Observed Mixing Processes in Prydz Bay, Antarctica, during Austral Summer

Abstract Prydz Bay is a major Antarctic Bottom Water production region adjacent to a major cold cavity ice shelf. Its underlying mixing processes are little known, although they determine the intensity of water mass transformation. Using microstructure measurements, we reveal detailed regional variations of dissipation and mixing in Prydz Bay during summer. In the upper layer, turbulence dominates the continental shelf, presenting weak dissipation rate of thermal variance χ θ [ O (10 −10 )°C 2 s −1 ], but showing the elevated dissipation rate of turbulent kinetic energy ε [ O (10 −8 ) W kg −1 ] and thermal eddy diffusivity K θ [ O (10 −4 ) m 2 s −1 ]. On the continental slope, diffusive convection prevails, with one-order greater χ θ and one-order smaller ε and K θ . On the shelf break, turbulence and diffusive convection coexist, with elevated ε , χ θ , and K θ reaching the orders of 10 −8 W kg −1 , 10 −8 °C 2 s −1 , and 10 −4 m 2 s −1 , respectively. Tidal current’s encountering with the ice shelf, background current’s impinging on the rough topography, and the intrusion of modified Circumpolar Deep Water all contribute to these spatial variations. The dissipation ratios Γ for both turbulence (0.05) and diffusive convection (0.14) are statistically smaller than those in waters of mid- and low latitudes because water in Prydz Bay is primarily stratified by salinity; hence, the strong temperature gradient is associated with a weak stratification. Based on these features, we propose a new indicator to differentiate turbulence and double diffusion. This study is helpful for better understanding Antarctic mixing processes, and their contributions to local water mass transformation and to global ocean circulation and climate. Significance Statement Prydz Bay is important in producing Antarctic Bottom Water and hence shaping global ocean circulation and climate. This bottom water production and spreading are catalyzed by microscale mixing, yet the local mixing processes remain unclear. Based on field measurements, we reveal the regional differences in dissipation/mixing intensities and its drivers in upper Prydz Bay during the austral summer. Turbulence and diffusive convection dominate the water in and away from the continental shelf, respectively, leading to spatially contrasted mixing intensities. Notably, an important parameter, dissipation ratio, in Prydz Bay is clearly smaller than that of mid- and low latitudes. This is directly linked to the vertical transition of water masses. This study improves our understanding of mixing in Prydz Bay and its influences on other multiscale dynamics.