal time scales, heat flux was shown as the main driver of buoyancy gain in all regions. In the SAZ and MIZ, shallow mixed layers and strong stratification enhanced mixed layer buoyancy gain by trapping incoming heat, while buoyancy loss resulted primarily from the entrainment of cold, salty water from below. In the PFZ, rapid mixing linked to Southern Ocean storms set persistently deep mixed layers and suppressed mixed layer intraseasonal thermohaline variability. In the polar regions, lateral stirring of meltwater from seasonal sea-ice melt dominated daily mixed layer salinity variability. We propose that these meltwater fronts are advected to the PFZ during late summer, indicating the potential for seasonal sea-ice freshwater to impact a region where the upwelling limb of overturning circulation reaches the surface. This study reveals a regional dependence of how the mixed layer thermohaline properties respond to small spatiotemporal processes, emphasizing the importance of surface forcing occurring between 1 and 10 days on the mixed layer water mass transformation in the Southern Ocean.
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19.
  • Dugstad, Johannes S., et al. (author)
  • Vertical Structure and Seasonal Variability of the Inflow to the Lofoten Basin Inferred From High-Resolution Lagrangian Simulations
  • 2019
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 124:12, s. 9384-9403
  • Journal article (peer-reviewed)abstract
    • The Lofoten Basin in the eastern Nordic Seas plays a central role in modifying the warm Atlantic Water inflow toward the Arctic Ocean. Here, the Atlantic Water experiences increased residence times, cooling, and substantial transformation. In this study, we investigate the Atlantic Water inflow pathways to the Lofoten Basin and their vertical and seasonal variations using 2-D and 3-D Lagrangian simulations forced by a high-resolution ocean model. Atlantic Water enters the basin from all directions, but we find two main inflow pathways at all vertical levels, one close to the Lofoten Escarpment in the southeast, associated with the Slope Current, and another close to the Helgeland Ridge in the southwest, associated with the Front Current. The surface inflow exhibits a stronger seasonal forcing than the inflow at depth as well as a stronger heat loss that is dominated by water masses entering the basin from the south. At deeper levels, the warm inflow from the east cools, while the relatively colder inflow from the west warms. The 2-D and 3-D synthetic trajectories show similar pathways. However, they are affected differently by the seasonal signal, giving different heat exchange patterns. Our results have implications for how results from Lagrangian observations in the region should be interpreted.
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20.
  • Fransner, Filippa, et al. (author)
  • Non-Redfieldian Dynamics Explain Seasonal pCO2 Drawdown in the Gulf of Bothnia
  • 2018
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 123:1, s. 166-188
  • Journal article (peer-reviewed)abstract
    • High inputs of nutrients and organic matter make coastal seas places of intense air-sea CO2 exchange. Due to their complexity, the role of coastal seas in the global air-sea CO2 exchange is, however, still uncertain. Here, we investigate the role of phytoplankton stoichiometric flexibility and extracellular DOC production for the seasonal nutrient and CO2 partial pressure (pCO2) dynamics in the Gulf of Bothnia, Northern Baltic Sea. A 3-D ocean biogeochemical-physical model with variable phytoplankton stoichiometry is for the first time implemented in the area and validated against observations. By simulating non-Redfieldian internal phytoplankton stoichiometry, and a relatively large production of extracellular dissolved organic carbon (DOC), the model adequately reproduces observed seasonal cycles in macronutrients and pCO2. The uptake of atmospheric CO2 is underestimated by 50% if instead using the Redfield ratio to determine the carbon assimilation, as in other Baltic Sea models currently in use. The model further suggests, based on the observed drawdown of pCO2, that observational estimates of organic carbon production in the Gulf of Bothnia, derived with the method, may be heavily underestimated. We conclude that stoichiometric variability and uncoupling of carbon and nutrient assimilation have to be considered in order to better understand the carbon cycle in coastal seas.
  •  
21.
  • Fredriksson, Sam, 1966, et al. (author)
  • Surface shear stress dependence of gas transfer velocity parameterizations using DNS
  • 2016
  • In: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291 .- 0148-0227 .- 2156-2202. ; 121:10, s. 7369-7389
  • Journal article (peer-reviewed)abstract
    • Air-water gas-exchange is studied in direct numerical simulations (DNS) of free-surface flows driven by natural convection and weak winds. The wind is modeled as a constant surface-shear-stress and the gas-transfer is modeled via a passive scalar. The simulations are characterized via a Richardson number Ri=Bν/u*4 where B, ν, and u* are the buoyancy flux, kinematic viscosity, and friction velocity respectively. The simulations comprise 0Ric or kg=Ashearu*Sc-n, Ri
  •  
22.
  • Geoffroy, Gaspard, et al. (author)
  • Global Mapping of the Nonstationary Semidiurnal Internal Tide Using Argo Data
  • 2022
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 127:4
  • Journal article (peer-reviewed)abstract
    • Data from Argo floats equipped with Iridium communications are used to obtain a global map of the total amplitude (or variance) of the semidiurnal internal tide at 1,000 dbar. The results are confirmed by a comparison with data from an historical collection of moored instruments. The obtained amplitude is in turn compared with the High-Resolution Empirical Tide (HRET) model, based on satellite altimetry. While HRET only contains the stationary component, with a fixed phase difference to the astronomical tide, the present results capture the total amplitude, including the nonstationary component. We estimate the global average ratio of total (Argo) to stationary (HRET) semidiurnal internal tide variance to be 6.5, and the amplitude ratio to be 3.6. Our estimate of the stationary fraction of the semidiurnal internal tide is subject to significant uncertainties. In particular, HRET is thought to mainly represent baroclinic mode-1 waves, while Argo data contain contributions from all modes.
