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  • 1. Chambault, Philippine
    et al.
    de Thoisy, Benoit
    Heerah, Karine
    Conchon, Anna
    Barrioz, Sébastien
    Dos Reis, Virginie
    Berzins, Rachel
    Kelle, Laurent
    Picard, Baptiste
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Le Maho, Yvon
    Chevallier, Damien
    The influence of oceanographic features on the foraging behavior of the olive ridley sea turtle Lepidochelys olivacea along the Guiana coast2016In: Progress in Oceanography, ISSN 0079-6611, E-ISSN 1873-4472, Vol. 142, p. 58-71Article, review/survey (Refereed)
    Abstract [en]

    The circulation in the Western Equatorial Atlantic is characterized by a highly dynamic mesoscale activity that shapes the Guiana continental shelf. Olive ridley sea turtles (Lepidochelys olivacea) nesting in French Guiana cross this turbulent environment during their post-nesting migration. We studied how oceanographic and biological conditions drove the foraging behavior of 18 adult females, using satellite telemetry, remote sensing data (sea surface temperature, sea surface height, current velocity and euphotic depth), simulations of micronekton biomass (pelagic organisms) and in situ records (water temperature and salinity). The occurrence of foraging events throughout migration was located using Residence Time analysis, while an innovative proxy of the hunting time within a dive was used to identify and quantify foraging events during dives. Olive ridleys migrated northwestwards using the Guiana current and remained on the continental shelf at the edge of eddies formed by the North Brazil retroflection, an area characterized by low turbulence and high micronekton biomass. They performed mainly pelagic dives, hunting for an average 77% of their time. Hunting time within a dive increased with shallower euphotic depth and with lower water temperatures, and mean hunting depth increased with deeper thermocline. This is the first study to quantify foraging activity within dives in olive ridleys, and reveals the crucial role played by the thermocline on the foraging behavior of this carnivorous species. This study also provides novel and detailed data describing how turtles actively use oceanographic structures during post-nesting migration.

  • 2. Chambault, Philippine
    et al.
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Benhamou, Simon
    Baudena, Alberto
    Pauthenet, Etienne
    Stockholm University, Faculty of Science, Department of Meteorology .
    de Thoisy, Benoît
    Bonola, Marc
    Dos Reis, Virginie
    Crosson, Rodrigue
    Brucker, Mathieu
    Le Maho, Yvon
    Chevallier, Damien
    The Gulf Stream frontal system: A key oceanographic feature in the habitat selection of the leatherback turtle?2017In: Deep Sea Research Part I: Oceanographic Research Papers, ISSN 0967-0637, E-ISSN 1879-0119, Vol. 123, p. 35-47Article in journal (Refereed)
    Abstract [en]

    Although some associations between the leatherback turtle Dermochelys coriacea and the Gulf Stream current have been previously suggested, no study has to date demonstrated strong affinities between leatherback movements and this particular frontal system using thorough oceanographic data in both the horizontal and vertical dimensions. The importance of the Gulf Stream frontal system in the selection of high residence time (HRT) areas by the North Atlantic leatherback turtle is assessed here for the first time using state-of-the-art ocean reanalysis products. Ten adult females from the Eastern French Guianese rookery were satellite tracked during post-nesting migration to relate (1) their horizontal movements to physical gradients (Sea Surface Temperature (SST), Sea Surface Height (SSH) and filaments) and biological variables (micronekton and chlorophyll a), and (2) their diving behaviour to vertical structures within the water column (mixed layer, thermocline, halocline and nutricline). All the turtles migrated northward towards the Gulf Stream north wall. Although their HRT areas were geographically remote (spread between 80-30 degrees W and 28-45 degrees N), all the turtles targeted similar habitats in terms of physical structures, i.e. strong gradients of SST, SSH and a deep mixed layer. This close association with the Gulf Stream frontal system highlights the first substantial synchronization ever observed in this species, as the HRTs were observed in close match with the autumn phytoplankton bloom. Turtles remained within the enriched mixed layer at depths of 38.5 +/- 7.9 m when diving in HRT areas, likely to have an easier access to their prey and maximize therefore the energy gain. These depths were shallow in comparison to those attained within the thermocline (82.4 +/- 5.6 m) while crossing the nutrient-poor subtropical gyre, probably to reach cooler temperatures and save energy during the transit. In a context of climate change, anticipating the evolution of such frontal structure under the influence of global warming is crucial to ensure the conservation of this vulnerable species.

  • 3. de lavergne, C.
    et al.
    Madec, G.
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Holmes, R. M.
    McDougall, T. J.
    Abyssal ocean overturning shaped by seafloor distribution2017In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 551, no 7679, p. 181-186Article in journal (Refereed)
    Abstract [en]

    The abyssal ocean is broadly characterized by northward flow of the densest waters and southward flow of less-dense waters above them. Understanding what controls the strength and structure of these interhemispheric flows-referred to as the abyssal overturning circulation-is key to quantifying the ocean's ability to store carbon and heat on timescales exceeding a century. Here we show that, north of 32 degrees S, the depth distribution of the seafloor compels dense southernorigin waters to flow northward below a depth of about 4 kilometres and to return southward predominantly at depths greater than 2.5 kilometres. Unless ventilated from the north, the overlying mid-depths (1 to 2.5 kilometres deep) host comparatively weak mean meridional flow. Backed by analysis of historical radiocarbon measurements, the findings imply that the geometry of the Pacific, Indian and Atlantic basins places a major external constraint on the overturning structure.

