Along-isopycnal Salinity Variability: Deducing Horizontal Stirring in the Global Ocean from Argo
Overview: Diffusion along density surfaces is a significant factor in determining the distribution of tracers in the ocean. Diffusion is determined by the eddy field with eddies of all sizes horizontally stirring tracers in the ocean. One signature of horizontal stirring is fluctuations of temperature and salinity along density surfaces, a frequently observed but under-analyzed tracer. Recent fine-scale observations show that these along-density surfaces fluctuations are organized into regions of elevated fluctuations that are temporally persistent over several years; this suggests that the underlying processes of eddy stirring are geographically varied and steady in time with much to be gained by investigating temperature and salinity fluctuations on a global scale. Intellectual Merits: This project will use Argo float data to investigate salinity variability along density surfaces known as isopycnals. Mixing length and horizontal diffusivity estimates, as well as an assessment of the processes that influence horizontal diffusion, will be made from the surface to 1000-2000 m depth in the near-global ocean. The variance of along-isopycnal salinity fluctuations will be compared to horizontal and vertical gradients of mean salinity. A correlation with horizontal gradients in particular indicates that the gradient available to stir is of primary importance and a mixing length framework is a valid approach for estimating horizontal diffusivity. In the near global ocean, horizontal diffusivity will be estimated using mixing length and velocity fluctuations derived from satellite altimetry, as well as additional estimates in the mixed layer, western boundary current regions, and near 1000 m depth (the later utilizing velocity fluctuations derived from Argo float drifts). Geographic variations in along-isopycnal salinity fluctuations, mixing length, and horizontal diffusivity will show how horizontal stirring varies with location, depth, mean velocity and tracer fields, and other relevant features of the ocean. Broader Impacts: Results from this study will directly improve our understanding of tracer distributions, ocean circulation, and earth's climate. The depth structure of mixing length and horizontal diffusivity will provide an observational estimate of how eddy stirring changes with depth that can be compared against modeling studies, and used to improve their parameterization in numerical models. An assessment of which processes predominantly control the evolution of tracer fluctuations will allow the methods of other current or future horizontal diffusivity estimates to be evaluated, especially estimates below the ocean surface. The methods utilized in this study can be generalized in the future to include additional information from other tracers. This award will also support an early career oceanographer in her first independent research project.