Mapping unstable manifolds using drifters/floats in a Southern Ocean field campaign

Ideas from dynamical systems theory have been used in an observational field campaign in the Southern Ocean to provide information on the mixing structure of the flow. Instantaneous snapshops of data from satellite altimetry provide information concerning surface currents at a scale of 100 km or so. We show that by using time-series of satellite altimetry we are able to deduce reliable information about the structure of the surface flow at scales as small as 10 km or so. This information was used in near-real time to provide an estimate of the location of stable and unstable manifolds in the vicinity of the Antarctic Circumpolar Current. As part of a large U.K./U.S. observational field campaign (DIMES: Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) a number of drifters and floats were then released (at the surface and at a depth of approximately 1 km) close to the estimated hyperbolic point at the intersection of the two manifolds, in several locations with apparently different dynamical characteristics. The subsequent trajectories of the drifters/floats has allowed the unstable manifolds to be tracked, and the relative separation of pairs of floats has allowed an estimation of Lyapunov exponents. The results of these deployments have given insight into the strengths and limitations of the satellite data which does not resolve small scales in the velocity field, and have elucidated the transport and mixing structure of the Southern Ocean at the surface and at depth.