  • 4. de Lavergne, Casimir
    et al.
    Madec, Gurvan
    Capet, Xavier
    Maze, Guillaume
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Getting to the bottom of the ocean2016In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 9, no 12, p. 857-858Article in journal (Refereed)
  • 5.
    Falahat, Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Thurnherr, Andreas M.
    Hibiya, Toshiyuki
    Comparison of calculated energy flux of internal tides with microstructure measurementsManuscript (preprint) (Other academic)
  • 6.
    Falahat, Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Thurnherr, Andreas M.
    Hibiya, Toshiyuki
    Comparison of calculated energy flux of internal tides with microstructure measurements2014In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 66, p. 23240-Article in journal (Refereed)
    Abstract [en]

    Vertical mixing caused by breaking of internal tides plays a major role in maintaining the deep-ocean stratification. This study compares observations of dissipation from microstructure measurements to calculations of the vertical energy flux from barotropic to internal tides, taking into account the temporal variation due to the spring-neap tidal cycle. The dissipation data originate from two surveys in the Brazil Basin Tracer Release Experiment (BBTRE), and one over the LArval Dispersal along the Deep East Pacific Rise (LADDER3), supplemented with a few stations above the North-Atlantic Ridge (GRAVILUCK) and in the western Pacific (IZU). A good correlation is found between logarithmic values of energy flux and local dissipation in BBTRE, suggesting that the theory is able to predict energy fluxes. For the LADDER3, the local dissipation is much smaller than the calculated energy flux, which is very likely due to the different topographic features of BBTRE and LADDER3. The East Pacific Rise consists of a few isolated seamounts, so that most of the internal wave energy can radiate away from the generation site, whereas the Brazil Basin is characterised by extended rough bathymetry, leading to a more local dissipation. The results from all four field surveys support the general conclusion that the fraction of the internal-tide energy flux that is dissipated locally is very different in different regions.

  • 7.
    Falahat, Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zarroug, Moundheur
    Stockholm University, Faculty of Science, Department of Meteorology .
    Global Calculation of Tidal Energy Conversion into Vertical Normal Modes2014In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 44, no 12, p. 3225-3244Article in journal (Refereed)
    Abstract [en]

    A direct calculation of the tidal generation of internal waves over the global ocean is presented. The calculation is based on a semianalytical model, assuming that the internal tide characteristic slope exceeds the bathymetric slope (subcritical slope) and the bathymetric height is small relative to the vertical scale of the wave, as well as that the horizontal tidal excursion is smaller than the horizontal topographic scale. The calculation is performed for the M-2 tidal constituent. In contrast to previous similar computations, the internal tide is projected onto vertical eigenmodes, which gives two advantages. First, the vertical density profile and the finite ocean depth are taken into account in a fully consistent way, in contrast to earlier work based on the WKB approximation. Nevertheless, the WKB-based total global conversion follows closely that obtained using the eigenmode decomposition in each of the latitudinal and vertical distributions. Second, the information about the distribution of the conversion energy over different vertical modes is valuable, since the lowest modes can propagate over long distances, while high modes are more likely to dissipate locally, near the generation site. It is found that the difference between the vertical distributions of the tidal conversion into the vertical modes is smaller for the case of very deep ocean than the shallow-ocean depth. The results of the present work pave the way for future work on the vertical and horizontal distribution of the mixing caused by internal tides.

  • 8.
    Falahat, Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zarroug, Moundheur
    Stockholm University, Faculty of Science, Department of Meteorology .
    Global calculation of tidal energy conversion rate into vertical normal modesManuscript (preprint) (Other academic)
  • 9. Ferreira, David
    et al.
    Cessi, Paola
    Coxall, Helen K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    de Boer, Agatha
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dijkstra, Henk A.
    Drijfhout, Sybren S.
    Eldevik, Tor
    Harnik, Nili
    McManus, Jerry F.
    Marshall, David P.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Schneider, Tapio
    Wills, Robert C.
    Atlantic-Pacific Asymmetry in Deep Water Formation2018In: Annual Review of Earth and Planetary Science, ISSN 0084-6597, E-ISSN 1545-4495, Vol. 46, p. 327-352Article, review/survey (Refereed)
    Abstract [en]

    While the Atlantic Ocean is ventilated by high-latitude deep water formation and exhibits a pole-to-pole overturning circulation, the Pacific Ocean does not. This asymmetric global overturning pattern has persisted for the past 2-3 million years, with evidence for different ventilation modes in the deeper past. In the current climate, the Atlantic-Pacific asymmetry occurs because the Atlantic is more saline, enabling deep convection. To what extent the salinity contrast between the two basins is dominated by atmospheric processes (larger net evaporation over the Atlantic) or oceanic processes (salinity transport into the Atlantic) remains an outstanding question. Numerical simulations have provided support for both mechanisms; observations of the present climate support a strong role for atmospheric processes as well as some modulation by oceanic processes. A major avenue for future work is the quantification of the various processes at play to identify which mechanisms are primary in different climate states.

