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The role of Southern Ocean fronts in the global climate system
Stockholm University, Faculty of Science, Department of Geological Sciences.ORCID iD: 0000-0003-0008-1886
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. However, until recently fronts have been poorly resolved by available data and climate models. In this thesis we utilise a combination of high resolution satellite data, model output and ARGO data to improve our basic understanding of fronts.

A method is derived whereby fronts are identified as local maxima in sea surface height gradients. In this way fronts are defined locally as jets, rather than continuous-circumpolar water mass boundaries. A new climatology of Southern Ocean fronts is presented. This climatology reveals a new interpretation of the Subtropical Front. The currents associated with the Subtropical Front correspond to the western boundary current extensions from each basin, and we name these the Dynamical Subtropical Front. Previous studies have instead suggested that the Subtropical Front is a continuous feature across the Southern Ocean associated with the super gyre boundary.

A comprehensive assessment of the relationship between front locations and wind stress is conducted. Firstly, the response of fronts to a southward shift in the westerly winds is tested using output from a 100 year climate change simulation on a high resolution coupled model. It is shown that there was no change in the location of fronts within the Antarctic Circumpolar Current as a result of a 1.3° southward shift in the westerly winds. Secondly, it is shown that the climatological position of the Subtropical Front is 5-10° north of the zero wind stress curl line, despite many studies assuming that the location of the Subtropical Front is determined by the zero wind stress curl.

Finally, we show that the nutrient supply at ocean fronts is primarily due to horizontal advection and not upwelling. Nutrients from coastal regions are entrained into western boundary currents and advected into the Southern Ocean along the Dynamical Subtropical Front. 

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University , 2014. , 41 p.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 355
Keyword [en]
Southern Ocean, fronts, jets, Antarctic Circumpolar Current, wind stress, chlorophyll, iron, Last Glacial Maximum
National Category
Climate Research Oceanography, Hydrology, Water Resources Geosciences, Multidisciplinary
Research subject
Marine Geology
Identifiers
URN: urn:nbn:se:su:diva-108736ISBN: 978-91-7447-991-1 (print)OAI: oai:DiVA.org:su-108736DiVA: diva2:760289
Public defence
2014-12-05, Ahlmannsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.

Available from: 2014-11-13 Created: 2014-11-03 Last updated: 2014-11-04Bibliographically approved
List of papers
1. Southern ocean fronts: controlled by wind or topography?
Open this publication in new window or tab >>Southern ocean fronts: controlled by wind or topography?
Show others...
2012 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, C08018- p.Article in journal (Refereed) Published
Abstract [en]

The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. Here we explore the relative importance of bottom topography and winds for the location of Southern Ocean fronts, using 100 years of a control and climate change simulation from the high resolution coupled climate model HiGEM. Topography has primary control on the number and intensity of fronts at each longitude. However, there is no strong relationship between the position or spacing of jets and underlying topographic gradients because of the effects of upstream and downstream topography. The Southern Hemisphere Westerlies intensify and shift south by 1.3 degrees in the climate change simulation, but there is no comparable meridional displacement of the Antarctic Circumpolar Current's (ACC) path or the fronts within its boundaries, even over flat topography. Instead, the current contracts meridionally and weakens. North of the ACC, the Subtropical Front (STF) shifts south gradually, even over steep topographic ridges. We suggest the STF reacts more strongly to the wind shift because it is strongly surface intensified. In contrast, fronts within the ACC are more barotropic and are therefore more sensitive to the underlying topography. An assessment of different methods for identifying jets reveals that maxima of gradients in the sea surface height field are the most reliable. Approximating the position of fronts using sea surface temperature gradients is ineffective at high latitudes while using sea surface height contours can give misleading results when studying the temporal variability of front locations.

National Category
Oceanography, Hydrology, Water Resources
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-81298 (URN)10.1029/2012JC007887 (DOI)000307731700001 ()
Note

AuthorCount:5;

Available from: 2012-10-29 Created: 2012-10-15 Last updated: 2017-12-07Bibliographically approved
2. The Dynamical Subtropical Front
Open this publication in new window or tab >>The Dynamical Subtropical Front
2013 (English)In: Journal of Geophysical Research - Oceans and Atmospheres, ISSN 0148-022A, Vol. 118, no 10, 5676-5685 p.Article in journal (Refereed) Published
Abstract [en]

The Southern Ocean Subtropical Front (STF) is thought to play a key role in the global climate system. Theory suggests that the latitude of the STF regulates the volume of saline Agulhas Leakage into the Atlantic Ocean from the Indian. Here we use satellite sea surface temperature (SST) and sea surface height (SSH) data to study the physical characteristics of the STF water mass boundary. We find that the strong currents in this region do not align with the surface water mass boundary. Therefore, we provide a new climatology for these currents which we define as the Dynamical STF (DSTF). The DSTF is the eastward extension of the western boundary current in each basin and is characterized by strong SST and SSH gradients and no seasonal cycle. At the center of each basin it merges with the Sub-Antarctic Front. On the eastern side of basins, the STF surface water mass boundary coincides with a separate region of multiple SST fronts. We call this the Subtropical Frontal Zone (STFZ). The fronts in the STFZ have a large seasonal cycle and no SSH signature. Despite lying close to the same water mass boundary, the DSTF and STFZ are completely unrelated. We therefore suggest the term STF only be used when referring to the surface water mass boundary. When studying the strong currents on the western side of basins the term DSTF is more relevant and, similarly, the term STFZ better describes the region of enhanced SST gradients towards the east.

