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Dissertation zugänglich unter
URN: urn:nbn:de:gbv:18-90625
URL: http://ediss.sub.uni-hamburg.de/volltexte/2018/9062/


Relevance of surface freshwater flux for global ocean circulation and Atlantic freshwater content variability

Relevanz des Oberflächenwasserflusses für die globale Ozeanzirkulation und die Variabilität des atlantischen Süßwassergehalts

Liu, Xin

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Freie Schlagwörter (Englisch): large-scale ocean circulation , freshwater content variability , freshwater flux , climate change , GECCO2
Basisklassifikation: 38.90
Institut: Geowissenschaften
DDC-Sachgruppe: Geowissenschaften
Dokumentart: Dissertation
Hauptberichter: Stammer, Detlef (Prof. Dr.)
Sprache: Englisch
Tag der mündlichen Prüfung: 01.02.2018
Erstellungsjahr: 2017
Publikationsdatum: 26.03.2018
Kurzfassung auf Englisch: Over the next century changes in the ocean will occur as a consequence of an intensified water cycle and the associated surface freshwater flux changes under global warming. One objective of this thesis is concerned with the dynamical ocean response to the associated surface volume flux anomalies. The other objective is to detect the contributions of surface freshwater flux and freshwater transport to the regional freshwater content variations in the Atlantic, specifically at interannual and decadal time scales.

To address the first objective, the model configuration of the German contribution to the Estimating the Circulation and Climate of the Ocean (GECCO2) is used with two salinity boundary conditions (virtual salt flux version and volume flux version). Both versions are forced first by the present-day freshwater flux and then by the additional freshwater flux anomalies, which are estimated over the period 2081−2100 relative to 1986−2005 under the Representative Concentration Pathway (RCP) 8.5 scenario from the Max-Planck-Institute Earth System Model.

The resulting differences between the two salinity boundary conditions show that the adjustment of ocean circulation involves a barotropic response as predicted from the Goldsbrough-Stommel theory. In comparison to the present-day Goldsbrough-Stommel circulation, the corresponding barotropic circulation driven by the RCP8.5 freshwater flux anomalies intensifies by about 20% globally with a stronger intensification about 50% in the Southern Ocean. The barotropic circulation anomaly induced by the intensified volume flux reaches to 0.6 Sv in the Antarctic Circumpolar Current region. The adjustment also involves changes in the meridional overturning circulation, mirroring the convergence and divergence of the mass transport driven by surface volume flux. The subsequent pathway of fresh water and the spreading of volume flux match with each other in the shallow cells but diverge substantially with depth. Associated with the circulation changes are the changes in the meridional heat and freshwater transports, which appear mostly as transport changes related to the meridional overturning circulation. Changes in the circulation also lead to a redistribution of temperature and salinity, which results in a significant contribution to the regional sea level changes in form of steric change. The sea level changes due to the projected freshwater flux anomalies are in the range of +/− 0.5 cm and can be largely attributed to the displacement of the isopycnals.

The second part of the thesis investigates the freshwater budgets of three regions in the Atlantic using the GECCO2 ocean synthesis over the period 1961−2014. The freshwater content changes in the subtropical and subpolar North Atlantic (23.5°N − 60°N) can be attributed to the combined impacts of the surface freshwater flux and the net freshwater transport across the boundaries. The strongly increased surface freshwater input over the last two decades is located to the north of 37°N and tends to enhance the southward transport at 37°N through the overturning circulation. The increased southward transport at 37°N turns out to be the dominant contribution to the freshwater content variability in the sub-region to the south of 37°N. Over the tropical region, the surface flux plays a role in changing the freshwater content in the northern box (0° − 23.5°N), but shows no significant impact in the southern box (23.5°S − 0°) because of its relatively small interannual variability. Overall, the results indicate the importance of the convergence/divergence of the freshwater transports across the boundaries to the freshwater content changes in every region. In addition, the shallow overturning cells in the tropical Atlantic are found to be important for the interannual variations of the freshwater transport, while the deep overturning circulation shows profound impacts on the decadal variations.

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