  • 10.
    Fransner, Filippa
    et al.
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Gustafsson, Erik
    Stockholms universitet, Baltic Nest Institute.
    Tedesco, Letizia
    Vichi, Marcello
    Hordoir, Robinson
    Swedish Meteorological and Hydrological Institute, Sweden.
    Roquet, Fabien
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Spilling, Kristian
    Kuznetsov, Ivan
    Eilola, Kari
    Mörth, Carl-Magnus
    Stockholms universitet, Institutionen för geologiska vetenskaper.
    Humborg, Christoph
    Stockholms universitet, Baltic Nest Institute.
    Nycander, Jonas
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Non-Redfieldian Dynamics Explain Seasonal pCO2 Drawdown in the Gulf of Bothnia2018In: Journal of Geophysical Research - Oceans, ISSN 2169-9275, E-ISSN 2169-9291, Vol. 123, no 1, p. 166-188Article in journal (Refereed)
    Abstract [en]

    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.

  • 11.
    Fransner, Filippa
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gustafsson, Erik
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Tedesco, Letizia
    Vichi, Marcello
    Hordoir, Robinson
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Spilling, Kristian
    Kuznetsov, Ivan
    Eilola, Kari
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute. University of Helsinki, Finland.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Non-Redfieldian Dynamics Explain Seasonal pCO2 Drawdown in the Gulf of Bothnia2018In: Journal of Geophysical Research - Oceans, ISSN 2169-9275, E-ISSN 2169-9291, Vol. 123, no 1, p. 166-188Article in journal (Refereed)
    Abstract [en]

    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.

  • 12. Hindell, Mark A.
    et al.
    McMahon, Clive R.
    Bester, Marthan N.
    Boehme, Lars
    Costa, Daniel
    Fedak, Mike A.
    Guinet, Christophe
    Herraiz-Borreguero, Laura
    Harcourt, Robert G.
    Huckstadt, Luis
    Kovacs, Kit M.
    Lydersen, Christian
    McIntyre, Trevor
    Muelbert, Monica
    Patterson, Toby
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Williams, Guy
    Charrassin, Jean-Benoit
    Circumpolar habitat use in the southern elephant seal: implications for foraging success and population trajectories2016In: Ecosphere, ISSN 2150-8925, E-ISSN 2150-8925, Vol. 7, no 5, article id e01213Article in journal (Refereed)
    Abstract [en]

    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.

  • 13. Kitade, Yujiro
    et al.
    Shimada, Keishi
    Tamura, Takeshi
    Williams, Guy D.
    Aoki, Shigeru
    Fukamachi, Yasushi
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hindell, Mark
    Ushio, Shuki
    Ohshima, Kay I.
    Antarctic Bottom Water production from the Vincennes Bay Polynya, East Antarctica2014In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 41, no 10, p. 3528-3534Article in journal (Refereed)
    Abstract [en]

    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.

  • 14. Labrousse, Sara
    et al.
    Vacquie-Garcia, Jade
    Heerah, Karine
    Guinet, Christophe
    Sallee, Jean-Baptiste
    Authier, Matthieu
    Picard, Baptiste
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bailleul, Frederic
    Hindell, Mark
    Charrassin, Jean-Benoit
    Winter use of sea ice and ocean water mass habitat by southern elephant seals: The length and breadth of the mystery2015In: Progress in Oceanography, ISSN 0079-6611, E-ISSN 1873-4472, Vol. 137, p. 52-68Article, review/survey (Refereed)
    Abstract [en]

    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.

  • 15. Mallett, Helen K. W.
    et al.
    Boehme, Lars
    Fedak, Mike
    Heywood, Karen J.
    Stevens, David P.
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology . University of Gothenburg, Sweden.
    Variation in the Distribution and Properties of Circumpolar Deep Water in the Eastern Amundsen Sea, on Seasonal Timescales, Using Seal-Borne Tags2018In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 10, p. 4982-4990Article in journal (Refereed)
    Abstract [en]

    In the Amundsen Sea, warm saline Circumpolar Deep Water (CDW) crosses the continental shelf toward the vulnerable West Antarctic ice shelves, contributing to their basal melting. Due to lack of observations, little is known about the spatial and temporal variability of CDW, particularly seasonally. A new data set of 6,704 seal tag temperature and salinity profiles in the easternmost trough between February and December 2014 reveals a CDW layer on average 49dbar thicker in late winter (August to October) than in late summer (February to April), the reverse seasonality of that seen at moorings in the western trough. This layer contains more heat in winter, but on the 27.76 kg/m(3) density surface CDW is 0.32 degrees C warmer in summer than in winter, across the northeastern Amundsen Sea, which may indicate that wintertime shoaling offshelf changes CDW properties onshelf. In Pine Island Bay these seasonal changes on density surfaces are reduced, likely by gyre circulation. Plain Language Summary In the Amundsen Sea, Antarctica, warm salty water crosses the continental shelf from the deep open ocean, toward the vulnerable West Antarctic ice shelves, bringing heat to help melt them from underneath. Due to lack of observations, little is known about how this flow of warm water varies in space and time, particularly seasonally. Between February and December 2014, in a trough in the eastern Amundsen Sea, 6,704 profiles were collected by sensors attached to seals, measuring temperature and salinity as the seals return from dives up to 1,200m deep. These data showed that this warm (similar to 1 degrees C) deep layer is on average similar to 50m thicker in late winter (August to October) than in late summer (February to April), the reverse seasonality of that seen within a trough in the western Amundsen Sea. This warm layer contains more heat in winter but on a surface of constant density is 0.32 degrees C warmer in summer than in winter, across the northeastern Amundsen Sea. This may indicate that in winter the deep waters offshelf rise, allowing different water onto the continental shelf. In Pine Island Bay these seasonal changes on density surfaces are reduced, probably because here the water circulates and mixes.

