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.2 kg m −3 , respectively. (c),(d) As in (a) and (b), but for the sections in FILT, along with the core layer density σ FILT = 24.8 kg m −3 . 4. Mechanism for STMW response a. STMW formation region To investigate the mechanisms underlying the impact of MOA coupling on STMW, the time-averaged formation map of STMW derived from instantaneous map ( Maze et al. 2009 ) is an indication of where the STMW is formed, which focuses on water masses within a particular density class with no
.2 kg m −3 , respectively. (c),(d) As in (a) and (b), but for the sections in FILT, along with the core layer density σ FILT = 24.8 kg m −3 . 4. Mechanism for STMW response a. STMW formation region To investigate the mechanisms underlying the impact of MOA coupling on STMW, the time-averaged formation map of STMW derived from instantaneous map ( Maze et al. 2009 ) is an indication of where the STMW is formed, which focuses on water masses within a particular density class with no
Sea, and the Bellingshausen Sea ( Raphael et al. 2016 ). The ASL significantly influences the formation and properties of two major water masses in the southeast Pacific: Southeast Pacific Subantarctic Mode Water (SEPSAMW) and Antarctic Intermediate Water (AAIW) ( Close et al. 2013 , hereafter C13 ). Subantarctic Mode Waters (SAMWs) are produced in several locations in the Indian and Pacific Oceans when the upper-ocean mixed layer deepens in winter (e.g., Aoki et al. 2007 ; McCartney 1982
Sea, and the Bellingshausen Sea ( Raphael et al. 2016 ). The ASL significantly influences the formation and properties of two major water masses in the southeast Pacific: Southeast Pacific Subantarctic Mode Water (SEPSAMW) and Antarctic Intermediate Water (AAIW) ( Close et al. 2013 , hereafter C13 ). Subantarctic Mode Waters (SAMWs) are produced in several locations in the Indian and Pacific Oceans when the upper-ocean mixed layer deepens in winter (e.g., Aoki et al. 2007 ; McCartney 1982
intense vertical mixing in the North Atlantic and Southern Oceans ( Desbruyères et al. 2017 ; Häkkinen et al. 2016 ). The existing studies suggest that mode waters, by their ability to store heat, play a key role in the climate regulation ( Gao et al. 2018 ). However, no driver or mechanism has been clearly identified to explain decadal variability in the water masses of the Southern Hemisphere oceans (SHOs). The water masses of the Southern Ocean play an important role in the global climate system
intense vertical mixing in the North Atlantic and Southern Oceans ( Desbruyères et al. 2017 ; Häkkinen et al. 2016 ). The existing studies suggest that mode waters, by their ability to store heat, play a key role in the climate regulation ( Gao et al. 2018 ). However, no driver or mechanism has been clearly identified to explain decadal variability in the water masses of the Southern Hemisphere oceans (SHOs). The water masses of the Southern Ocean play an important role in the global climate system
in water masses as defined by their temperature and salinity and material changes in seawater temperature. We will describe in section 3 how this theory is translated into a practical method to estimate material changes in water masses and map these into geographical space. We present an application of this minimum transformation method to recent data over the Argo period in section 4 and give results in section 5 . We discuss the results and compare them with existing work in section 6
in water masses as defined by their temperature and salinity and material changes in seawater temperature. We will describe in section 3 how this theory is translated into a practical method to estimate material changes in water masses and map these into geographical space. We present an application of this minimum transformation method to recent data over the Argo period in section 4 and give results in section 5 . We discuss the results and compare them with existing work in section 6
1. Introduction The global ocean waters are traditionally divided into distinct water masses defined by their origin, their physicochemical properties (in particular their temperature and salinity), and their vertical position. Tracking the position and properties of water masses provides a powerful way for monitoring the ocean circulation and climate variability. The ocean, seen as a network of numerous water masses that are formed, transformed, mixed, and subducted, is however complex to
1. Introduction The global ocean waters are traditionally divided into distinct water masses defined by their origin, their physicochemical properties (in particular their temperature and salinity), and their vertical position. Tracking the position and properties of water masses provides a powerful way for monitoring the ocean circulation and climate variability. The ocean, seen as a network of numerous water masses that are formed, transformed, mixed, and subducted, is however complex to
