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Drivers of variability in Southern Ocean water mass formation regions

Drivers of variability in Southern Ocean water mass formation regions
Drivers of variability in Southern Ocean water mass formation regions
Subantarctic Mode Water (SAMW) forms on the northern side of the Subantarctic Front, in regions of deep winter mixed layers. The water mass is responsible for absorbing a significant proportion of the anthropogenic CO2 and heat that is taken up by the global ocean. Two modes of SAMW are present in the Pacific, a lighter central Pacific mode (CPSAMW), and a denser Southeast Pacific mode (SEPSAMW). Antarctic Intermediate Water (AAIW), characterised by a salinity minimum, forms to the south of SAMW, via the subduction of northward flowing Antarctic Surface Water. Each water mass has experienced significant interannual variability in recent years. In this study, mixed layer temperature and salinity budgets are computed in the SAMW formation regions, to determine the processes responsible for variability in the properties of the mixed layers that ultimately subduct as SAMW. The results of the budgets are used to determine the drivers of interannual variability in the Southern Ocean water mass formation regions. The dominant drivers in the variability of both temperature and salinity of the SAMW formation region mixed layers are shown to be surface buoyancy fluxes, horizontal advection and entrainment.

Salinity advection in each water mass formation region is found to be strongly correlated with sea ice area in the northern Ross Sea, with the strongest impact occurring at lags of up to two years. Correlation is also found between meridional salinity advection in the southeast Pacific water mass formation regions, and sea ice area in the northern Amundsen/Bellingshausen Sea, suggesting freshwater fluxes due to sea ice melt reach the SEPSAMW formation region over six months. Patterns in Ross Sea sea ice appear to be related to those in the El Nino Southern Oscillation (ENSO), which drives a 10-15 ˜year cycle that is also seen in salinity advection, with the strongest signal in the AAIW formation regions.

In 2016, the winter mixed layer in the southeast Pacific SAMW formation region was anomalously shallow, warm and fresh. Reduced surface cooling during autumn/winter was found to be the main cause of the mixed layer failing to deepen. However, the mixed layer was already anomalously fresh due to northward advection of freshwater, linked to increased sea ice melt in the Amundsen/Bellingshausen Sea, acting to increase the stability of the mixed layer. The increased sea ice was the result of a simultaneously positive ENSO and Southern Annular Mode (SAM) the previous winter. Both climate modes then switched to negative, and the high frequency variability in the SAM drove anomalies in both zonal and meridional winds over the southeast Pacific.
University of Southampton
Sanders, Rachael, Nathalie Charlotte
3fd6718d-49d8-4ed8-9ccc-3d0bc46d8e38
Sanders, Rachael, Nathalie Charlotte
3fd6718d-49d8-4ed8-9ccc-3d0bc46d8e38
Naveira Garabato, Alberto
97c0e923-f076-4b38-b89b-938e11cea7a6

Sanders, Rachael, Nathalie Charlotte (2021) Drivers of variability in Southern Ocean water mass formation regions. University of Southampton, Doctoral Thesis, 222pp.

Record type: Thesis (Doctoral)

Abstract

Subantarctic Mode Water (SAMW) forms on the northern side of the Subantarctic Front, in regions of deep winter mixed layers. The water mass is responsible for absorbing a significant proportion of the anthropogenic CO2 and heat that is taken up by the global ocean. Two modes of SAMW are present in the Pacific, a lighter central Pacific mode (CPSAMW), and a denser Southeast Pacific mode (SEPSAMW). Antarctic Intermediate Water (AAIW), characterised by a salinity minimum, forms to the south of SAMW, via the subduction of northward flowing Antarctic Surface Water. Each water mass has experienced significant interannual variability in recent years. In this study, mixed layer temperature and salinity budgets are computed in the SAMW formation regions, to determine the processes responsible for variability in the properties of the mixed layers that ultimately subduct as SAMW. The results of the budgets are used to determine the drivers of interannual variability in the Southern Ocean water mass formation regions. The dominant drivers in the variability of both temperature and salinity of the SAMW formation region mixed layers are shown to be surface buoyancy fluxes, horizontal advection and entrainment.

Salinity advection in each water mass formation region is found to be strongly correlated with sea ice area in the northern Ross Sea, with the strongest impact occurring at lags of up to two years. Correlation is also found between meridional salinity advection in the southeast Pacific water mass formation regions, and sea ice area in the northern Amundsen/Bellingshausen Sea, suggesting freshwater fluxes due to sea ice melt reach the SEPSAMW formation region over six months. Patterns in Ross Sea sea ice appear to be related to those in the El Nino Southern Oscillation (ENSO), which drives a 10-15 ˜year cycle that is also seen in salinity advection, with the strongest signal in the AAIW formation regions.

In 2016, the winter mixed layer in the southeast Pacific SAMW formation region was anomalously shallow, warm and fresh. Reduced surface cooling during autumn/winter was found to be the main cause of the mixed layer failing to deepen. However, the mixed layer was already anomalously fresh due to northward advection of freshwater, linked to increased sea ice melt in the Amundsen/Bellingshausen Sea, acting to increase the stability of the mixed layer. The increased sea ice was the result of a simultaneously positive ENSO and Southern Annular Mode (SAM) the previous winter. Both climate modes then switched to negative, and the high frequency variability in the SAM drove anomalies in both zonal and meridional winds over the southeast Pacific.

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Published date: 6 December 2021

Identifiers

Local EPrints ID: 452918
URI: http://eprints.soton.ac.uk/id/eprint/452918
PURE UUID: 1181cf6d-753e-4102-85be-dcd117495dfe
ORCID for Alberto Naveira Garabato: ORCID iD orcid.org/0000-0001-6071-605X

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Date deposited: 06 Jan 2022 17:50
Last modified: 17 Mar 2024 03:04

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Contributors

Author: Rachael, Nathalie Charlotte Sanders
Thesis advisor: Alberto Naveira Garabato ORCID iD

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