  • 16. Mensah, Vigan
    et al.
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology . University of Gothenburg, Sweden.
    Siegelman-Charbit, Lia
    Picard, Baptiste
    Pauthenet, Etienne
    Stockholm University, Faculty of Science, Department of Meteorology .
    Guinet, Christophe
    A Correction for the Thermal Mass-Induced Errors of CTD Tags Mounted on Marine Mammals2018In: Journal of Atmospheric and Oceanic Technology, ISSN 0739-0572, E-ISSN 1520-0426, Vol. 35, no 6, p. 1237-1252Article in journal (Refereed)
    Abstract [en]

    The effect of thermal mass on the salinity estimate from conductivity-temperature-depth (CTD) tags sensor mounted on marine mammals is documented, and a correction scheme is proposed to mitigate its impact. The algorithm developed here allows for a direct correction of the salinity data, rather than a correction of the sample's conductivity and temperature. The amplitude of the thermal mass-induced error on salinity and its correction are evaluated via comparison between data from CTD tags and from Sea-Bird Scientific CTD used as a reference. Thermal mass error on salinity appears to be generally O(10(-2)) g kg(-1), it may reach O(10(-1)) g kg(-1), and it tends to increase together with the magnitude of the cumulated temperature gradient (T-HP) within the water column. The correction we propose yields an error decrease of up to similar to 60% if correction coefficients specific to a certain tag or environment are calculated, and up to 50% if a default value for the coefficients is provided. The correction with the default coefficients was also evaluated using over 22 000 in situ dive data from five tags deployed in the Southern Ocean and is found to yield significant and systematic improvements on the salinity data, including for profiles whose T-HP was weak and the error small. The correction proposed here yields substantial improvements in the density estimates, although a thermal mass-induced error in temperature measurements exists for very large T-HP and has yet to be corrected.

  • 17. Nakanowatari, Takuya
    et al.
    Ohshima, Kay I.
    Mensah, Vigan
    Mitani, Yoko
    Hattori, Kaoru
    Kobayashi, Mari
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sakurai, Yasunori
    Mitsudera, Humio
    Wakatsuchi, Masaaki
    Hydrographic observations by instrumented marine mammals in the Sea of Okhotsk2017In: Polar Science, ISSN 1873-9652, E-ISSN 1876-4428, Vol. 13, p. 56-65Article in journal (Refereed)
    Abstract [en]

    The Sea of Okhotsk is a challenging environment for obtaining in situ data and satellite observation in winter due to sea ice cover. In this study, we evaluated the validity of hydrographic observations by marine mammals (e.g., seals and sea lions) equipped with oceanographic conductivity-temperaturedepth (CTD) sensors. During 4-yr operations from 2011 to 2014, we obtained total of 997 temperature-salinity profiles in and around the Soya Strait, Iony Island, and Urup Strait. The hydrographic data were mainly obtained from May to August and the maximum profile depth in shelf regions almost reaches to the seafloor, while valuable hydrographic data under sea ice cover were also obtained. In strong thermoclines, the seal-derived data sometimes showed positive biases in salinity with spikelike signal. For these salinity biases, we applied a new thermal mass inertia correction scheme, effectively reducing spurious salinity biases in the seasonal thermocline. In the Soya Strait and the adjacent region, the detailed structure of the Soya Warm Current including the cold-water belt was well identified. Dense water up to 27.0 sigma(theta), which can be a potential source of Okhotsk Sea Intermediate Water, has flowed from the Soya Strait into the Sea of Okhotsk in mid-winter (February). In summer, around the Iony Island and Urup Strait, remarkable cold and saline waters are localized in the surface layers. These regions are also characterized by weak stratification, suggesting the occurrence of tidally induced vertical mixing. Thus, CTD-tag observations have a great potential in monitoring data-sparse regions in the Sea of Okhotsk.

  • 18.
    Nycander, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hieronymus, Magnus
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    The nonlinear equation of state of sea water and the global water mass distribution2015In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 18, p. 7714-7721Article in journal (Refereed)
    Abstract [en]

    The role of nonlinearities of the equation of state (EOS) of seawater for the distribution of water masses in the global ocean is examined through simulations with an ocean general circulation model with various manipulated versions of the EOS. A simulation with a strongly simplified EOS, which contains only two nonlinear terms, still produces a realistic water mass distribution, demonstrating that these two nonlinearities are indeed the essential ones. Further simulations show that each of these two nonlinear terms affects a specific aspect of the water mass distribution: the cabbeling term is crucial for the formation of Antarctic Intermediate Water and the thermobaric term for the layering of North Atlantic Deep Water and Antarctic Bottom Water.

