Antarctic Intermediate Water and Subantarctic Mode Water Formation in the Southeast Pacific: The Role of Turbulent Mixing

Abstract

During the 2005 austral winter (late August–early October) and 2006 austral summer (February–mid-March) two intensive hydrographic surveys of the southeast Pacific sector of the Southern Ocean were completed. In this study the turbulent kinetic energy dissipation rate ε, diapycnal diffusivity κ, and buoyancy flux Jb are estimated from the CTD/O2 and XCTD profiles for each survey. Enhanced κ of O(10−3 to 10−4 m2 s−1) is found near the Subantarctic Front (SAF) during both surveys. During the winter survey, enhanced κ was also observed north of the “subduction front,” the northern boundary of the winter deep mixed layer north of the SAF. In contrast, the summer survey found enhanced κ across the entire region north of the SAF below the shallow seasonal mixed layer. The enhanced κ below the mixed layer decays rapidly with depth. A number of ocean processes are considered that may provide the energy flux necessary to support the observed diffusivity. The observed buoyancy flux (4.0 × 10−8 m2 s−3) surrounding the SAF during the summer survey is comparable to the mean buoyancy flux (0.57 × 10−8 m2 s−3) associated with the change in the interior stratification between austral summer and autumn, determined from Argo profiles. The authors suggest that reduced ocean stratification during austral summer and autumn, by interior mixing, preconditions the water column for the rapid development of deep mixed layers and efficient Antarctic Intermediate Water and Subantarctic Mode Water formation during austral winter and early spring.

Mixing Variability in the Southern Ocean

“…Recent work suggests that in some regions of the Southern Ocean, the interaction between the Antarctic Circumpolar Current, or tidal flows, and rough topography is a significant source of internal waves (e.g., Naveira Garabato et al 2004b;Nikurashin and Ferrari 2010). There is growing evidence that the resulting enhanced mixing over regions of rough topography affects the abyssal stratification, the circulation of the Southern Ocean, and the global overturning circulation (Naveira Garabato et al 2004b;Sloyan et al 2010;Nikurashin and Ferrari 2010). This work investigates small-scale diapycnal turbulent mixing and its sources north of the Kerguelen Plateau, a major topographic feature in the Southern Ocean.…”

“…The quantitative contribution of either diapycnal mixing or buoyancy fluxes is unknown, but diapycnal mixing at least partly closes the Southern Ocean meridional circulation by upwelling tracers across isopycnals (Huang 1999). As such, the intensity and distribution of diapycnal mixing in the Southern Ocean plays a major role in the global overturning circulation (Polzin et al 1997;Sloyan and Rintoul 2001;Wunsch and Ferrari 2004;Ferrari 2014).…”

Mechanisms for enhanced turbulence in the Drake Passage region of the Southern Ocean

“…A growing body of work has used hydrographic and velocity finescale data to estimate diapycnal mixing rates in this region (Naveira Garabato et al (2004), Thompson et al (2007), Sloyan et al (2010), Wu et al (2011), Whalen et al (2012). Thorpe scale analysis (Thorpe, 1977) and shear/strain based methods (Kunze et al (1990)) are most commonly used to estimate microscale quantities.…”

“…Naveira Garabato et al (2004) attributed high values of diffusivity ( ∼ 10 −4 m 2 s −1 between 500-1000m) to bottom-generated lee-waves. Multiple studies (Wu et al (2011), Sloyan et al (2010, Thompson et al (2007), Kilbourne and Girton (2015)) suggest wind excited near-inertial waves radiate into the interior and break, contributing to high values of mixing below the surface layer. However, direct evidence of this process is still lacking.…”

Model bias for South Atlantic Antarctic intermediate water in CMIP5

“…The formation process of AAIW is still poorly understood, although several mechanisms have been proposed. These processes include ice-sea and air-sea exchange of freshwater and heat (Santoso and England 2004), Ekman transport over the layer outcrop (Ribbe 2001;Rintoul and England 2002), interior mixing across the SAF (Molinelli 1981;Sloyan et al 2010) and the deep winter convection in the southeastern Pacific Ocean mixed layer (McCartney 1977). The AAIW in the Atlantic is formed originally in the southwestern basin where the AAIW/ Subantarctic Mode Water (SAMW) from the southeast Pacific through Drake Passage joins the southward Brazil Current (McCartney 1977).…”