Realistic depiction of Southern Ocean eddy dynamics is critical in models used for climate prediction. Mixing generated by mesoscale eddies is believed to play an important role in the transfer of water masses and tracers across the Antarctic Circumpolar Current (ACC). In turn, these water mass transfers are thought to control the global overturning circulation and inter-ocean exchange. The central goals of this project are to quantify Lagrangian isopycnal eddy diffusivities in the Southern Ocean and to characterize their horizontal and vertical distributions, and to assess their ability to parameterize isopycnal eddy tracer fluxes across the ACC. These questions will be addressed using the 0.1 degree, 42-level global configuration of the Los Alamos National Laboratory (LANL) Parallel Ocean Program (POP).
Intellectual Merit: Large uncertainty exists in the representation of Southern Ocean eddy mixing processes in standard climate scale ocean models. The effects of eddies are commonly parameterized using eddy diffusion, and climate scale models are very sensitive to the magnitude of the eddy diffusion coefficients in the Southern Ocean. Whether the sensitivity of the ocean circulation to surface forcing is altered when using constant eddy diffusion coefficients rather than variable eddy diffusion coefficients remains unclear, and, more fundamentally, it is unclear if local, or even zonally integrated tracer transports are simulated adequately by the eddy diffusion model. A related fundamental issue is to determine the horizontal and vertical distributions of the eddy diffusion coefficients. Different methods have resulted in conflicting magnitudes and spatial distributions in the Southern Ocean. Lagrangian floats provide a way to test the applicability of the diffusion model. Of order 10,000 numerical floats will be deployed in the ocean model, at various locations within and outside the ACC and at a range of depth levels to estimate Lagrangian diffusivities and their spatial distributions. The skill of these Lagrangian diffusivities in parameterizing zonally integrated as well as local eddy tracer transport in the ACC will be assessed. This project will contribute to ongoing efforts to reconcile conflicting diffusivity estimates for the Southern Ocean and will ultimately allow us to assess how much the spatial structure of the diffusivities matters in simulating Southern Ocean eddy fluxes accurately.
Broader Impacts: The project will contribute to the early career development of a postdoctoral researcher. The research will explore different ways to parameterize mixing coefficients, which will be testable in predictive climate models. A detailed assessment of Lagrangian methods and their relation to eddy fluxes and diffusivity estimates from other methods will benefit efforts to analyze data from floats deployed all over the ocean. In particular, the methods investigated in this study will contribute to the interpretation of in situ data returned by the 180acoustic floats that are being deployed as part of the NSF-funded Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) experiment.
This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) program.
