Estimating cross-stream isopycnal eddy diffusivity from mooring observations

Abstract. We present a method to derive distributions of cross-stream isopycnal eddy diffusivities in the Antarctic Circumpolar Current (ACC) from data measured by a mooring. This method transforms the time series measured by the mooring to spatial (cross-stream) distributions using dynamic height as the cross-stream coordinate. For this transformation, the relation between dynamic height and cross-stream location must be inferred from a spatial data set, such as hydrographic section measurements or reanalysis data along a transect through the mooring. From the distribution of temperature as a function of neutral density and cross-stream distance, the isopycnal temperature properties and fluctuations can be determined, and a mixing length and eddy diffusivity can be inferred. We apply this method to a mooring in Drake Passage and compare the resulting distributions with those derived from hydrographic section data. The mooring and section data yield very similar distributions of the mean isopycnal temperature field as well as the root mean square (rms) temperature fluctuations along isopycnals, which provide a metric of isopycnal stirring. The mooring-derived eddy diffusivity distributions capture some key features predicted by kinematic theories: reduced diffusivities at the Sub-Antarctic Front jet and increased diffusivities at mid-depths, both in line with mean flow suppression theory. The results presented here show that the methodology can be a valuable tool to study cross-stream isopycnal mixing properties from mooring data, especially in equivalent-barotropic systems and for moorings with a high vertical resolution.