| 21. |
- Gülk, Birte, 1994, et al.
(författare)
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Impacts of Vertical Convective Mixing Schemes and Freshwater Forcing on the 2016-2017 Maud Rise Polynya Openings in a Regional Ocean Simulation
- 2024
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Ingår i: JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS. - 1942-2466. ; 16:5
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Tidskriftsartikel (refereegranskat)abstract
- The correct representation of the Maud Rise open-ocean polynya in the Weddell Sea remains a challenge for ocean models. Here we reproduce the most recent polynya openings in 2016-2017 using a regional configuration, and assess their dependencies on vertical convective mixing schemes and freshwater forcing, both separately and in combination. We test three vertical convective mixing schemes: the enhanced vertical diffusion (EVD), the Eddy-Diffusivity Mass-Flux (EDMF) parameterization, and a modified version of EDMF accounting for thermobaric effects. Using simulations for the period 2007-2017, we find that the modified EDMF reproduces the observed climatological evolution of the mixed layer depth better than the original EDMF and the EVD, but a polynya fails to open due to excessive freshwater forcing. We thus use the modified EDMF to perform sensitivity experiments with reduced precipitation during 2012-2017. The imposed freshwater forcing strongly affects the number of years with polynyas. The simulation with the best representation of the 2016-2017 polynyas is analyzed to evaluate the triggering mechanisms. The 2016 polynya was induced by the action of thermobaric instabilities on a weak ambient stratification. This opening preconditioned the water column for 2017, which produced a stronger polynya. By examining the impacts of the different convective mixing schemes, we show that the modified EDMF generates more realistic patterns of deep convection. Our results highlight the importance of surface freshwater forcing and thermobaricity in governing deep convection around Maud Rise, and the need to represent thermobaric instabilities to realistically model Maud Rise polynyas. We investigate the impacts of representing numerical vertical mixing and surface freshwater forcing in a regional ocean model on polynyas (large openings in the pack ice) at Maud Rise, Southern Ocean. Maud Rise is prone to hosting polynyas, often associated with deep convection, which is a local vertical mixing process homogenizing the water column between surface and depths of several hundred meters. Numerical models often use simplistic strategies to represent this process, but improved parameterizations have recently become available. In this work, we test the impact of the representation of convective mixing in a particularly sensitive region. The last Maud Rise polynyas were observed in 2016 and 2017. Our regional simulation is capable of reproducing these polynyas, which has long been a challenge for ocean-sea ice models. We show that the 2016 polynya resulted from the action of a vertical instability at depth acting on weak ambient stratification. This event preconditioned the stronger 2017 polynya and deep convection. We conclude that representing convective plumes as a sub-grid scale process in models leads to a more realistic representation of open-ocean polynyas and associated convection events. The Eddy-Diffusivity Mass-Flux (EDMF) parameterization is tested in a regional simulation of the ocean around Maud Rise Thermobaric effects on convective plumes are enabled by modifying the EDMF parameterization Simulations of Maud Rise polynyas are highly sensitive to freshwater forcing and mixing schemes
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| 22. |
- Gülk, Birte, 1994, et al.
(författare)
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Variability and Remote Controls of the Warm-Water Halo and Taylor Cap at Maud Rise
- 2023
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Ingår i: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291. ; 128:7
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Tidskriftsartikel (refereegranskat)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.
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| 23. |
- Harcourt, R., et al.
