| 1. |
- Akhoudas, Camille Hayatte, 1992-, et al.
(författare)
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Isotopic evidence for an intensified hydrological cycle in the Indian sector of the Southern Ocean
- 2023
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Ingår i: Nature Communications. - 2041-1723. ; 14
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Tidskriftsartikel (refereegranskat)abstract
- The hydrological cycle is expected to intensify in a warming climate. However, observational evidence of such changes in the Southern Ocean is difficult to obtain due to sparse measurements and a complex superposition of changes in precipitation, sea ice, and glacial meltwater. Here we disentangle these signals using a dataset of salinity and seawater oxygen isotope observations collected in the Indian sector of the Southern Ocean. Our results show that the atmospheric water cycle has intensified in this region between 1993 and 2021, increasing the salinity in subtropical surface waters by 0.06 ± 0.07 g kg−1 per decade, and decreasing the salinity in subpolar surface waters by -0.02 ± 0.01 g kg−1 per decade. The oxygen isotope data allow to discriminate the different freshwater processes showing that in the subpolar region, the freshening is largely driven by the increase in net precipitation (by a factor two) while the decrease in sea ice melt is largely balanced by the contribution of glacial meltwater at these latitudes. These changes extend the growing evidence for an acceleration of the hydrological cycle and a melting cryosphere that can be expected from global warming.
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| 2. |
- Chambault, Philippine, et al.
(författare)
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The Gulf Stream frontal system : A key oceanographic feature in the habitat selection of the leatherback turtle?
- 2017
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Ingår i: Deep Sea Research Part I. - : Elsevier BV. - 0967-0637 .- 1879-0119. ; 123, s. 35-47
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- 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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| 4. |
- Mensah, Vigan, et al.
(författare)
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A Correction for the Thermal Mass-Induced Errors of CTD Tags Mounted on Marine Mammals
- 2018
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Ingår i: Journal of Atmospheric and Oceanic Technology. - 0739-0572 .- 1520-0426. ; 35:6, s. 1237-1252
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Pauthenet, Etienne, et al.
(författare)
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A Linear Decomposition of the Southern Ocean Thermohaline Structure
- 2017
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Ingår i: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 47:1, s. 29-47
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Pauthenet, Etienne, 1991-, et al.
(författare)
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Seasonal Meandering of the Polar Front Upstream of the Kerguelen Plateau
- 2018
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 45:18, s. 9774-9781
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
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| 8. |
- Pauthenet, Etienne, 1991-
(författare)
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Unraveling the thermohaline structure of the Southern Ocean using functional data analysis
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Southern Ocean connects the Indian, Pacific and Atlantic Oceans and provides a direct pathway to exchange mass, heat and salt across the Global Ocean, therefore playing an important role in the global climate system. Due to the complexity of its structure and the general inadequacy of its sampling, both in time and space, it remains a challenge to describe and visualize the three dimensional pattern of its circulation and the associated tracer distribution (temperature, salinity, oxygen or nutrients). This thesis contributes to the understanding of the thermohaline structure of the ocean and especially of the remote Southern Ocean by introducing a novel decomposition method, the Functional Principal Component Analysis applied on vertical profiles of temperature and salinity. To this end, we first normalize hydrographic profiles by using a functional spline representation. Then the statistical method of dimension reduction and feature extraction reveals the main spatial patterns of the temperature and salinity variations. The first two vertical modes contribute to 90% of the combined variance and are related to very robust structures of the Global Ocean. The first mode is mainly controlled by temperature and the second by salinity. In the Southern Ocean, the vertical modes present circumpolar patterns that can be closely related to the stratification regimes that define the circumpolar fronts. Notably the Polar Front is located at the natural boundary between the region controlled by the first (thermal) mode to the north and the second (haline) mode to the south. A mapping of the fundamental zonation is provided with an estimate of the width of the water mass boundaries. As a validation of this method, the Antarctic Polar Front is investigated further in the Indian sector using the same statistical framework. We show that the Polar Front latitudinal position varies seasonally upstream of the Kerguelen Plateau. This meandering is confirmed by hydrographic data gathered by elephant seals equipped with miniaturized sensors. The proposed statistical method provides an objective way to define water mass boundaries and their spatial variability. It offers a useful framework for representing the density structure of the ocean in a reduced-dimension space while maximizing the variance explained. The functional approach also provides a robust way to validate model outputs against observations from any platforms.
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