Keyword
ubtropical front, Agulhas Leakage, Southern Ocean, wind stress curl, sea surface temperature, sea surface height
National Category
Oceanography, Hydrology, Water Resources
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-97487 (URN)10.1002/jgrc.20408 (DOI)000327380100058 ()
Note

AuthorCount: 2;

Available from: 2013-12-12 Created: 2013-12-12 Last updated: 2017-12-06Bibliographically approved
3. The control of the Southern Hemisphere Westerlies on the position of the Subtropical Front
Open this publication in new window or tab >>The control of the Southern Hemisphere Westerlies on the position of the Subtropical Front
2013 (English)In: Journal of Geophysical Research - Oceans and Atmospheres, ISSN 0148-022A, Vol. 118, no 10, 5669-5675 p.Article in journal (Refereed) Published
Abstract [en]

In recent years the latitudinal position of the Subtropical Front (STF) has emerged as a key parameter in the global climate. A poleward positioned front is thought to allow a greater salt flux from the Indian to the Atlantic Ocean and so drive a stronger Atlantic Meridional Overturning Circulation. Here the common view that the STF aligns with the zero wind stress curl (WSC) is challenged. Based on the STF climatologies of Orsi et al. (1995), Belkin and Gordon (1996), Graham and De Boer (2013), and on satellite scatterometry winds, we find that the zero WSC contour lies on average ∼10°, ∼8°, and ∼5° poleward of the front for the three climatologies, respectively. The circulation in the region between the Subtropical Gyres and the zero WSC contour is not forced by the WSC but rather by the strong bottom pressure torque that is a result of the interaction of the Antarctic Circumpolar Current with the ocean floor topography. The actual control of the position of the STF is crucially dependent on whether the front is regarded as simply a surface water mass boundary or a dynamical front. For the Agulhas Leakage problem, the southern boundary of the so-called Super Gyre may be the most relevant property but this cannot easily be identified in observations.

Keyword
Subtropical Front, wind stress curl, Southern Ocean, satellite data, fronts, Dynamical Subtropical Front
National Category
Oceanography, Hydrology, Water Resources
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-97489 (URN)10.1002/jgrc.20407 (DOI)000327380100057 ()
Note

AuthorCount: 4;

Available from: 2013-12-12 Created: 2013-12-12 Last updated: 2017-12-06Bibliographically approved
4. Identifying sources and transport pathways of iron in the Southern Ocean
Open this publication in new window or tab >>Identifying sources and transport pathways of iron in the Southern Ocean
(English)In: Deep Sea Research Part I: Oceanographic Research Papers, ISSN 0967-0637, E-ISSN 1879-0119Article in journal (Refereed) Submitted
Abstract [en]

Over large regions of the global ocean primary productivity is limited by the availability of dissolved iron. Changes in the supply of iron to these regions could have major impacts on primary productivity and the carbon cycle. One of the largest sources of dissolved iron to the ocean is thought to be from shelf sediments, and this source is often parameterized in biogeochemical models as a depth dependent iron flux through the seafloor. Using the knowledge that Southern Ocean surface waters are iron limited, we infer source regions of iron to the Southern Ocean by identifying where the most intense chlorophyll blooms develop. We further derive surface current patterns from satellite sea surface height fields to assess the role of the ocean circulation in transporting iron away from these source regions. We find a tight relationship between satellite chlorophyll concentrations and sea surface height. Large chlorophyll blooms develop on the shelf and where the western boundary currents detach from the continental shelves and turn eastward into the Southern Ocean. This is likely due to shelf supplied iron becoming entrained into western boundary currents and advected into the Southern Ocean along the Dynamical Subtropical Front. The most intense chlorophyll blooms are located along coastal margins of islands and continents. Blooms do not develop over submerged seamounts or plateaus in the open ocean. This suggests that shelf sediments in coastal regions act as large bioavailable iron sources to the Southern Ocean. We recommend that a more accurate method of parameterizing the shelf sediment iron flux could be to prescribe this flux only through grid cells neighboring coastlines. Finally, we hypothesize how changes in sea level during glacial-interglacial cycles may have altered the distribution of shelf sediment iron sources in the Southern Ocean and helped to drive export production anomalies in the Sub-Antarctic Zone.

National Category
Geochemistry Geosciences, Multidisciplinary Oceanography, Hydrology, Water Resources
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-108739 (URN)
Available from: 2014-11-03 Created: 2014-11-03 Last updated: 2017-12-05

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