(författare)
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Animal-borne telemetry: An integral component of the ocean observing toolkit
- 2019
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Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 6:JUN
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Tidskriftsartikel (refereegranskat)abstract
- Animal telemetry is a powerful tool for observing marine animals and the physical environments that they inhabit, from coastal and continental shelf ecosystems to polar seas and open oceans. Satellite-linked biologgers and networks of acoustic receivers allow animals to be reliably monitored over scales of tens of meters to thousands of kilometers, giving insight into their habitat use, home range size, the phenology of migratory patterns and the biotic and abiotic factors that drive their distributions. Furthermore, physical environmental variables can be collected using animals as autonomous sampling platforms, increasing spatial and temporal coverage of global oceanographic observation systems. The use of animal telemetry, therefore, has the capacity to provide measures from a suite of essential ocean variables (EOVs) for improved monitoring of Earth's oceans. Here we outline the design features of animal telemetry systems, describe current applications and their benefits and challenges, and discuss future directions. We describe new analytical techniques that improve our ability to not only quantify animal movements but to also provide a powerful framework for comparative studies across taxa. We discuss the application of animal telemetry and its capacity to collect biotic and abiotic data, how the data collected can be incorporated into ocean observing systems, and the role these data can play in improved ocean management. © 2019 Harcourt, Sequeira, Zhang, Roquet, Komatsu, Heupel, McMahon, Whoriskey, Meekan, Carroll, Brodie, Simpfendorfer, Hindell, Jonsen, Costa, Block, Muelbert, Woodward, Weise, Aarestrup, Biuw, Boehme, Bograd, Cazau, Charrassin, Cooke, Cowley, de Bruyn, Jeanniard du Dot, Duarte, Eguíluz, Ferreira, Fernández-Gracia, Goetz, Goto, Guinet, Hammill, Hays, Hazen, Hückstädt, Huveneers, Iverson, Jaaman, Kittiwattanawong, Kovacs, Lydersen, Moltmann, Naruoka, Phillips, Picard, Queiroz, Reverdin, Sato, Sims, Thorstad, Thums, Treasure, Trites, Williams, Yonehara and Fedak.
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| 24. |
- Hindell, Mark A., et al.
(författare)
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Circumpolar habitat use in the southern elephant seal : implications for foraging success and population trajectories
- 2016
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Ingår i: Ecosphere. - : Wiley. - 2150-8925 .- 2150-8925. ; 7:5
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Tidskriftsartikel (refereegranskat)abstract
- In the Southern Ocean, wide-ranging predators offer the opportunity to quantify how animals respond to differences in the environment because their behavior and population trends are an integrated signal of prevailing conditions within multiple marine habitats. Southern elephant seals in particular, can provide useful insights due to their circumpolar distribution, their long and distant migrations and their performance of extended bouts of deep diving. Furthermore, across their range, elephant seal populations have very different population trends. In this study, we present a data set from the International Polar Year project; Marine Mammals Exploring the Oceans Pole to Pole for southern elephant seals, in which a large number of instruments (N = 287) deployed on animals, encompassing a broad circum-Antarctic geographic extent, collected in situ ocean data and at-sea foraging metrics that explicitly link foraging behavior and habitat structure in time and space. Broadly speaking, the seals foraged in two habitats, the relatively shallow waters of the Antarctic continental shelf and the Kerguelen Plateau and deep open water regions. Animals of both sexes were more likely to exhibit area-restricted search (ARS) behavior rather than transit in shelf habitats. While Antarctic shelf waters can be regarded as prime habitat for both sexes, female seals tend to move northwards with the advance of sea ice in the late autumn or early winter. The water masses used by the seals also influenced their behavioral mode, with female ARS behavior being most likely in modified Circumpolar Deepwater or northerly Modified Shelf Water, both of which tend to be associated with the outer reaches of the Antarctic Continental Shelf. The combined effects of (1) the differing habitat quality, (2) differing responses to encroaching ice as the winter progresses among colonies, (3) differing distances between breeding and haul-out sites and high quality habitats, and (4) differing long-term -regional trends in sea ice extent can explain the differing population trends observed among elephant seal colonies.
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| 25. |
- Kitade, Yujiro, et al.