  •  
23.
  • Graham, Robert M., 1988-, et al. (author)
  • The Dynamical Subtropical Front
  • 2013
  • In: Journal of Geophysical Research - Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 118:10, s. 5676-5685
  • Journal article (peer-reviewed)abstract
    • The Southern Ocean Subtropical Front (STF) is thought to play a key role in the global climate system. Theory suggests that the latitude of the STF regulates the volume of saline Agulhas Leakage into the Atlantic Ocean from the Indian. Here we use satellite sea surface temperature (SST) and sea surface height (SSH) data to study the physical characteristics of the STF water mass boundary. We find that the strong currents in this region do not align with the surface water mass boundary. Therefore, we provide a new climatology for these currents which we define as the Dynamical STF (DSTF). The DSTF is the eastward extension of the western boundary current in each basin and is characterized by strong SST and SSH gradients and no seasonal cycle. At the center of each basin it merges with the Sub-Antarctic Front. On the eastern side of basins, the STF surface water mass boundary coincides with a separate region of multiple SST fronts. We call this the Subtropical Frontal Zone (STFZ). The fronts in the STFZ have a large seasonal cycle and no SSH signature. Despite lying close to the same water mass boundary, the DSTF and STFZ are completely unrelated. We therefore suggest the term STF only be used when referring to the surface water mass boundary. When studying the strong currents on the western side of basins the term DSTF is more relevant and, similarly, the term STFZ better describes the region of enhanced SST gradients towards the east.
  •  
24.
  • Gülk, Birte, 1994, et al. (author)
  • Variability and Remote Controls of the Warm-Water Halo and Taylor Cap at Maud Rise
  • 2023
  • In: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291. ; 128:7
  • Journal article (peer-reviewed)abstract
    • The region of Maud Rise, a seamount in the Weddell Sea, is known for the occurrence of irregular polynya openings during the winter months. Hydrographic observations have shown the presence of a warmer water mass below the mixed layer along the seamount's flanks, commonly termed the warm-water Halo, surrounding a colder region above the rise, the Taylor Cap. Here we use two observational data sets, an eddy-permitting reanalysis product and regional high-resolution simulations, to investigate the interannual variability of the Halo and Taylor Cap for the period 2007–2022. Observations include novel hydrographic profiles obtained in the Maud Rise area in January 2022, during the first SO-CHIC cruise. It is demonstrated that the temperature of deep waters around Maud Rise exhibits strong interannual variability within the Halo and Taylor Cap, occasionally to such an extent that the two features become indistinguishable. A warming of deep waters by as much as 0.8°C is observed in the Taylor Cap during the years preceding the opening of a polynya in 2016 and 2017, starting in 2011. By analyzing regional simulations, we show that most of the observed variability in the Halo is forced remotely by advection of deep waters from the Weddell Gyre into the region surrounding Maud Rise. Our highest-resolution simulation indicates that mesoscale eddies subsequently transfer the properties of the Halo's deep waters onto the Taylor Cap. The eddies responsible for such transfer originate in an abrupt retroflection along the inner flank of the Halo.
  •  
25.
  • Gustafsson, Erik, 1977, et al. (author)
  • The air-water CO2 exchange of a coastal sea— A sensitivity study on factors that influence the absorption and outgassing of CO2 in the Baltic Sea
  • 2015
  • In: Journal of Geophysical Research - Oceans. - 0148-0227 .- 2156-2202 .- 2169-9275 .- 2169-9291. ; 120:8, s. 5342-5357
  • Journal article (peer-reviewed)abstract
    • In this study, the BALTSEM model is used to estimate how air-water CO2 fluxes in the Baltic Sea respond to parameterizations of organic alkalinity (Aorg), gas transfer, and phytoplankton growth, and further to changes in river loads. The forcing data include the most complete compilation of Baltic river loads for dissolved inorganic and organic carbon (DIC and DOC) and total alkalinity (TA). In addition, we apply the most recent estimates of internal TA generation in the system. Our results clearly demonstrate how airwater CO2 fluxes of a coastal sea depend on river loads of carbon, TA, and nutrients as well the freshwater import itself. Long-term changes in DIC loads are shown to be compensated by corresponding changes in air-water CO2 exchange. By adding Aorg, a discrepancy in the carbonate system calculations was removed, and the simulated net CO2 absorption of the system decreased by 11%. A new parameterization for cyanobacteria growth significantly improved the seasonal development of pCO2 in the central Baltic Sea, although the net effect on CO2 fluxes was below 5%. By applying either a linear, quadratic, or cubic wind speed dependence for gas transfer, the long-term net CO2 exchange was adjusted by less than 5%. There is no clear indication that any one of these parameterizations provides a more accurate estimate of CO2 fluxes than the other two. Our findings are applicable in other coastal areas that are heavily influenced by river loads of TA, DIC, and DOC.
  •  
26.
