Ocean dynamics of the shelf and bays of the eastern Agulhas bank: a process-oriented numerical modelling study
There is a general need for a broader base of information on the finer detail of ocean processes in the Eastern Agulhas Bank region. In this thesis, the Regional Ocean Modelling System (ROMS) was applied to resolve high resolution (~1km) bay and shelf scale ocean processes along the south eastern co...
Main Author: | |
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Format: | Doctoral or Postdoctoral Thesis |
Language: | English |
Published: |
Nelson Mandela University
2020
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Subjects: | |
Online Access: | http://hdl.handle.net/10948/49038 http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:41595 |
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record_format |
openpolar |
institution |
Open Polar |
collection |
SEALS Digital Commons (South East Academic Libraries System, South Africa) |
op_collection_id |
ftsealsdc |
language |
English |
topic |
Oceanography -- Research Marine biology Marine meteorology Hydrography |
spellingShingle |
Oceanography -- Research Marine biology Marine meteorology Hydrography Bailey, Dylan Francis Ocean dynamics of the shelf and bays of the eastern Agulhas bank: a process-oriented numerical modelling study |
topic_facet |
Oceanography -- Research Marine biology Marine meteorology Hydrography |
description |
There is a general need for a broader base of information on the finer detail of ocean processes in the Eastern Agulhas Bank region. In this thesis, the Regional Ocean Modelling System (ROMS) was applied to resolve high resolution (~1km) bay and shelf scale ocean processes along the south eastern coastline of South Africa using an offline multi-nested approach. Experimental and reference ocean model runs were used with in situ instrument mooring data to investigate how currents and temperature in the bays and the adjacent shelf are affected by the bathymetry, Agulhas Current (AC), regional winds, and coastal trapped waves (CTW). Focus was given to temperature, circulation, and coastal trapped waves, given the abundance of hourly in situ mooring data for these variables, making them suitable for comparison with the ocean model. SST variability trends were seasonally and spatially predictable, being greatest in the summer months and smallest in the winter months. A region of high SST variability extended from Port Alfred toward the south-west, indicating entrainment of nearshore upwelled water and instabilities in the inshore front of the AC. Shelf bottom temperature variability had a more spatially diverse seasonal trend, with coastal variability the highest (lowest) and offshore variability the lowest (highest) during the summer (winter) months. A distinct asymmetry in mean seasonal shelf bottom temperatures evident to the south of Algoa Bay and St Francis Bay was shown to be dependent on the profile of the shelf margin in relation to the inshore front of the AC using an experimental model run. Bay-scale anti-cyclonic circulation patterns dominate the bays throughout the year, with regions of strong mean surface currents occurring south of Cape Recife and Cape St Francis. General circulation patterns over the shelf are strongly governed by regional wind regimes, with the strongest flows being south-westward occurring during north-easterly wind events. Shelf bottom flows typically follow the expected Ekman-veering response, apart from near coastal flows that are topographically steered. A region of consistently strong south-westward shelf bottom flow extends from the east of Port Alfred to the shelf margin south of Bird Island. This flow is predominantly driven by the AC, although its velocity is strongly governed by regional winds. Circulation patterns during powerful eastward moving wind systems are strongly governed by the passage of associated CTW. Comparisons with an experimental model run where surface winds were removed revealed that the shelf wave was responsible for 61% of water volume displacement during a CTW event, and that the coastal shear turbulence caused by this displacement has a direct effect on the mixed layer depth. The links made between the complex shelf and bay ocean processes indicate that subtle changes to the ocean state will have far reaching effects on the regional ocean environment and its biota. Given the challenges of climate change, this is of paramount importance if we are to quantify these effects and mitigate their impacts, particularly with respect to the socio-economic dependency of those who rely on the ocean for its living resources. |
format |
Doctoral or Postdoctoral Thesis |
author |
Bailey, Dylan Francis |
author_facet |
Bailey, Dylan Francis |
author_sort |
Bailey, Dylan Francis |
title |
Ocean dynamics of the shelf and bays of the eastern Agulhas bank: a process-oriented numerical modelling study |
title_short |
Ocean dynamics of the shelf and bays of the eastern Agulhas bank: a process-oriented numerical modelling study |
title_full |
Ocean dynamics of the shelf and bays of the eastern Agulhas bank: a process-oriented numerical modelling study |
title_fullStr |