  • 19. Ohshima, Kay I.
    et al.
    Fukamachi, Yasushi
    Williams, Guy D.
    Nihashi, Sohey
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kitade, Yujiro
    Tamura, Takeshi
    Hirano, Daisuke
    Herraiz-Borreguero, Laura
    Field, Iain
    Hindell, Mark
    Aoki, Shigeru
    Wakatsuchi, Masaaki
    Antarctic BottomWater production by intense sea-ice formation in the Cape Darnley polynya2013In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 6, no 3, p. 235-240Article in journal (Refereed)
    Abstract [en]

    The formation of Antarctic Bottom Water-the cold, dense water that occupies the abyssal layer of the global ocean-is a key process in global ocean circulation. This water mass is formed as dense shelf water sinks to depth. Three regions around Antarctica where this process takes place have been previously documented. The presence of another source has been identified in hydrographic and tracer data, although the site of formation is not well constrained. Here we document the formation of dense shelf water in the Cape Darnley polynya (65 degrees -69 degrees E) and its subsequent transformation into bottom water using data from moorings and instrumented elephant seals (Mirounga leonina). Unlike the previously identified sources of Antarctic Bottom Water, which require the presence of an ice shelf or a large storage volume, bottom water production at the Cape Darnley polynya is driven primarily by the flux of salt released by sea-ice formation. We estimate that about 0.3-0.7 x 10(6) m(3) s(-1) of dense shelf water produced by the Cape Darnley polynya is transformed into Antarctic BottomWater. The transformation of this water mass, which we term Cape Darnley BottomWater, accounts for 6-13% of the circumpolar total.

  • 20.
    Pauthenet, Etienne
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology . University of Gothenburg, Sweden.
    Madec, G.
    Guinet, C.
    Hindell, M.
    McMahon, C. R.
    Harcourt, R.
    Nerini, D.
    Seasonal Meandering of the Polar Front Upstream of the Kerguelen Plateau2018In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 18, p. 9774-9781Article in journal (Refereed)
    Abstract [en]

    The location of the Antarctic Polar Front (PF) is mapped in the Southern Indian Ocean bydecomposing the shape of temperature and salinity profiles into vertical modes using a functional PrincipalComponent Analysis. We define the PF as the northernmost minimum of temperature at the subsurface andrepresent it as a linear combination of the first three modes. This method is applied on an ocean reanalysisdata set and on in situ observations, revealing a seasonal variability of the PF latitudinal position that ismost pronounced between the Conrad Rise and the Kerguelen Plateau. This shift coincides with variationsin the transport across the Northern Kerguelen Plateau. We suggest that seasonal changes of the upperstratification may drive the observed variability of the PF, with potentially large implications for thepathways and residence time of water masses over the plateau and the phytoplankton bloom extendingsoutheast of the Kerguelen Islands.

  • 21.
    Pauthenet, Etienne
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Madec, Gurvan
    Nerini, David
    A Linear Decomposition of the Southern Ocean Thermohaline Structure2017In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 47, no 1, p. 29-47Article in journal (Refereed)
    Abstract [en]

    The thermohaline structure of the Southern Ocean is deeply influenced by the presence of the Antarctic Circumpolar Current (ACC), where water masses of the World Ocean are advected, transformed, and redistributed to the other basins. It remains a challenge to describe and visualize the complex 3D pattern of this circulation and its associated tracer distribution. Here, a simple framework is presented to analyze the Southern Ocean thermohaline structure. A functional principal component analysis (PCA) is applied to temperature u and salinity S profiles to determine the main spatial patterns of their variations. Using the Southern Ocean State Estimate (SOSE), this study determines the vertical modes describing the Southern Ocean thermohaline structure between 5 and 2000 m. The first two modes explain 92% of the combined theta-S variance, thus providing a surprisingly good approximation of the thermohaline properties in the Southern Ocean. The first mode (72% of total variance) accurately describes the north-south property gradients. The secondmode (20%) mostly describes salinity at 500m in the region of Antarctic Intermediate Water formation. These two modes present circumpolar patterns that can be closely related with standard frontal definitions. By projecting any given hydrographic profile onto the SOSE-based modes, it is possible to determine its position relative to the fronts. The projection is successfully applied on the hydrographic profiles of the WOCE SR3 section. The Southern Ocean thermohaline decomposition provides an objective way to define water mass boundaries and their spatial variability and has useful application for comparing model output with observations.

  • 22.
    Pauthenet, Etienne
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Madec, Gurvan
    Nerini, David
    The thermohaline structure of the ocean in a reduced-dimension spaceManuscript (preprint) (Other academic)
  • 23. Pellichero, Violaine
    et al.
    Sallée, Jean-Baptiste
    Schmidtko, Sunke
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Charrassin, Jean-Benoît
    The ocean mixed layer under Southern Ocean sea-ice: Seasonal cycle and forcing2017In: Journal of Geophysical Research - Oceans, ISSN 2169-9275, E-ISSN 2169-9291, Vol. 122, no 2, p. 1608-1633Article in journal (Refereed)
    Abstract [en]

    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.