  • Hepworth, Ehlke, et al. (author)
  • Synoptic-Scale Extreme Variability of Winter Antarctic Sea-Ice Concentration and Its Link to Southern Ocean Extratropical Cyclones
  • 2024
  • In: Journal of Geophysical Research - Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 129:6
  • Journal article (peer-reviewed)abstract
    • This study provides a systematic analysis of extreme variability in winter Antarctic sea-ice concentration (SIC) and its relationship with extratropical cyclones. We characterize sea-ice variability and cyclone activity in different Southern Ocean sectors using atmospheric reanalyzes and a cyclone-tracking algorithm, and then quantify the proportion of extreme sea-ice variability engendered by cyclones of different intensities. The regions with relatively lower sea-ice area (SIA) (the King Haakon VII, East Antarctic, and Bellingshausen sectors) have an even distribution of cyclones within all intensity ranges, while, in the sectors with higher SIA, the Ross/Amundsen displays a higher number of intense and weak cyclones, and the Weddell sector has the majority of weak cyclones. Our systematic analysis reveals a significant link between extreme variability in winter SIC and: (a) all cyclones in the Ross/Amundsen sector; (b) all but the weakest cyclones in the King Haakon VII, East Antarctic, and Bellingshausen sectors; and (c) all but the most intense cyclones in the Weddell sector. The latter result is explained by the fact that the Weddell sector experiences more frequent, weaker cyclones than the other sectors. Cumulatively, approximately 30%–40% of the extreme sea-ice variability is caused by extratropical cyclones within all regions.
  •  
27.
  • Klocker, A., et al. (author)
  • Generation of the Internal Pycnocline in the Subpolar Southern Ocean by Wintertime Sea Ice Melting
  • 2023
  • In: Journal of Geophysical Research-Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 128:3
  • Journal article (peer-reviewed)abstract
    • The ocean's internal pycnocline is a layer of elevated stratification that separates the well-ventilated upper ocean from the more slowly renewed deep ocean. Despite its pivotal role in organizing ocean circulation, the processes governing the formation of the internal pycnocline remain little understood. Classical theories on pycnocline formation have been couched in terms of temperature and it is not clear how the theory applies in the high-latitude Southern Ocean, where stratification is dominated by salinity. Here we assess the mechanisms generating the internal pycnocline at southern high latitudes through the analysis of a high-resolution, realistic, global sea ice-ocean model. We show evidence suggesting that the internal pycnocline's formation is associated with sea ice-ocean interactions in two distinct ice-covered regions, fringing the Antarctic continental slope and the winter sea-ice edge. In both areas, winter-persistent sea-ice melt creates strong, salinity-based stratification at the base of the winter mixed layer. The resulting sheets of high stratification subsequently descend into the ocean interior at fronts of the Antarctic Circumpolar Current, and connect seamlessly to the internal pycnocline in areas further north in which pycnocline stratification is determined by temperature. Our findings thus suggest an important role of localized sea ice-ocean interactions in configuring the vertical structure of the Southern Ocean.
  •  
28.
  • Linders, Johanna, et al. (author)
  • The melt-freeze cycle of the Arctic Ocean ice cover and its dependence on ocean stratification
  • 2013
  • In: Journal of Geophysical Research-Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 118:11, s. 5963-5976
  • Journal article (peer-reviewed)abstract
    • A time-dependent, 1-D coupled ice-ocean model is used to quantify the impact of ocean stratification on the Arctic ice cover. The model results show that the ice growth during winter equals the ice melt in summer for areas with a well-developed cold halocline layer (CHL), provided that the initial ice thickness is around 3 m, while thinner initial ice thickness results in net growth. Areas with weak salt stratification can have a negative annual thickness change irrespective of the initial ice thickness and are thus dependent on ice import in order to remain ice covered. The model results also show that ocean stratification is mostly important for ice-thickness development during the growing season. Areas with weak stratification have an ocean heat flux up to 8 W m(-2) reaching the ice during the growing season, while areas with a CHL have an average of about 0.7 W m(-2). In the extreme area, north of Svalbard, the ocean heat fluxes are typically around 25 W m(-2) but can be up to 400 W m(-2) during the initial adjustment, when the warm Atlantic water has direct contact with the ice. A general outcome of the study is that, depending on ocean stratification, the ice cover of Arctic Ocean can be divided into one part with net ice growth (the major part) and another part with net ice melt (mainly in the Nansen Basin).
  •  
29.
  • Luhar, M., et al. (author)
  • Seagrass blade motion under waves and its impact on wave decay
  • 2017
  • In: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291.
  • Journal article (peer-reviewed)abstract
    • © 2017. American Geophysical Union. All Rights Reserved.The hydrodynamic drag generated by seagrass meadows can dissipate wave-energy, causing wave decay. It is well known that this drag depends on the relative motion between the water and the seagrass blades, yet the impact of blade motion on drag and wave-energy dissipation remains to be fully characterized. In this experimental study, we examined the impact of blade motion on wave decay by concurrently recording blade posture during a wave cycle and measuring wave decay over a model seagrass meadow. We also identified a scaling law that predicts wave decay over the model meadow for a range of seagrass blade density, wave period, wave height, and water depth scaled from typical field conditions. Blade flexibility led to significantly lower drag and wave decay relative to theoretical predictions for rigid, upright blades. To quantify the impact of blade motion on wave decay, we employed an effective blade length, le, defined as the rigid blade length that leads to equivalent wave-energy dissipation. We estimated le directly from images of blade motion. Consistent with previous studies, these estimates showed that the effective blade length depends on the dimensionless Cauchy number, which describes the relative magnitude of the wave hydrodynamic drag and the restoring force due to blade rigidity. As the hydrodynamic forcing increases, the blades exhibit greater motion. Greater blade motion leads to smaller relative velocities, reducing drag, and wave-energy dissipation (i.e., smaller le).
  •  
30.
