Lagrangian studies, circulation and mixing in the Southern Ocean

Oceans play a vital role as one of the major components of Earth's climate system. The study of oceanic processes and the complexity inherent in dynamic ows is essential for understanding their regulatory character on the climate's variability. A key region for the study of such intrinsic oceanic variability is the Southern Ocean. In the form of a wind-driven, zonally unbounded, strong eastward ow, the Antarctic Circumpolar Current (ACC) circumnavigates the Antarctic continent connecting each of the ocean basins. The dynamics of the ACC, which is characterised by the absence of land barriers, apart from when crossing Drake Passage, have long been a topic of debate [Rintoul et al., 2001]. The main interests of this study focus on inferring and mapping the dynamic variability the ACC exhibits by means of transient disturbances [Hughes, 2005] (such as mesoscale eddies) and subsequent mixing from Lagrangian trajectories. The distribution of eddy transport and intensity, the mixing of conservative quantities and ow dynamics through to the interaction of eddy kinetic energy, mean ow and topography are examined. The sparseness of observations in the Southern Ocean and the necessity to understand the role of the oceanic circulation in the climate by a holistic approach highlights computational ocean circulation models as indispensable. In the context of this study, output from the run401 of the Ocean Circulation and Climate Advance Model (OCCAM) 1/12� ocean model, developed at the U.K. National Oceanography Centre, is utilised. In order to deduce the temporal and spatial variability of the ow dynamics, as well as its vertical distribution, simulation of monthly releases of passive particles using di�erent schemes (i.e. cluster or linear alignment) on isobaric and isoneutral surfaces was conducted. An analysis of the Lagrangian trajectories reveals the characteristics of the dynamics that control the ow and depict regions of enhanced eddy activity and mixing. The model's ability to simulate real oceanic ows is established through comparison with a purposeful release of the tracer CF3SF5, which is conducted as part of the DIMES experiment (http://dimes.ucsd.edu/). We �nd that topography plays a fundamental role in the context of Southern Ocean mixing through the association of high EKE regions, where the interaction of vortical elements and multi �lamented jets in non-parallel ows supports an e�ective mechanism for eddy stirring, resulting in the enhanced dispersion of particles. Suppression of mixingin regions where the ow is delineated by intensi�ed and coherent, both in space and time, jets (strong PV gradients) signifying the separation of the ow in di�erentiated kinematic environments, is illustrated. The importance of a local approximation to mixing instead of the construction of zonal averages is presented. We present the caveats of classical di�usion theory in the presence of persistent structures and �nd that values of 1000-2000 m2