Estimating and Analyzing the Southern Ocean State

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Intellectual Merit The central purpose of this project is to attain an estimate of the state of the Southern Ocean from January 2005 through December 2009. The resulting estimate will be more consistent with observations than the previous solution, and will also include the IPY time period, making it useful to many researchers spanning multiple disciplines. The time-varying, eddy-permitting estimate will be dynamically realistic (i.e. no unphysical nudging or relaxation terms will be implemented), and thus a justifiable tool for quantifying the circulation and investigating dynamical balances. Using the Southern Ocean state estimate the volume and potential vorticity (PV) budgets will be analyzed in order to answer the following two questions. Where is diapycnal mixing significant in the Southern Ocean? What is the three-dimensional potential vorticity (PV) budget? The volume budget analysis involves diagnosing the continuity equation in isopycnal space. Terms in this budget will reveal transport pathways (via the along-isopycnal thickness advection terms), the buoyancy budget (via the air-sea flux source term), abyssal mixing (via the diapycnal transport term), and the PV budget (via the thickness tendency term). All four phenomena are worthy of investigation, though in keeping with the science questions posed, the analysis will primarily focus on the latter two terms. Maps of the diapycnal mixing rate will highlight regions of increased ocean turbulence and water mass evolution. Local process studies will provide information regarding uncertainty in the proposed work, while our results will put the localprocess studies in a larger context. Mapping the PV distribution in isopycnal space will shed light on water mass transport pathways, and analyzing the PV budget will illuminate the dynamics of these pathways. This project intends to evolve the present vertically-integrated understanding of the Southern Ocean PV budget into three-dimensions. PV exchange through vortex stretching is believed to be the mechanism allowing the Antarctic Circumpolar Current system to pass over ocean-ridges and maintain its massive zonal transport. A key goal of the project is to determine the partitioning of this stretching amongst density levels.Broader Impact Collaboration with International Polar Year (IPY) programs will enhance this project. Likewise, making a Southern Ocean state estimate available to the scientific community will add value to many other funded projects. For example, use of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean project tracer release observations will benefit this project by better constraining the estimate, and also by allowing the determination of a reference tracer diffusivity. In return, the state estimate will provide researchers in the DIMES program with a constrained background state for their analysis. Determination of an accurate background mean state is vital to the DIMES project goal of determining local diapycnal and isopycnal mixing. Synthesizing sparse and distinct observations into an eddy permitting state estimate is a great resource for growing understanding of the Southern Ocean.