  • Löptien, Ulrike, et al. (author)
  • Long-term characteristics of simulated ice deformation in the Baltic Sea (1962–2007)
  • 2013
  • In: Journal of Geophysical Research - Oceans. - : American Geophysical Union (AGU). - 2169-9291 .- 2169-9275. ; 118:2, s. 801-815
  • Journal article (peer-reviewed)abstract
    • The North Atlantic Oscillation (NAO) index is a frequently used measure for the mean winter conditions in Northern Europe. A positive, high index is associated with strong westerlies and anomalous warm temperatures. The effects on sea ice conditions in the Baltic Sea are twofold. Warm temperatures prevent sea ice formation. If ice is present nevertheless, the strong winds can promote the formation of ice ridges which hinders ship traffic. We use an ocean-sea ice model to investigate the NAO impact on the ridged ice area fraction in the Baltic during 1962–2007. Our simulations indicate that in the northern Bothnian Bay, a high NAO index is related to an anomalous accumulation of ridges, while in the rest of the Baltic Sea, the relationship is contrary. The NAO explains locally at most only 20–25% of the ridged ice fraction interannual variability which indicates the systems complexity. However, we find high skill with local correlations around 0.8 for annually averaged ridged ice fraction reconstructed from multilinear regression using winter averaged wind extremes, surface air temperature, and sea surface temperature (SST). This suggests that the amount of ridged ice in late winter can be derived from these routinely measured quantities. In large parts of the basin, it is sufficient to use the atmospheric parameters as a predictor, while in the eastern Bothnian Bay and southern Gulf of Finland, the SST is required to reconstruct the bulk of the ridged ice fraction.
  •  
31.
  • Melet, Angélique, et al. (author)
  • Internal tide generation by abyssal hills using analytical theory
  • 2013
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 118:11, s. 6303-6318
  • Journal article (peer-reviewed)abstract
    • Internal tide driven mixing plays a key role in sustaining the deep ocean stratification and meridional overturning circulation. Internal tides can be generated by topographic horizontal scales ranging from hundreds of meters to tens of kilometers. State of the art topographic products barely resolve scales smaller than ∼10 km in the deep ocean. On these scales abyssal hills dominate ocean floor roughness. The impact of abyssal hill roughness on internal-tide generation is evaluated in this study. The conversion of M2 barotropic to baroclinic tidal energy is calculated based on linear wave theory both in real and spectral space using the Shuttle Radar Topography Mission SRTM30_PLUS bathymetric product at 1/120° resolution with and without the addition of synthetic abyssal hill roughness. Internal tide generation by abyssal hills integrates to 0.1 TW globally or 0.03 TW when the energy flux is empirically corrected for supercritical slope (i.e., ∼10% of the energy flux due to larger topographic scales resolved in standard products in both cases). The abyssal hill driven energy conversion is dominated by mid-ocean ridges, where abyssal hill roughness is large. Focusing on two regions located over the Mid-Atlantic Ridge and the East Pacific Rise, it is shown that regionally linear theory predicts an increase of the energy flux due to abyssal hills of up to 100% or 60% when an empirical correction for supercritical slopes is attempted. Therefore, abyssal hills, unresolved in state of the art topographic products, can have a strong impact on internal tide generation, especially over mid-ocean ridges.
  •  
32.
  • Parampil, Sindu Raj, et al. (author)
  • Observed subseasonal variability of heat flux and the SST response of the tropical Indian Ocean
  • 2016
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 121:10, s. 7290-7307
  • Journal article (peer-reviewed)abstract
    • We develop an experimental daily surface heat flux data set based on satellite observations to study subseasonal variability (periods shorter than 90 days) in the tropical Indian Ocean. We use incoming shortwave and longwave radiation from the International Satellite Cloud Climatology Project, and sea surface temperature (SST) from microwave sensors, to estimate net radiative flux. Latent and sensible heat fluxes are estimated from scatterometer winds and near-surface air temperature and specific humidity from Atmospheric Infrared Sounder (AIRS) observations calibrated to buoy data. Seasonal biases in net heat flux are generally within 10 W m(-2) of estimates from moorings, and the phases and amplitudes of subseasonal variability of heat fluxes are realistic. We find that the contribution of subseasonal changes in air-sea humidity gradients to latent heat flux equals or exceeds the contribution of subseasonal changes in wind speed in all seasons. SST responds coherently to subseasonal oscillations of net heat flux associated with active and suppressed phases of atmospheric convection in the summer hemisphere. Thus, subseasonal SST changes are mainly forced by heat flux in the northeast Indian Ocean in northern summer, and in the 15 degrees S-5 degrees N latitude belt in southern summer. In the winter hemisphere, subseasonal SST changes are not a one-dimensional response to heat flux, implying that they are mainly due to oceanic advection, entrainment, or vertical mixing. The coherent evolution of subseasonal SST variability and surface heat flux suggests active coupling between SST and large-scale, organized tropical convection in the summer season.
  •  
33.
