At the latitudes of the Antarctic Circumpolar Current (ACC), waters from the Atlantic, Indian and 28 Pacific Oceans are brought to the surface by the Roaring Forties to be transformed into Subantarctic 29 Mode Waters to the north and Antarctic Bottom Waters to the south (Marshall and Speer 2012). 30 This global transformation of water masses is achieved by intense air-sea exchange of heat, fresh 31 water, carbon, and other chemical tracers in the Southern Ocean and exerts a strong control on 32 Earth’s climate. Above the sill depth of the Drake Passage, the circulation is dominated zonally by 33 the ACC and meridionally by the sum of a wind-driven meridonal overturning circulation (MOC) 34 plus a MOC driven by the turbulent eddies generated through instabilities of the ACC (Johnson and 35 Bryden 1989; Speer et al. 2000; Marshall and Radko 2003). The air-sea fluxes and Earth’s climate 36 are therefore very sensitive to oceanic turbulence in the Southern Ocean. The current debate as to 37 whether Southern Ocean carbon uptake will increase or decrease in a warming climate stems from 38 different assumptions about the changes in oceanic turbulence (Russell et al. 2006; Abernathey 39 et al. 2011). 40

Despite its importance for climate studies, there have not been direct observational estimates 41 of the rate of mixing which drives the eddy-induced circulation across the ACC. Indirect esti-42 mates have been made, for example, by Stammer (1998) who used scaling laws and the surface 43 geostrophic velocity from altimetry, and by Marshall et al.(2006) who drove numerical tracers by 44 the altimetric velocity field. Phillips and Rintoul (2000) attempted to estimate the fluxes of heat 45 and momentum from mooring data, but not the rate at which tracers are mixed. Here we present 46 the first direct measurements based on the spreading of a tracer deliberately released as part of 47