Investigating the sensitivity of one-dimensional turbulence schemes in the sub-Antarctic Southern Ocean

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dc.contributor.advisorSmit, Jacoba Een_ZA
dc.contributor.advisorVichi, Marcelloen_ZA
dc.contributor.authorBoodhraj, Kirodhen_ZA
dc.date.accessioned2018-05-03T12:36:56Z
dc.date.available2018-05-03T12:36:56Z
dc.date.issued2018en_ZA
dc.description.abstractThe sub-Antarctic Zone (SAZ) is a zone of vigorous vertical mixing in the Southern Ocean where it is difficult to obtain data for model validation on the turbulence conditions. In this study, a onedimensional configuration from the Nucleus for the European Modelling of the Ocean (NEMO) model was implemented in order to determine the sensitivity and turbulence response of an idealized SAZ water column. Various turbulence scheme parameterizations that are available for ocean models were tested. Furthermore, the number of vertical levels were varied in order to ascertain the sensitivity of the grid. The forcing data were obtained from various reanalyses (ERA-Interim, NASA, NCEP and JRA55) and were likewise tested. Different turbulence diagnostics and univariate indicators were chosen to ascertain the turbulence response and to analyse the energetics of the water column. It was found that using different reanalyses produced different tracer (salinity and temperature) results. Even though the results varied considerably, very high correlations were found for the potential energy anomaly between reanalyses and insignificant correlations were found for the other indicators. This suggested that it was a valuable descriptor which captured the buoyancy fluxes and wind stress information and can be efficiently used to assess the vertical turbulent state with data such as ARGO profiles. It was further found that for a single reanalysis, the turbulence schemes had produced similar results (with small variability and not to the extent as changing the reanalysis) for the turbulence diagnostics and univariate indicators. An important finding of an entrapped warm water parcel beneath cooler waters was found in simulation outputs as well as ARGO validation data. For realistic conditions observed from the ARGO floats, as the season progressed, there were no more instances of a warm water parcel. There was no reason however, to why there should not have been eddies passing by the region. In simulations, the warm water parcel persisted throughout the season for simulated data, likely causing the early stratification that affects ocean models in the SAZ. The stratification was found to have an approximate one month early onset observed from comparing the ARGO data profiles to simulated profiles. The Brunt Väisälä frequency, potential energy anomaly as well as the buoyancy flux were analysed and these diagnostics indicated that an approximate one month early stratification was found during November. It was likely that this false stratification signal may have influenced the summer stratification leading to a poor representation of the Mixed Layer Depth (MLD) and various other indicators. It was found that during the austral winter months, the model simulated comparable MLD's to the ARGO float data as well as theWinter Cruise data (obtained from the SA Agulhas II), capturing the winter dynamics well.en_ZA
dc.identifier.apacitationBoodhraj, K. (2018). <i>Investigating the sensitivity of one-dimensional turbulence schemes in the sub-Antarctic Southern Ocean</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Oceanography. Retrieved from http://hdl.handle.net/11427/27913en_ZA
dc.identifier.chicagocitationBoodhraj, Kirodh. <i>"Investigating the sensitivity of one-dimensional turbulence schemes in the sub-Antarctic Southern Ocean."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Oceanography, 2018. http://hdl.handle.net/11427/27913en_ZA
dc.identifier.citationBoodhraj, K. 2018. Investigating the sensitivity of one-dimensional turbulence schemes in the sub-Antarctic Southern Ocean. University of Cape Town.en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Boodhraj, Kirodh AB - The sub-Antarctic Zone (SAZ) is a zone of vigorous vertical mixing in the Southern Ocean where it is difficult to obtain data for model validation on the turbulence conditions. In this study, a onedimensional configuration from the Nucleus for the European Modelling of the Ocean (NEMO) model was implemented in order to determine the sensitivity and turbulence response of an idealized SAZ water column. Various turbulence scheme parameterizations that are available for ocean models were tested. Furthermore, the number of vertical levels were varied in order to ascertain the sensitivity of the grid. The forcing data were obtained from various reanalyses (ERA-Interim, NASA, NCEP and JRA55) and were likewise tested. Different turbulence diagnostics and univariate indicators were chosen to ascertain the turbulence response and to analyse the energetics of the water column. It was found that using different reanalyses produced different tracer (salinity and temperature) results. Even though the results varied considerably, very high correlations were found for the potential energy anomaly between reanalyses and insignificant correlations were found for the other indicators. This suggested that it was a valuable descriptor which captured the buoyancy fluxes and wind stress information and can be efficiently used to assess the vertical turbulent state with data such as ARGO profiles. It was further found that for a single reanalysis, the turbulence schemes had produced similar results (with small variability and not to the extent as changing the reanalysis) for the turbulence diagnostics and univariate indicators. An important finding of an entrapped warm water parcel beneath cooler waters was found in simulation outputs as well as ARGO validation data. For realistic conditions observed from the ARGO floats, as the season progressed, there were no more instances of a warm water parcel. There was no reason however, to why there should not have been eddies passing by the region. In simulations, the warm water parcel persisted throughout the season for simulated data, likely causing the early stratification that affects ocean models in the SAZ. The stratification was found to have an approximate one month early onset observed from comparing the ARGO data profiles to simulated profiles. The Brunt Väisälä frequency, potential energy anomaly as well as the buoyancy flux were analysed and these diagnostics indicated that an approximate one month early stratification was found during November. It was likely that this false stratification signal may have influenced the summer stratification leading to a poor representation of the Mixed Layer Depth (MLD) and various other indicators. It was found that during the austral winter months, the model simulated comparable MLD's to the ARGO float data as well as theWinter Cruise data (obtained from the SA Agulhas II), capturing the winter dynamics well. DA - 2018 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2018 T1 - Investigating the sensitivity of one-dimensional turbulence schemes in the sub-Antarctic Southern Ocean TI - Investigating the sensitivity of one-dimensional turbulence schemes in the sub-Antarctic Southern Ocean UR - http://hdl.handle.net/11427/27913 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/27913
dc.identifier.vancouvercitationBoodhraj K. Investigating the sensitivity of one-dimensional turbulence schemes in the sub-Antarctic Southern Ocean. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Oceanography, 2018 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/27913en_ZA
dc.language.isoengen_ZA
dc.publisher.departmentDepartment of Oceanographyen_ZA
dc.publisher.facultyFaculty of Scienceen_ZA
dc.publisher.institutionUniversity of Cape Town
dc.subject.otherOcean and Atmospheric Sciencesen_ZA
dc.titleInvestigating the sensitivity of one-dimensional turbulence schemes in the sub-Antarctic Southern Oceanen_ZA
dc.typeMaster Thesis
dc.type.qualificationlevelMasters
dc.type.qualificationnameMScen_ZA
uct.type.filetypeText
uct.type.filetypeImage
uct.type.publicationResearchen_ZA
uct.type.resourceThesisen_ZA
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