  • Pellichero, Violaine, et al. (author)
  • The ocean mixed layer under Southern Ocean sea-ice : Seasonal cycle and forcing
  • 2017
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 122:2, s. 1608-1633
  • Journal article (peer-reviewed)abstract
    • The oceanic mixed layer is the gateway for the exchanges between the atmosphere and the ocean; in this layer, all hydrographic ocean properties are set for months to millennia. A vast area of the Southern Ocean is seasonally capped by sea-ice, which alters the characteristics of the ocean mixed layer. The interaction between the ocean mixed layer and sea-ice plays a key role for water mass transformation, the carbon cycle, sea-ice dynamics, and ultimately for the climate as a whole. However, the structure and characteristics of the under-ice mixed layer are poorly understood due to the sparseness of in situ observations and measurements. In this study, we combine distinct sources of observations to overcome this lack in our understanding of the polar regions. Working with elephant seal-derived, ship-based, and Argo float observations, we describe the seasonal cycle of the ocean mixed-layer characteristics and stability of the ocean mixed layer over the Southern Ocean and specifically under sea-ice. Mixed-layer heat and freshwater budgets are used to investigate the main forcing mechanisms of the mixed-layer seasonal cycle. The seasonal variability of sea surface salinity and temperature are primarily driven by surface processes, dominated by sea-ice freshwater flux for the salt budget and by air-sea flux for the heat budget. Ekman advection, vertical diffusivity, and vertical entrainment play only secondary roles. Our results suggest that changes in regional sea-ice distribution and annual duration, as currently observed, widely affect the buoyancy budget of the underlying mixed layer, and impact large-scale water mass formation and transformation with far reaching consequences for ocean ventilation.
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34.
  • Pemberton, Per, et al. (author)
  • The response of the central Arctic Ocean stratification to freshwater perturbations
  • 2016
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 121:1, s. 792-817
  • Journal article (peer-reviewed)abstract
    • Using a state-of-the-art coupled ice-ocean-circulation model, we perform a number of sensitivity experiments to examine how the central Arctic Ocean stratification responds to changes in river runoff and precipitation. The simulations yield marked changes in the cold halocline and the Arctic Atlantic layer. Increased precipitation yields a warming of the Atlantic layer, which primarily is an advective signal, propagated through the St. Anna Trough, reflecting air-sea heat flux changes over the Barents Sea. As the freshwater supply is increased, the anticyclonic Beaufort Gyre is weakened and a greater proportion of the Arctic Ocean freshwater is exported via the Fram Strait, with nearly compensating export decreases through the Canadian Arctic Archipelago. The corresponding reorganization of the freshwater pool appears to be controlled by advective processes, rather than by the local changes in the surface freshwater flux. A simple conceptual model of the Arctic Ocean, based on a geostrophically controlled discharge of the low-salinity water, is introduced and compared with the simulations. Key predictions of the conceptual model are that the halocline depth should decrease with increasing freshwater input and that the Arctic Ocean freshwater storage should increase proportionally to the square root of the freshwater input, which are in broad qualitative agreement with the sensitivity experiments. However, the model-simulated rate of increase of the freshwater storage is weaker, indicating that effects related to wind forcing and rerouting of the freshwater-transport pathways play an important role for the dynamics of the Arctic Ocean freshwater storage.
  •  
35.
  • Qiao, Wenli, et al. (author)
  • Momentum Flux Balance at the Air-Sea Interface
  • 2021
  • In: Journal of Geophysical Research - Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 126:2
  • Journal article (peer-reviewed)abstract
    • Abstract Ocean waves can spatiotemporally redistribute the momentum flux at the air-sea interface, which varies with the sea state. Traditional atmosphere-ocean coupled systems assume the ocean-side stress (Ï„oc) to be identical to the air-side stress (Ï„a); consequently, the role of ocean waves is neglected. In this study, the wave impacts on the air-sea momentum flux are investigated based on 1-year high-resolution model simulations in the Baltic Sea using an atmosphere-wave coupled model (Uppsala University-Coupled Model, UU-CM). The simulation results show that Ï„oc can differ significantly from Ï„a in both direction and magnitude. The direction difference between Ï„oc and Ï„a (DD(Ï„oc, Ï„a)) and the normalized momentum flux () decrease with increasing inverse wave age. In general, and DD(Ï„oc, Ï„a) are pronounced under wind-following swell and wind-crossing swell conditions, respectively. The occurrence frequencies of large and DD(Ï„oc, Ï„a) are higher nearer the coast; statistically, both decrease significantly with increasing water depth because of the joint effect of dissipation processes. Based on four selected areas, we find that alongshore winds (winds blowing parallel to the coastline) are favorable for large angular differences between Ï„oc and Ï„a (DD(Ï„oc, Ï„a) > 5°). However, onshore winds predominate at . The Ï„a in the wave model is generally less than that obtained from the atmospheric model under low-moderate wind conditions if the wave model feeds only the Charnock coefficient (roughness length) back to the atmospheric model in coupled systems.
  •  
36.
  • Rosen, P. O., et al. (author)
  • Ice export from the Laptev and East Siberian Sea derived from O-18 values
  • 2015
  • In: Journal of Geophysical Research-Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 120:9, s. 5997-6007
  • Journal article (peer-reviewed)abstract
    • Ice export from the vast Arctic Siberian shelf is calculated using O-18 values and salinity data for water samples collected during the International Siberian Shelf Study between August and September 2008 (ISSS-08). The samples represent a wide range of salinities and O-18 values due to river water inputs and sea ice removal. We estimate the fraction of water that has been removed as ice by interpreting observed O-18 values and salinities as a result of mixing between river water and sea water end-members as well as to fractional ice removal. This method does not assume an ice end-member of fixed composition, which is especially important when applied on samples with large differences in salinity. The results show that there is net transport of ice from both the Laptev and the Eastern Siberian Seas, and in total 3000 km(3) of sea ice is exported from the shelf. The annual total export of ice from the entire region, calculated from the residence time of water on the shelf, is estimated to be 860 km(3) yr(-1). Thus, changes in ice production on the shelf may have great impact on sea ice export from the Arctic Ocean.
  •  
37.
  •  
38.