and surface wind datasets are provided by the fifth-generation ECMWF atmospheric reanalysis (ERA5) of the global climate. The data for the period 2004–18 is used in this study. b. Method 1) Potential vorticity Mode water refers to a thick layer of water with homogeneous physical properties covering a large area of the ocean. Thus, mode water is featured with low potential vorticity (PV), which stands out from the surrounding water masses as a PV minimum. PV provides an excellent tracer for mode
and surface wind datasets are provided by the fifth-generation ECMWF atmospheric reanalysis (ERA5) of the global climate. The data for the period 2004–18 is used in this study. b. Method 1) Potential vorticity Mode water refers to a thick layer of water with homogeneous physical properties covering a large area of the ocean. Thus, mode water is featured with low potential vorticity (PV), which stands out from the surrounding water masses as a PV minimum. PV provides an excellent tracer for mode
across-isothermal formation rate of water masses defined in temperature space using surface heat fluxes in the North Atlantic. Such a framework was extended to temperature–salinity space by Speer (1993) , who depicts the intensity and direction of water mass transformation due to surface buoyancy forcing as a transformation vector. Moreover, Hieronymus et al. (2014) estimated the effect of subsurface mixing terms on the water mass formation rate. The water mass framework was also applied in
across-isothermal formation rate of water masses defined in temperature space using surface heat fluxes in the North Atlantic. Such a framework was extended to temperature–salinity space by Speer (1993) , who depicts the intensity and direction of water mass transformation due to surface buoyancy forcing as a transformation vector. Moreover, Hieronymus et al. (2014) estimated the effect of subsurface mixing terms on the water mass formation rate. The water mass framework was also applied in
estimate in Eulerian coordinates is applied in sections 6 and 7 and appendix C . 5. Thermodynamic estimate of surface water-mass transformation Water-mass transformation (WMT) is the exchange of volume between water masses with different properties, such as salinity, temperature, or buoyancy. The surface water-mass transformation rate F and formation rate Δ F , driven by the air–sea buoyancy fluxes, are estimated here following the approach of Maze et al. (2009) and Cerovečki et al. (2013
estimate in Eulerian coordinates is applied in sections 6 and 7 and appendix C . 5. Thermodynamic estimate of surface water-mass transformation Water-mass transformation (WMT) is the exchange of volume between water masses with different properties, such as salinity, temperature, or buoyancy. The surface water-mass transformation rate F and formation rate Δ F , driven by the air–sea buoyancy fluxes, are estimated here following the approach of Maze et al. (2009) and Cerovečki et al. (2013
. 2017 ). Previous studies showed that the water masses, including the SAMW in the Southern Ocean, are poorly represented across climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and differ significantly from one another ( Downes et al. 2010 ; Sallée et al. 2013b ). The poor simulation of the SAMW is related to the unrealistic air–sea fluxes on the ocean surface ( Sallée et al. 2013b ). As the SAMW is key to the ocean interior structure in the Southern Hemisphere, the
. 2017 ). Previous studies showed that the water masses, including the SAMW in the Southern Ocean, are poorly represented across climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and differ significantly from one another ( Downes et al. 2010 ; Sallée et al. 2013b ). The poor simulation of the SAMW is related to the unrealistic air–sea fluxes on the ocean surface ( Sallée et al. 2013b ). As the SAMW is key to the ocean interior structure in the Southern Hemisphere, the
). A general theory describing how water masses are formed from air–sea fluxes of heat and freshwater was developed by Walin (1982) and Tziperman (1986) and expanded upon by several papers (e.g., Speer and Tziperman 1992 ; Marshall et al. 1999 ; Nurser et al. 1999 ). Water mass transformation (WMT) is the mass transport of seawater through a surface of constant property or class (e.g., density interval) and water mass formation (WMF) is the creation or destruction of water masses of a
). A general theory describing how water masses are formed from air–sea fluxes of heat and freshwater was developed by Walin (1982) and Tziperman (1986) and expanded upon by several papers (e.g., Speer and Tziperman 1992 ; Marshall et al. 1999 ; Nurser et al. 1999 ). Water mass transformation (WMT) is the mass transport of seawater through a surface of constant property or class (e.g., density interval) and water mass formation (WMF) is the creation or destruction of water masses of a