  • 24. Pollmann, Friederike
    et al.
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Madec, Gurvan
    Effects of the Asymmetry between Surface and Interior Flow on the Dynamics of a Thermohaline Loop2015In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 45, no 10, p. 2544-2563Article in journal (Refereed)
    Abstract [en]

    Large-scale overturning cells in the ocean typically combine an essentially horizontal surface branch and an interior branch below, where the circulation spans both horizontal and vertical scales. The aim of this study is to analyze the impact of this asymmetry between the two branches by folding a one-dimensional thermohaline loop, such that its lower part remains vertical while its upper part is folded down into the horizontal plane. It is found that both the transitory response and the distribution of thermohaline properties are modified significantly when the loop is folded. In some cases, velocity oscillations are induced during the spinup that were not seen in the unfolded case. This is because a circular loop allows for compensations between the density torques produced above and below the heat forcing level, while such compensations are not possible in the folded loop because of the horizontal direction of the surface circulation. Furthermore, the dynamical effects associated with nonlinearities of the equation of state are significantly altered by the folding. Cabbeling tends to decelerate the flow in the folded loop, instead of accelerating it as in the circular case, and can also act to dampen velocity oscillations. Thermobaricity also alters the loop circulation, although comparatively less.

  • 25.
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Dynamical Potential Energy: A New Approach to Ocean Energetics2013In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 43, no 2, p. 457-476Article in journal (Refereed)
    Abstract [en]

    The concept of available potential energy is supposed to indicate which part of the potential energy is available to transform into kinetic energy. Yet it is impossible to obtain a unique definition of available potential energy for the real ocean because of nonlinearities of the equation of state, rendering its usefulness largely hypothetical. In this paper, the conservation of energy is first reformulated in terms of horizontal anomalies of density and pressure for a simplified ocean model using the Boussinesq and hydrostatic approximations. This framework introduces the concept of dynamical potential energy, defined as the horizontal anomaly of potential energy, to replace available potential energy. Modified conservation equations are derived that make it much simpler to identify oceanic power input by buoyancy and mechanical forces. Closed budgets of energy are presented for idealized circulations obtained with a general circulation model, comparing spatial patterns of power inputs generated by wind and thermal forcings. Finally, a generalization of the framework to compressible fluids is presented, opening the way to applications in atmosphere energetics.

  • 26.
    Roquet, Fabien
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lindqvist, Rickard
    Stockholm University, Faculty of Science, Department of Meteorology .
    Pollmann, Friederike
    Ferreira, David
    Madec, Gurvan
    Stability of the thermohaline circulation examined with a one-dimensional fluid loop2017In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 69, no 1, article id 1380490Article in journal (Refereed)
    Abstract [en]

    The Stommel box model elegantly demonstrates that the oceanic response to mixed boundary conditions, combining a temperature relaxation with a fixed salt flux forcing, is non-linear owing to the so-called salt advection feedback. This non-linearity produces a parameter range of bi-stability associated with hysteresis effects characterised by a fast thermally driven mode and a slow salinity-driven mode. Here, we investigate whether a similar dynamical behaviour can be found in the thermohaline loop model, a one-dimensional analogue of the box model. Asemi-analytical method to compute possible steady states of the loop model is presented, followed by a linear stability analysis carried out for a large range of loop configurations. While the salt advection feedback is found as in the box model, a major difference is obtained for the fast mode: an oscillatory instability is observed near the turning point of the fast mode branch, such that the range of bi-stability is systematically reduced, or even removed, in some cases. The oscillatory instability originates from a salinity anomaly that grows exponentially as it turns around the loop, a situation that may occur only when the salinity torque is directed against the loop flow. Factors such as mixing intensity, the relative strength of thermal and haline forcings, the non-linearity of the equation of state or the loop geometry can strongly affect the stability properties of the loop.

  • 27.
    Roquet, Fabien
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Madec, G.
    McDougall, Trevor J.
    Barker, Paul M.
    Accurate polynomial expressions for the density and specific volume of seawater using the TEOS-10 standard2015In: Ocean Modelling, ISSN 1463-5003, E-ISSN 1463-5011, Vol. 90, p. 29-43Article in journal (Refereed)
    Abstract [en]

    A new set of approximations to the standard TEOS-10 equation of state are presented. These follow a polynomial form, making it computationally efficient for use in numerical ocean models. Two versions are provided, the first being a fit of density for Boussinesq ocean models, and the second fitting specific volume which is more suitable for compressible models. Both versions are given as the sum of a vertical reference profile (6th-order polynomial) and an anomaly (52-term polynomial, cubic in pressure), with relative errors of similar to 0.1% on the thermal expansion coefficients. A 75-term polynomial expression is also presented for computing specific volume, with a better accuracy than the existing TEOS-10 48-term rational approximation, especially regarding the sound speed, and it is suggested that this expression represents a valuable approximation of the TEOS-10 equation of state for hydrographic data analysis. In the last section, practical aspects about the implementation of TEOS-10 in ocean models are discussed.