  • Sjöblom, Anna, et al. (author)
  • The turbulent kinetic energy budget in the marine atmospheric surface layer
  • 2002
  • In: Journal of Geophysical Research - Oceans and Atmospheres. - 2169-9275 .- 2169-9291. ; 107:C10, s. 616-618
  • Journal article (peer-reviewed)abstract
    • [1] The terms in the turbulent kinetic energy (TKE) budget have been analyzed according to stability, wave age, and wind speed, using long-term measurements over the sea. The measurements were performed at the island Östergarnsholm in the middle of the Baltic Sea. The results show that there is an imbalance between normalized production and normalized dissipation, also in neutral conditions, and that this imbalance depends not only on stability, which has been previously suggested, but also on wave age and wind speed. For small wave ages and high wind speeds, production is larger than dissipation at neutral conditions. For saturated waves and moderate wind speeds, the sea surface resembles a land surface, while for swell and low wind speeds, dissipation strongly exceeds production. The normalized pressure transport becomes significant during swell conditions, and is not balanced by the normalized turbulent transport. “Inactive” turbulence, where energy is being brought down to the surface from higher levels, is probably the reason for these high values of the pressure transport. The traditional “inertial dissipation method,” where the sum of the transport terms is assumed small and neglected, therefore needs to be corrected for an imbalance between production and dissipation.
  •  
39.
  • Sun, Xiaole, 1983-, et al. (author)
  • Large-Scale Summertime Variability of Carbonate Chemistry Across the East Siberian Sea: Primary Production Versus Ikaite Dissolution
  • 2024
  • In: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291. ; 129:1
  • Journal article (peer-reviewed)abstract
    • Sea-ice dynamics can affect carbon cycling in polar oceans, with sea-ice ikaite acting as a potentially important carbon pump. However, there is no large-scale direct field evidence to support this. Here we used a unique data set that combined continuous measurements of atmospheric and water CO2 concentrations with water chemistry data collected over 1,200km along the East Siberian Sea, the widest Arctic shelf sea. Our results reveal large spatial heterogeneity of sea-ice ikaite contents, which directly interact with carbonate chemistry in the water column. Our findings demonstrate that the CO2 drawdown by sea-ice ikaite dissolution could be as important as that by primary production. We suggest that the role of ikaite in regulating the seasonal carbon cycle on a regional scale could be more important than we previously thought. Effects of the warmer climate on sea ice loss might also play a role in the ikaite inventory.
  •  
40.
  • ten Doeschate, A., et al. (author)
  • Upper Ocean Response to Rain Observed From a Vertical Profiler
  • 2019
  • In: Journal of Geophysical Research - Oceans. - : AMER GEOPHYSICAL UNION. - 2169-9275 .- 2169-9291. ; 124:6, s. 3664-3681
  • Journal article (peer-reviewed)abstract
    • Rainfall induces a vertical salinity gradient directly below the ocean surface, the strength and lifetime of which depend on the size of the rain event, the availability of mixing, and the air-sea heat fluxes. The presence of rain in turn influences the near-surface turbulent mixing and air-sea exchange processes. During a campaign in the midlatitude North Atlantic, the Air-Sea Interaction Profiler (ASIP) was used to investigate changes in the vertical distribution of salinity (S), temperature (T), and turbulent kinetic energy dissipation rate (E) caused by four rain events. During one of the rain events a strong shallow stratification was formed. The buoyancy effect of this freshwater lens changes the dominant wind-driven turbulent mixing. The surface momentum flux was limited to a shallow layer, and below it E is reduced by 2 orders of magnitude. For a different rain event of higher-peak rain rate, the salinity anomaly is smaller and is dispersed deeper into the water column. The difference in ocean response shows that the upper ocean is sensitive to changes in the atmospheric forcing associated with the rain events. The observed salinity anomalies as a function of rain rate and wind speed are compared to relationships from studies with the 1-D turbulence model GOTM and satellite validation. The observations suggest that the vertical salinity anomaly is best described as a function of total rain. A higher-resolution prognostic model for sea surface salinity and temperature is shown to perform well in predicting the observed S and T anomalies.
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41.
  • Toppaladoddi, S., et al. (author)
  • Seasonal Evolution of the Arctic Sea Ice Thickness Distribution
  • 2023
  • In: Journal of Geophysical Research - Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 128:5
  • Journal article (peer-reviewed)abstract
    • The Thorndike et al. (1975, ) theory of the ice thickness distribution, g(h), treats the dynamic and thermodynamic aggregate properties of the ice pack in a novel and physically self-consistent manner. Therefore, it has provided the conceptual basis of the treatment of sea-ice thickness categories in climate models. The approach, however, is not mathematically closed due to the treatment of mechanical deformation using the redistribution function ?, the authors noting "The present theory suffers from a burdensome and arbitrary redistribution function ?." Toppaladoddi and Wettlaufer (2015, ) showed how ? can be written in terms of g(h), thereby solving the mathematical closure problem and writing the theory in terms of a Fokker-Planck equation, which they solved analytically to quantitatively reproduce the observed winter g(h). Here, we extend this approach to include open water by formulating a new boundary condition for their Fokker-Planck equation, which is then coupled to the observationally consistent sea-ice growth model of Semtner (1976, ) to study the seasonal evolution of g(h). We find that as the ice thins, g(h) transitions from a single- to a double-peaked distribution, which is in agreement with observations. To understand the cause of this transition, we construct a simpler description of the system using the equivalent Langevin equation formulation and solve the resulting stochastic ordinary differential equation numerically. Finally, we solve the Fokker-Planck equation for g(h) under different climatological conditions to study the evolution of the open-water fraction.Plain Language Summary A quantitative understanding of the evolution of the thickness distribution of sea ice is necessary to accurately predict changes in the Arctic ice cover. In the original formulation of the governing equation for the thickness distribution by Thorndike et al., the treatment of the redistribution term-which represents the mechanical deformation of ice by rafting and ridging-is referred to as "arbitrar y" and "burdensome." Using an analogy with Brownian motion, we have recast the redistribution term, closed the original theory and incorporated the process of open water formation to produce seasonal predictions of the thickness distribution. Using our theory we show that a second peak in the thickness distribution emerges in summer, which is consistent with observations. Furthermore, we explore how the greenhouse gas and oceanic heat flux forcings impact the open-water fraction and mean thickness, and the relative sensitivities to these forcings.
