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Unraveling the thermohaline structure of the Southern Ocean using functional data analysis
Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).ORCID-id: 0000-0002-0531-3810
2018 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

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.

sted, utgiver, år, opplag, sider
Stockholm: Department of Meteorology, Stockholm University , 2018. , s. 42
Emneord [en]
Southern Ocean, Front, Water mass, Antarctic Polar Front, Descriptive Physical Oceanography, Functional Principal Component Analysis, Elephant Seal, Biologging
HSV kategori
Forskningsprogram
atmosfärvetenskap och oceanografi
Identifikatorer
URN: urn:nbn:se:su:diva-161034ISBN: 978-91-7797-496-3 (tryckt)ISBN: 978-91-7797-497-0 (digital)OAI: oai:DiVA.org:su-161034DiVA, id: diva2:1255795
Disputas
2018-11-30, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 14:00 (engelsk)
Opponent
Veileder
Merknad

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.

Tilgjengelig fra: 2018-11-07 Laget: 2018-10-15 Sist oppdatert: 2018-11-06bibliografisk kontrollert
Delarbeid
1. A Linear Decomposition of the Southern Ocean Thermohaline Structure
Åpne denne publikasjonen i ny fane eller vindu >>A Linear Decomposition of the Southern Ocean Thermohaline Structure
2017 (engelsk)Inngår i: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 47, nr 1, s. 29-47Artikkel i tidsskrift (Fagfellevurdert) Published
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.

HSV kategori
Forskningsprogram
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-141311 (URN)10.1175/JPO-D-16-0083.1 (DOI)000393300700003 ()
Tilgjengelig fra: 2017-04-03 Laget: 2017-04-03 Sist oppdatert: 2020-01-23bibliografisk kontrollert
2. Seasonal Meandering of the Polar Front Upstream of the Kerguelen Plateau
Åpne denne publikasjonen i ny fane eller vindu >>Seasonal Meandering of the Polar Front Upstream of the Kerguelen Plateau
Vise andre…
2018 (engelsk)Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, nr 18, s. 9774-9781Artikkel i tidsskrift (Fagfellevurdert) Published
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.

Emneord
functional principal component analysis, antarctic polar front, southern indian ocean, kerguelen
HSV kategori
Forskningsprogram
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-160774 (URN)10.1029/2018GL079614 (DOI)000447761300049 ()
Tilgjengelig fra: 2018-10-04 Laget: 2018-10-04 Sist oppdatert: 2020-01-23bibliografisk kontrollert
3. The thermohaline structure of the ocean in a reduced-dimension space
Åpne denne publikasjonen i ny fane eller vindu >>The thermohaline structure of the ocean in a reduced-dimension space
(engelsk)Manuskript (preprint) (Annet vitenskapelig)
Emneord
global ocean, thermohaline structure, functional principal component analysis
HSV kategori
Forskningsprogram
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-160895 (URN)
Tilgjengelig fra: 2018-10-10 Laget: 2018-10-10 Sist oppdatert: 2018-10-16bibliografisk kontrollert
4. A Correction for the Thermal Mass-Induced Errors of CTD Tags Mounted on Marine Mammals
Åpne denne publikasjonen i ny fane eller vindu >>A Correction for the Thermal Mass-Induced Errors of CTD Tags Mounted on Marine Mammals
Vise andre…
2018 (engelsk)Inngår i: Journal of Atmospheric and Oceanic Technology, ISSN 0739-0572, E-ISSN 1520-0426, Vol. 35, nr 6, s. 1237-1252Artikkel i tidsskrift (Fagfellevurdert) Published
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.

Emneord
Data processing, In situ oceanic observations, Instrumentation, sensors, Profilers, oceanic
HSV kategori
Forskningsprogram
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-159163 (URN)10.1175/JTECH-D-17-0141.1 (DOI)000438020400005 ()
Tilgjengelig fra: 2018-08-24 Laget: 2018-08-24 Sist oppdatert: 2020-01-23bibliografisk kontrollert

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