  • 28.
    Roquet, Fabien
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Madec, Gurvan
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Defining a Simplified Yet "Realistic" Equation of State for Seawater2015In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 45, no 10, p. 2564-2579Article in journal (Refereed)
    Abstract [en]

    There is a growing realization that the nonlinear nature of the equation of state has a deep impact on the global ocean circulation; however, the understanding of the global effects of these nonlinearities remains elusive. This is partly because of the complicated formulation of the seawater equation of state making it difficult to handle in theoretical studies. In this paper, a hierarchy of polynomial equations of state of increasing complexity, optimal in a least squares sense, is presented. These different simplified equations of state are then used to simulate the ocean circulation in a global 2 degrees-resolution configuration. Comparisons between simulated ocean circulations confirm that nonlinear effects are of major importance, in particular influencing the circulation through determination of the static stability below the mixed layer, thus controlling rates of exchange between the atmosphere and the ocean interior. It is found that a simple polynomial equation of state, with a quadratic term in temperature (for cabbeling), a temperature-pressure product term (for thermobaricity), and a linear term in salinity, that is, only four tuning parameters, is enough to simulate a reasonably realistic global circulation. The best simulation is obtained when the simplified equation of state is forced to have an accurate thermal expansion coefficient near the freezing point, highlighting the importance of polar regions for the global stratification. It is argued that this simplified equation of state will be of great value for theoretical studies and pedagogical purposes.

  • 29.
    Roquet, Fabien
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Williams, Guy
    Hindell, Mark A.
    Harcourt, Rob
    McMahon, Clive
    Guinet, Christophe
    Charrassin, Jean-Benoit
    Reverdin, Gilles
    Boehme, Lars
    Lovell, Phil
    Fedak, Mike
    A Southern Indian Ocean database of hydrographic profiles obtained with instrumented elephant seals2014In: Scientific Data, E-ISSN 2052-4463, Vol. 1, article id 140028Article in journal (Refereed)
    Abstract [en]

    The instrumentation of southern elephant seals with satellite-linked CTD tags has offered unique temporal and spatial coverage of the Southern Indian Ocean since 2004. This includes extensive data from the Antarctic continental slope and shelf regions during the winter months, which is outside the conventional areas of Argo autonomous floats and ship-based studies. This landmark dataset of around 75,000 temperature and salinity profiles from 20–140 °E, concentrated on the sector between the Kerguelen Islands and Prydz Bay, continues to grow through the coordinated efforts of French and Australian marine research teams. The seal data are quality controlled and calibrated using delayed-mode techniques involving comparisons with other existing profiles as well as cross-comparisons similar to established protocols within the Argo community, with a resulting accuracy of ±0.03 °C in temperature and ±0.05 in salinity or better. The data offer invaluable new insights into the water masses, oceanographic processes and provides a vital tool for oceanographers seeking to advance our understanding of this key component of the global ocean climate.

  • 30.
    Roquet, Fabien
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Wunsch, Carl
    Forget, Gael
    Heimbach, Patrick
    Guinet, Christophe
    Reverdin, Gilles
    Charrassin, Jean-Benoit
    Bailleul, Frederic
    Costa, Daniel P.
    Huckstadt, Luis A.
    Goetz, Kimberly T.
    Kovacs, Kit M.
    Lydersen, Christian
    Biuw, Martin
    Nost, Ole A.
    Bornemann, Horst
    Ploetz, Joachim
    Bester, Marthan N.
    McIntyre, Trevor
    Muelbert, Monica C.
    Hindell, Mark A.
    McMahon, Clive R.
    Williams, Guy
    Harcourt, Robert
    Field, Iain C.
    Chafik, Leon
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nicholls, Keith W.
    Boehme, Lars
    Fedak, Mike A.
    Estimates of the Southern Ocean general circulation improved by animal-borne instruments2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 23, p. 6176-6180Article in journal (Refereed)
    Abstract [en]

    Over the last decade, several hundred seals have been equipped with conductivity-temperature-depth sensors in the Southern Ocean for both biological and physical oceanographic studies. A calibrated collection of seal-derived hydrographic data is now available, consisting of more than 165,000 profiles. The value of these hydrographic data within the existing Southern Ocean observing system is demonstrated herein by conducting two state estimation experiments, differing only in the use or not of seal data to constrain the system. Including seal-derived data substantially modifies the estimated surface mixed-layer properties and circulation patterns within and south of the Antarctic Circumpolar Current. Agreement with independent satellite observations of sea ice concentration is improved, especially along the East Antarctic shelf. Instrumented animals efficiently reduce a critical observational gap, and their contribution to monitoring polar climate variability will continue to grow as data accuracy and spatial coverage increase.

  • 31. Stewart, K. D.
    et al.
    Haine, T. W. N.
    Hogg, A. McC.
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    On Cabbeling and Thermobaricity in the Surface Mixed Layer2017In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 47, no 7, p. 1775-1787Article in journal (Refereed)
    Abstract [en]

    The surface mixed layer (ML) governs atmosphere-ocean fluxes, and thereby affects Earth's climate. Accurate representation of ML processes in ocean models remains a challenge, however. The O(100) m deep ML exhibits substantial horizontal thermohaline gradients, despite being near-homogenous vertically, making it an ideal location for processes that result from the nonlinearity of the equation of state, such as cabbeling and thermobaricity. Traditional approaches to investigate these processes focus on their roles in interior water-mass transformation and are ill suited to examine their influence on the ML. However, given the climatic significance of the ML, quantifying the extent to which cabbeling and thermobaricity influence the ML density field offers insight into improving ML representations in ocean models. A recent simplified equation of state of seawater allows the local effects of cabbeling and thermobaric processes in the ML to be expressed analytically as functions of the local temperature gradient and ML depth. These simplified expressions are used to estimate the extent to which cabbeling and thermobaricity contribute to local ML density differences. These estimates compare well with values calculated directly using the complete nonlinear equation of state. Cabbeling and thermobaricity predominantly influence the ML density field poleward of 30 degrees. Mixed layer thermobaricity is basin-scale and winter intensified, while ML cabbeling is perennial and localized to intense, zonally coherent regions associated with strong temperature fronts, such as the Antarctic Circumpolar Current and the Kuroshio and Gulf Stream Extensions. For latitudes between 40 degrees and 50 degrees in both hemispheres, the zonally averaged effects of ML cabbeling and ML thermobaricity can contribute on the order of 10% of the local ML density difference.