Ocean dynamics of the shelf and bays of the eastern Agulhas bank: a process-oriented numerical modelling study |
title_full_unstemmed |
Ocean dynamics of the shelf and bays of the eastern Agulhas bank: a process-oriented numerical modelling study |
title_sort |
ocean dynamics of the shelf and bays of the eastern agulhas bank: a process-oriented numerical modelling study |
publisher |
Nelson Mandela University |
publishDate |
2020 |
url |
http://hdl.handle.net/10948/49038 http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:41595 |
long_lat |
ENVELOPE(-38.060,-38.060,-54.004,-54.004) ENVELOPE(-55.715,-55.715,52.567,52.567) ENVELOPE(-100.146,-100.146,58.893,58.893) |
geographic |
Bird Island Cape St. Francis Francis Bay |
geographic_facet |
Bird Island Cape St. Francis Francis Bay |
genre |
Bird Island |
genre_facet |
Bird Island |
op_relation |
http://hdl.handle.net/10948/49038 vital:41595 http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:41595 |
op_rights |
Nelson Mandela University |
_version_ |
1766379126039511040 |
spelling |
ftsealsdc:vital:41595 2023-05-15T15:44:45+02:00 Ocean dynamics of the shelf and bays of the eastern Agulhas bank: a process-oriented numerical modelling study Bailey, Dylan Francis 2020 xxiii, 144 leaves pdf http://hdl.handle.net/10948/49038 http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:41595 English eng Nelson Mandela University Faculty of Science http://hdl.handle.net/10948/49038 vital:41595 http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:41595 Nelson Mandela University Oceanography -- Research Marine biology Marine meteorology Hydrography Thesis Doctoral DPhil 2020 ftsealsdc 2021-05-14T07:49:29Z There is a general need for a broader base of information on the finer detail of ocean processes in the Eastern Agulhas Bank region. In this thesis, the Regional Ocean Modelling System (ROMS) was applied to resolve high resolution (~1km) bay and shelf scale ocean processes along the south eastern coastline of South Africa using an offline multi-nested approach. Experimental and reference ocean model runs were used with in situ instrument mooring data to investigate how currents and temperature in the bays and the adjacent shelf are affected by the bathymetry, Agulhas Current (AC), regional winds, and coastal trapped waves (CTW). Focus was given to temperature, circulation, and coastal trapped waves, given the abundance of hourly in situ mooring data for these variables, making them suitable for comparison with the ocean model. SST variability trends were seasonally and spatially predictable, being greatest in the summer months and smallest in the winter months. A region of high SST variability extended from Port Alfred toward the south-west, indicating entrainment of nearshore upwelled water and instabilities in the inshore front of the AC. Shelf bottom temperature variability had a more spatially diverse seasonal trend, with coastal variability the highest (lowest) and offshore variability the lowest (highest) during the summer (winter) months. A distinct asymmetry in mean seasonal shelf bottom temperatures evident to the south of Algoa Bay and St Francis Bay was shown to be dependent on the profile of the shelf margin in relation to the inshore front of the AC using an experimental model run. Bay-scale anti-cyclonic circulation patterns dominate the bays throughout the year, with regions of strong mean surface currents occurring south of Cape Recife and Cape St Francis. General circulation patterns over the shelf are strongly governed by regional wind regimes, with the strongest flows being south-westward occurring during north-easterly wind events. Shelf bottom flows typically follow the expected Ekman-veering response, apart from near coastal flows that are topographically steered. A region of consistently strong south-westward shelf bottom flow extends from the east of Port Alfred to the shelf margin south of Bird Island. This flow is predominantly driven by the AC, although its velocity is strongly governed by regional winds. Circulation patterns during powerful eastward moving wind systems are strongly governed by the passage of associated CTW. Comparisons with an experimental model run where surface winds were removed revealed that the shelf wave was responsible for 61% of water volume displacement during a CTW event, and that the coastal shear turbulence caused by this displacement has a direct effect on the mixed layer depth. The links made between the complex shelf and bay ocean processes indicate that subtle changes to the ocean state will have far reaching effects on the regional ocean environment and its biota. Given the challenges of climate change, this is of paramount importance if we are to quantify these effects and mitigate their impacts, particularly with respect to the socio-economic dependency of those who rely on the ocean for its living resources. Doctoral or Postdoctoral Thesis Bird Island SEALS Digital Commons (South East Academic Libraries System, South Africa) Bird Island ENVELOPE(-38.060,-38.060,-54.004,-54.004) Cape St. Francis ENVELOPE(-55.715,-55.715,52.567,52.567) Francis Bay ENVELOPE(-100.146,-100.146,58.893,58.893) |