Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength
The Southern Ocean hosts a winter deep mixing band (DMB) near the Antarctic Circumpolar Current's (ACC) northern boundary, playing a pivotal role in Subantarctic Mode Water formation. Here, we investigate what controls the presence and geographical extent of the DMB. Using observational data, w...
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ftcopernicus:oai:publications.copernicus.org:egusphere115454 2024-06-23T07:47:11+00:00 Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength Caneill, Romain Roquet, Fabien Nycander, Jonas 2024-04-19 application/pdf https://doi.org/10.5194/egusphere-2023-2404 https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2404/ eng eng doi:10.5194/egusphere-2023-2404 https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2404/ eISSN: Text 2024 ftcopernicus https://doi.org/10.5194/egusphere-2023-2404 2024-06-13T01:23:50Z The Southern Ocean hosts a winter deep mixing band (DMB) near the Antarctic Circumpolar Current's (ACC) northern boundary, playing a pivotal role in Subantarctic Mode Water formation. Here, we investigate what controls the presence and geographical extent of the DMB. Using observational data, we construct seasonal climatologies of surface buoyancy fluxes, Ekman buoyancy transport, and upper stratification. The strength of the upper-ocean stratification is determined using the columnar buoyancy index, defined as the buoyancy input necessary to produce a 250 m deep mixed layer. It is found that the DMB lies precisely where the autumn–winter buoyancy loss exceeds the columnar buoyancy found in late summer. The buoyancy loss decreases towards the south, while in the north the stratification is too strong to produce deep mixed layers. Although this threshold is also crossed in the Agulhas Current and East Australian Current regions, advection of buoyancy is able to stabilise the stratification. The Ekman buoyancy transport has a secondary impact on the DMB extent due to the compensating effects of temperature and salinity transports on buoyancy. Changes in surface temperature drive spatial variations in the thermal expansion coefficient (TEC). These TEC variations are necessary to explain the limited meridional extent of the DMB. We demonstrate this by comparing buoyancy budgets derived using varying TEC values with those derived using a constant TEC value. Reduced TEC in colder waters leads to decreased winter buoyancy loss south of the DMB, yet substantial heat loss persists. Lower TEC values also weaken the effect of temperature stratification, partially compensating for the effect of buoyancy loss damping. TEC modulation impacts both the DMB characteristics and its meridional extent. Text Antarc* Antarctic Southern Ocean Copernicus Publications: E-Journals Antarctic Southern Ocean The Antarctic |
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Open Polar |
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Copernicus Publications: E-Journals |
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ftcopernicus |
language |
English |
description |
The Southern Ocean hosts a winter deep mixing band (DMB) near the Antarctic Circumpolar Current's (ACC) northern boundary, playing a pivotal role in Subantarctic Mode Water formation. Here, we investigate what controls the presence and geographical extent of the DMB. Using observational data, we construct seasonal climatologies of surface buoyancy fluxes, Ekman buoyancy transport, and upper stratification. The strength of the upper-ocean stratification is determined using the columnar buoyancy index, defined as the buoyancy input necessary to produce a 250 m deep mixed layer. It is found that the DMB lies precisely where the autumn–winter buoyancy loss exceeds the columnar buoyancy found in late summer. The buoyancy loss decreases towards the south, while in the north the stratification is too strong to produce deep mixed layers. Although this threshold is also crossed in the Agulhas Current and East Australian Current regions, advection of buoyancy is able to stabilise the stratification. The Ekman buoyancy transport has a secondary impact on the DMB extent due to the compensating effects of temperature and salinity transports on buoyancy. Changes in surface temperature drive spatial variations in the thermal expansion coefficient (TEC). These TEC variations are necessary to explain the limited meridional extent of the DMB. We demonstrate this by comparing buoyancy budgets derived using varying TEC values with those derived using a constant TEC value. Reduced TEC in colder waters leads to decreased winter buoyancy loss south of the DMB, yet substantial heat loss persists. Lower TEC values also weaken the effect of temperature stratification, partially compensating for the effect of buoyancy loss damping. TEC modulation impacts both the DMB characteristics and its meridional extent. |
format |
Text |
author |
Caneill, Romain Roquet, Fabien Nycander, Jonas |
spellingShingle |
Caneill, Romain Roquet, Fabien Nycander, Jonas Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength |
author_facet |
Caneill, Romain Roquet, Fabien Nycander, Jonas |
author_sort |
Caneill, Romain |
title |
Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength |
title_short |
Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength |
title_full |
Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength |
title_fullStr |
Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength |
title_full_unstemmed |
Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength |
title_sort |
southern ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength |
publishDate |
2024 |
url |
https://doi.org/10.5194/egusphere-2023-2404 https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2404/ |
geographic |
Antarctic Southern Ocean The Antarctic |
geographic_facet |
Antarctic Southern Ocean The Antarctic |
genre |
Antarc* Antarctic Southern Ocean |
genre_facet |
Antarc* Antarctic Southern Ocean |
op_source |
eISSN: |
op_relation |
doi:10.5194/egusphere-2023-2404 https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2404/ |
op_doi |
https://doi.org/10.5194/egusphere-2023-2404 |
_version_ |
1802651271107182592 |