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42.
  • Wu, Lichuan, et al. (author)
  • Swell impact on wind stress and atmospheric mixing in a regional coupled atmosphere-wave model
  • 2016
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 121:7, s. 4633-4648
  • Journal article (peer-reviewed)abstract
    • Over the ocean, the atmospheric turbulence can be significantly affected by swell waves. Change in the atmospheric turbulence affects the wind stress and atmospheric mixing over swell waves. In this study, the influence of swell on atmospheric mixing and wind stress is introduced into an atmosphere-wave-coupled regional climate model, separately and combined. The swell influence on atmospheric mixing is introduced into the atmospheric mixing length formula by adding a swell-induced contribution to the mixing. The swell influence on the wind stress under wind-following swell, moderate-range wind, and near-neutral and unstable stratification conditions is introduced by changing the roughness length. Five year simulation results indicate that adding the swell influence on atmospheric mixing has limited influence, only slightly increasing the near-surface wind speed; in contrast, adding the swell influence on wind stress reduces the near-surface wind speed. Introducing the wave influence roughness length has a larger influence than does adding the swell influence on mixing. Compared with measurements, adding the swell influence on both atmospheric mixing and wind stress gives the best model performance for the wind speed. The influence varies with wave characteristics for different sea basins. Swell occurs infrequently in the studied area, and one could expect more influence in high-swell-frequency areas (i.e., low-latitude ocean). We conclude that the influence of swell on atmospheric mixing and wind stress should be considered when developing climate models.
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43.
  • Wu, Lichuan, et al. (author)
  • Upper-ocean mixing due to surface gravity waves
  • 2015
  • In: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 120:12, s. 8210-8228
  • Journal article (peer-reviewed)abstract
    • Surface gravity waves play an important role in the lower layer of the atmosphere and the upper layer of the ocean. Surface waves effect upper-ocean mixing mainly through four processes: wave breaking, Stokes drift interaction with the Coriolis force, Langmuir circulation, and stirring by nonbreaking waves. We introduce the impact of these four processes into a 1-D  ocean turbulence model. The parameterizations used are based mainly on existing investigations. Comparison of simulation results and measurements demonstrates that considering all the effects of waves, rather than just one effect, significantly improves model performance. The nonbreaking-wave-induced mixing and Langmuir turbulence are the most important terms when considering the impact of waves on upper-ocean mixing. Under high-wave conditions, the turbulent mixing induced by nonbreaking waves can be of the same order of magnitude as the viscosity induced by other terms at the surface. Nonbreaking waves contribute very little to shear production and their impact is negligible in the models. Sensitivity experiments demonstrate that the vertical profile of the Stokes drift calculated from the 2-D wave spectrum improves model performance significantly compared with other methods of introducing wave effects.
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44.
  • Wu, Lichuan, et al. (author)
  • Wind Profile in the Wave Boundary Layer and Its Application in a Coupled Atmosphere-Wave Model
  • 2022
  • In: Journal of Geophysical Research - Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 127:2
  • Journal article (peer-reviewed)abstract
    • Current models cannot capture well the impacts of wind waves on the atmospheric boundary layer. Here, we proposed a new turbulence closure model to estimate the wind stress in the wave boundary layer from viscous stress, shear-induced turbulent stress, wind-sea induced stress, and swell-induced upward stress separately. The misalignment between the wind stress and wind is also considered in the model. Single-column simulations indicate that (a) the swell-induced upward momentum flux increases the surface wind and changes the wind direction, (b) the misalignment between the upward momentum flux and wind has a more significant impact on the wind profile than that from the downward momentum flux, and (c) the impact of swell-induced upward momentum flux decreases with atmospheric convection. The proposed closure scheme was implemented into an atmosphere-wave coupled model. A month-long simulation over the ocean off California shows that the surface wind can be altered up to 5% by ocean surface gravity waves.Plain Language SummaryAir-sea interactions are important for weather and climate predictions since they control the momentum and energy transfer between the atmosphere and the ocean. In current models, the momentum flux in the atmospheric boundary layer is estimated by turbulence closure models, which were developed heavily based on measurements over land. However, those turbulence closure models often fail to capture the momentum flux and wind profile in the marine atmospheric boundary layer due to wave impacts. In this study, we developed a new turbulence closure model that can capture the impacts of swell-induced upward momentum flux and the misalignment angle between the stress and wind as well as the swell-induced low-level wind jet. A month-long simulation indicates that considering the wave, impacts can alter the surface wind up to 5%. Thus, it is necessary to implement those wave impacts into ocean, weather, and climate models.
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45.