  • 32. Treasure, Anne M.
    et al.
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ansorge, Isabelle J.
    Bester, Marthan N.
    Boehme, Lars
    Bornemann, Horst
    Charrassin, Jean-Benoit
    Chevallier, Damien
    Costa, Daniel P.
    Fedak, Mike A.
    Guinet, Christophe
    Hammill, Mike O.
    Harcourt, Robert G.
    Hindell, Mark A.
    Kovacs, Kit M.
    Lea, Mary-Anne
    Lovell, Phil
    Lowther, Andrew D.
    Lydersen, Christian
    McIntyre, Trevor
    McMahon, Clive R.
    Muelbert, Monica M. C.
    Nicholls, Keith
    Picard, Baptiste
    Reverdin, Gilles
    Trites, Andrew W.
    Williams, Guy D.
    de Bruyn, P. J. Nico
    Marine Mammals Exploring the Oceans Pole to Pole A Review of the MEOP Consortium2017In: Oceanography, ISSN 1042-8275, Vol. 30, no 2, p. 132-138Article in journal (Refereed)
    Abstract [en]

    Polar oceans are poorly monitored despite the important role they play in regulating Earth's climate system. Marine mammals equipped with biologging devices are now being used to fill the data gaps in these logistically difficult to sample regions. Since 2002, instrumented animals have been generating exceptionally large data sets of oceanographic CTD casts (>500,000 profiles), which are now freely available to the scientific community through the MEOP data portal (http://meop.net). MEOP (Marine Mammals Exploring the Oceans Pole to Pole) is a consortium of international researchers dedicated to sharing animal-derived data and knowledge about the polar oceans. Collectively, MEOP demonstrates the power and cost-effectiveness of using marine mammals as data-collection platforms that can dramatically improve the ocean observing system for biological and physical oceanographers. Here, we review the MEOP program and database to bring it to the attention of the international community.

  • 33. Williams, G. D.
    et al.
    Herraiz-Borreguero, L.
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tamura, T.
    Ohshima, K. I.
    Fukamachi, Y.
    Fraser, A. D.
    Gao, L.
    Chen, H.
    McMahon, C. R.
    Harcourt, R.
    Hindell, M.
    The suppression of Antarctic bottom water formation by melting ice shelves in Prydz Bay2016In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 7, article id 12577Article in journal (Refereed)
    Abstract [en]

    A fourth production region for the globally important Antarctic bottom water has been attributed to dense shelf water formation in the Cape Darnley Polynya, adjoining Prydz Bay in East Antarctica. Here we show new observations from CTD-instrumented elephant seals in 2011-2013 that provide the first complete assessment of dense shelf water formation in Prydz Bay. After a complex evolution involving opposing contributions from three polynyas (positive) and two ice shelves (negative), dense shelf water (salinity 34.65-34.7) is exported through Prydz Channel. This provides a distinct, relatively fresh contribution to Cape Darnley bottom water. Elsewhere, dense water formation is hindered by the freshwater input from the Amery and West Ice Shelves into the Prydz Bay Gyre. This study highlights the susceptibility of Antarctic bottom water to increased freshwater input from the enhanced melting of ice shelves, and ultimately the potential collapse of Antarctic bottom water formation in a warming climate.

  • 34. Zhang, Xiyue
    et al.
    Thompson, Andrew F.
    Flexas, Mar M.
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bornemann, Horst
    Circulation and meltwater distribution in the Bellingshausen Sea: From shelf break to coast2016In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 12, p. 6402-6409Article in journal (Refereed)
    Abstract [en]

    West Antarctic ice shelves have thinned dramatically over recent decades. Oceanographic measurements that explore connections between offshore warming and transport across a continental shelf with variable bathymetry toward ice shelves are needed to constrain future changes in melt rates. Six years of seal-acquired observations provide extensive hydrographic coverage in the Bellingshausen Sea, where ship-based measurements are scarce. Warm but modified Circumpolar Deep Water floods the shelf and establishes a cyclonic circulation within the Belgica Trough with flow extending toward the coast along the eastern boundaries and returning to the shelf break along western boundaries. These boundary currents are the primary water mass pathways that carry heat toward the coast and advect ice shelf meltwater offshore. The modified Circumpolar Deep Water and meltwater mixtures shoal and thin as they approach the continental slope before flowing westward at the shelf break, suggesting the presence of the Antarctic Slope Current. Constraining meltwater pathways is a key step in monitoring the stability of the West Antarctic Ice Sheet.

1 - 34 of 34
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