(författare)
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Antarctic Bottom Water production from the Vincennes Bay Polynya, East Antarctica
- 2014
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 41:10, s. 3528-3534
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Tidskriftsartikel (refereegranskat)abstract
- One year moorings at depths greater than 3000m on the continental slope off Vincennes Bay, East Antarctica, reveal the cold (<-0.5 degrees C) and fresh (<34.64) signals of newly formed Antarctic Bottom Water (AABW). The signal appeared in June, 3 months after the onset of active sea-ice production in the nearby Vincennes Bay Polynya (VBP). The AABW signal continued for about 5 months at two moorings, with 1 month delay at the western site further downstream. Ship-based hydrographic data are in agreement, detecting the westward spread of new AABW over the continental slope from VBP. On the continental shelf, Dense Shelf Water (DSW) formation is observed by instrumented seals, in and around the VBP during autumn, and we estimate its transport to be 0.16 +/- 0.07 (x 106m3s-1). We conclude that the DSW formed in this region, albeit from a modest amount of sea-ice production, nonetheless contributes to the upper layer of AABW in Australian-Antarctic Basin.
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| 26. |
- Klocker, Andreas, et al.
(författare)
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Deep-Reaching Global Ocean Overturning Circulation Generated by Surface Buoyancy Forcing
- 2023
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Ingår i: Tellus, Series A: Dynamic Meteorology and Oceanography. - 0280-6495 .- 1600-0870. ; 75:1, s. 392-409
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Tidskriftsartikel (refereegranskat)abstract
- In contrast with the atmosphere, which is heated from below by solar radiation, the ocean is both heated and cooled from above. To drive a deep-reaching overturning circulation in this context, it is generally assumed that either intense interior mixing by winds and internal tides, or wind-driven upwelling is required; in their absence, the circulation is thought to collapse to a shallow surface cell. We demonstrate, using a primitive equation model with an idealized domain and no wind forcing, that the surface temperature forcing can in fact drive an interhemispheric overturning provided that there is an open channel unblocked in the zonal direction, such as in the Southern Ocean. With this geometry, rotating horizontal convection, in combination with asymmetric surface cooling between the north and south, drives a deep-reaching two-cell overturning circulation. The resulting vertical mid-depth stratification closely resembles that of the real ocean, suggesting that wind-driven pumping is not necessary to produce a deep-reaching overturning circulation, and that buoyancy forcing plays a more important role than is usually assumed.
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| 27. |
- Klocker, A., et al.
(författare)
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Generation of the Internal Pycnocline in the Subpolar Southern Ocean by Wintertime Sea Ice Melting
- 2023
-
Ingår i: Journal of Geophysical Research-Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 128:3
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Tidskriftsartikel (refereegranskat)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.
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| 28. |
- Kolbe, Marlen, et al.
(författare)
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Impact of Thermohaline Variability on Sea Level Changes in the Southern Ocean
- 2021
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Ingår i: Journal of Geophysical Research - Oceans. - 0148-0227 .- 2156-2202. ; 126:9
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Tidskriftsartikel (refereegranskat)abstract
- The Southern Ocean is responsible for the majority of the global oceanic heat uptake that contributes to global sea level rise. At the same time, ocean temperatures do not change at the same rate in all regions and sea level variability is also affected by changes in salinity. This study investigates 10 years of steric height variability (2008–2017) in the Southern Ocean (30°S to 70°S) by analyzing temperature and salinity variations obtained from the GLORYS-031 model provided by the European Copernicus Marine Environment Monitoring Service. The thermohaline variability is decomposed into thermohaline modes using a functional Principal Component Analysis. Thermohaline modes provide a natural basis to decompose the joint temperature-salinity vertical profiles into a sum of vertical modes weighted by their respective principal components that can be related to steric height. Interannual steric height trends are found to differ significantly between subtropical and subpolar regions, simultaneously with a shift from a thermohaline stratification dominated by the first “thermal” mode in the north to the second ’saline’ mode in the South. The Polar Front appears as a natural boundary between the two regions, where steric height variations are minimized. Despite higher melt rates and atmospheric temperatures, steric height in Antarctic waters (0–2,000m) has dropped since 2008 due to higher salt content in the surface and upper intermediate layer and partially colder waters, while subtropical waters farther north have mostly risen due to increased heat storage.
