The role of mixing in the large-scale ocean circulation

Abstract Irreversible mixing of tracers and momentum in the ocean occurs via diffusion and friction at the scale of molecules. That such molecular processes profoundly influence basin-scale ocean currents is counter-intuitive. Many successful theories of ocean circulation indeed ignore diffusive and frictional processes. Yet oceanographers have long recognised that turbulence can amplify irreversible mixing and its influence on large-scale flows. In recent years, substantial progress has been made in the mapping of mixing energised by three-dimensional and geostrophic turbulence. This progress not only helps to quantify connections between mixing and observed circulation systems, but also to better characterise these circulation systems. This chapter surveys the rapidly evolving understanding of the impacts of mixing on the strength and structure of major ocean gyres, overturning circulations and the Antarctic circumpolar current. Accumulating evidence suggests that global ocean circulation is shaped by energetic mixing near ocean boundaries, while either weak or largely adiabatic away from boundaries.