Tracking the spread of a passive tracer through Southern Ocean water masses

Abstract. A dynamically passive inert tracer was released in the interior South Pacific Ocean at latitudes of the Antarctic Circumpolar Current. Observational cross sections of the tracer were taken over four consecutive years as it drifted through Drake Passage and into the Atlantic Ocean. The tracer was released within a region of high salinityrelative to surrounding waters at the same density. In the absence of irreversible mixing a tracer remains at constant salinityand temperature on an isopycnalsurface. To investigate the process of irreversible mixing we analysed the tracer in potential densityversus salinity-anomaly coordinates. Observations of high tracer concentration tended to be collocated with isopycnal salinityanomalies. With time an initially narrow peak in tracer concentration as a function of salinityat constant density, broadened with the tracer being found at ever fresher salinities, consistent with diffusion-like behaviour in that coordinate system. The second moment of the tracer as a function of salinitysuggested an initial period of slow spreading for approximately 2 years in the Pacific, followed by more rapid spreading as the tracer entered Drake Passage and the ScotiaSea. Analysis of isopycnal salinitygradients based on the Argoprogramme suggests that part of this apparent change can be explained by changes in background salinitygradients while part of the change may be explained by geographical changes in background mixing.