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| 29. |
- Labrousse, S., et al.
(författare)
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Weddell seal behaviour during an exceptional oceanographic event in the Filchner-Ronne Ice Shelf in 2017
- 2021
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Ingår i: Antarctic Science. - : Cambridge University Press (CUP). - 0954-1020 .- 1365-2079. ; 33:3, s. 252-264
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Tidskriftsartikel (refereegranskat)abstract
- Rapid and regionally contrasting climate changes have been observed around Antarctica. However, our understanding of the impact of these changes on ecosystems remains limited, and there is an urgent need to better identify habitats of Antarctic species. The Weddell seal (Leptonychotes weddellii) is a circumpolar mesopredator and an indicative species of Antarctic marine communities. It has been extensively studied in the western Ross Sea and East Antarctica, and an understanding of its ecology in the Weddell Sea in the wintertime is emerging. We documented the behavioural response(s) of four Weddell seals from February to June in 2017 in the Filchner-Ronne Ice Shelf region and related these to unusual oceanographic conditions in 2017. Unexpectedly, we found that Weddell seals had the longest foraging effort within the outflow of Ice Shelf Water or at its turbulent boundary. They also foraged on the eastern side of the trough from April to June within the Modified Warm Deep Water and seem to take advantage of the unusual conditions of persistent inflow of warm waters through the winter. Linking animal behavioural responses to oceanographic conditions is informative for quantifying rarely recorded events and provides great insight into how predators may respond to changing conditions.
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| 30. |
- Labrousse, Sara, et al.
(författare)
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Winter use of sea ice and ocean water mass habitat by southern elephant seals : The length and breadth of the mystery
- 2015
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Ingår i: Progress in Oceanography. - : Elsevier BV. - 0079-6611 .- 1873-4472. ; 137, s. 52-68
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Forskningsöversikt (refereegranskat)abstract
- Understanding the responses of animals to the environment is crucial for identifying critical foraging habitat. Elephant seals (Mirounga leonine) from the Kerguelen Islands (49 degrees 20'S, 70 degrees 20'E) have several different foraging strategies. Why some individuals undertake long trips to the Antarctic continent while others utilize the relatively close frontal zones is poorly understood. Here, we investigate how physical properties within the sea ice zone are linked to foraging activities of southern elephant seals (SES). To do this, we first developed a new approach using indices of foraging derived from high temporal resolution dive and accelerometry data to predict foraging behaviour in an extensive, low resolution dataset from CTD-Satellite Relay Data Loggers (CTD-SRDLs). A sample of 37 post-breeding SES females were used to construct a predictive model applied to demersal and pelagic dive strategies relating prey encounter events (PEE) to dive parameters (dive duration, bottom duration, hunting-time, maximum depth, ascent speed, descent speed, sinuosity, and horizontal speed) for each strategy. We applied these models to a second sample of 35 seals, 20 males and 15 females, during the post-moult foraging trip to the Antarctic continental shelf between 2004 and 2013, which did not have fine-scale behavioural data. The females were widely distributed with important foraging activity south of the Southern Boundary Front, while males predominately travelled to the south-eastern part of the East Antarctica region. Combining our predictions of PEE with environmental features (sea ice concentration, water masses at the bottom phase of dives, bathymetry and slope index) we found higher foraging activity for females over shallower seabed depths and at the boundary between the overlying Antarctic Surface Water (AASW) and the underlying Modified Circumpolar Deep Water (MCDW). Increased biological activity associated with the upper boundary of MCDW, may provide overwintering areas for SES prey. Male foraging activity was strongly associated with pelagic dives within the Antarctic Slope Front where upwelling of nutrient rich Circumpolar Deep Water onto surface water may enhance and concentrate resources. A positive association between sea ice and foraging activity was found for both sexes where increased biological activity may sustain an under-ice ecosystem. Variability of the East Antarctic sea ice season duration is likely a crucial element to allow air-breathing predators to benefit from profitable prey patches within the pack ice habitat.
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