Temporal Variability of Surface Eddy Diffusivities in the Global Ocean from Altimetry

Mesoscale velocity fluctuations ('eddies'), are a ubiquitous phenomenon in the global ocean with high importance to the global ocean circulation. Eddy effects are important for the meridional overturning circulation, water mass formation, and tracer transports like heat, salt and anthropogenic carbon.
Due to the small scale of eddies, observational based estimates of their effects remain challenging to obtain, especially with global coverage and sufficient resolution in space and time.Here we use an observation-driven idealized model experiment combining velocities from altimetry and several tracer fields to diagnose lateral eddy diffusivites near the sea surface.
The diffusivities exhibit strong temporal variability in most ocean basins, with interannual variability ranging from 20% to over 100% of the local average in a majority of the global ocean. Perhaps even more importantly, correlation with the ENSO index in large parts of the Pacific basins is observed, suggesting a modulation of surface eddy diffusivities by large scale climate fluctuations.
This may constitute a feedback mechanism previously unaccounted for in global climate models.
Our study demonstrates how the complex surface velocity field modulates lateral surface mixing in the world ocean. We anticipate that the monthly data set of surface diffusivities will be valuable for the testing and development of future parameterizations for eddy effects in coarse resolution models.