  • Zhao, Biao, Dr, et al. (author)
  • A Numerical Study of Tropical Cyclone and Ocean Responses to Air‐Sea Momentum Flux at High Winds
  • 2024
  • In: Journal of Geophysical Research - Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 129:7
  • Journal article (peer-reviewed)abstract
    • The relationship between minimum sea level pressure (MSLP) and maximum wind speed, is a commonly employed metric for assessing tropical cyclone (TC) forecast skill. However, accurately reproducing this relationship in TC forecasts is challenging. By introducing a new air-sea momentum flux scheme considering both wave state and saturation (decrease) effect at high winds into a fully coupled atmosphere-ocean-wave model, our numerical results reveal that the maximum wind speed and MSLP respond oppositely to the air-sea momentum flux change. The simulated wind-pressure relationship aligns well with observations when the new flux scheme is used. This study highlights the large sensitivity of wind-pressure relationship to the air-sea momentum flux parameterization at high winds. Furthermore, the air-sea momentum flux has a significant effect on ocean wave characteristics and sea surface temperature (SST) cooling in TC simulations.
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46.
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47.
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48.
  • Zhou, Lu, 1993, et al. (author)
  • Sea Ice Production in the 2016 and 2017 Maud Rise Polynyas
  • 2023
  • In: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291. ; 128:2
  • Journal article (peer-reviewed)abstract
    • Sea ice production within polynyas, an outcome of the atmosphere-ice-ocean interaction, is a major source of dense water and hence key to the global overturning circulation, but is poorly quantified over open-ocean polynyas. Using the two recent extensive open-ocean polynyas within the wider Maud Rise region of the Weddell Sea in 2016 and 2017, we here explore the sea ice energy budget and estimate their sea ice production based on satellite retrievals, in-situ hydrographic observations and the Japanese 55-year Reanalysis. We find that the oceanic heat flux amounts to 36.1 and 30.7Wm−2 within the 2016 and 2017 polynyas, respectively. Especially the 2017 open-ocean polynya produced nearly 200km3 of new sea ice, which is comparable to the production in the largest Antarctic coastal polynyas. Finally, we determine that ice production is highly correlated with and sensitive to skin temperature and wind speed, which affect the turbulent fluxes. It is also strongly sensitive to uncertainties in the sea ice concentration and 1,000hPa temperature, which all urgently need to be better monitored at high latitudes. Lastly, more process-oriented campaigns are required to further elucidate the role of open-ocean polynya on the local and global ocean circulations.
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49.
  • Azaneu, M., et al. (author)
  • Influence of Shelf Break Processes on the Transport of Warm Waters Onto the Eastern Amundsen Sea Continental Shelf
  • 2023
  • In: Journal of Geophysical Research-Oceans. - 2169-9275. ; 128:5
  • Journal article (peer-reviewed)abstract
    • The heat transported onto the continental shelf by Circumpolar Deep Water (CDW) is the main driver of ice shelf basal melting in the Amundsen Sea. Here, we investigate the slope current system and the variability of the heat transported through the Pine Island-Thwaites central and eastern troughs using data from five moorings deployed in the region between 5 March 2012 and 7 February 2016. Substantial variability on intermonthly time scales (3-4 months) is observed in the onshore heat flux, driven primarily by zonal wind stress north of the shelf break. Heat content, onshore flow, and heat flux are highly correlated between central and eastern troughs, which are most likely dynamically linked by the zonal wind stress forcing. This is the first time this dynamic link between troughs is observed. In the eastern the Amundsen Sea, during the El Nino of 2015/2016, strong eastward winds led to lower temperatures over the continental shelf while the onshore heat flux is intensified. We hypothesize that this anti-correlation between heat content and heat flux results from a strengthened eastward undercurrent leading to upwelling of a colder and deeper CDW variety. These results highlight the complex and heterogeneous response of this region to environmental and the importance of velocity data for understanding the dynamics in this region. It also suggests that the hypothesized link between large-scale atmospheric forcing (e.g., El Nino-Southern Oscillation) and ice-shelf melt is not produced via changes in heat content, but instead via changes in onshore heat flux.
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50.
  • Biddle, Louise C., et al. (author)
  • Upper Ocean Distribution of Glacial Meltwater in the Amundsen Sea, Antarctica
  • 2019
  • In: Journal of Geophysical Research-Oceans. - 2169-9275. ; 124:10, s. 6854-6870
  • Journal article (peer-reviewed)abstract
    • Pine Island Ice Shelf, in the Amundsen Sea, is losing mass due to increased heat transport by warm ocean water penetrating beneath the ice shelf and causing basal melt. Tracing this warm deep water and the resulting glacial meltwater can identify changes in melt rate and the regions most affected by the increased input of this freshwater. Here, optimum multiparameter analysis is used to deduce glacial meltwater fractions from independent water mass characteristics (standard hydrographic observations, (NG), and oxygen isotopes), collected during a ship-based campaign in the eastern Amundsen Sea in February-March 2014. (NG) (neon, argon, krypton, and xenon) and oxygen isotopes are used to trace the glacial melt and meteoric water found in seawater, and we demonstrate how their signatures can be used to rectify the hydrographic trace of glacial meltwater, which provides a much higher-resolution picture. The presence of glacial meltwater is shown to mask the Winter Water properties, resulting in differences between the water mass analyses of up to 4-g/kg glacial meltwater content. This discrepancy can be accounted for by redefining the "pure" Winter Water endpoint in the hydrographic glacial meltwater calculation. The corrected glacial meltwater content values show a persistent signature between 150 and 400 m of the water column across all of the sample locations (up to 535 km from Pine Island Ice Shelf), with increased concentration toward the west along the coastline. It also shows, for the first time, the signature of glacial meltwater flowing off-shelf in the eastern channel.
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