Irreversible mixing induced by geostrophic turbulence over the global ocean

Abstract

Two recently proposed mixing diagnostics are employed to estimate the global surface irreversible mixing based on particle and tracer simulation driven by satellite-derived geostrophic velocities. These two novel diagnostics, similar to the traditional dispersion diffusivity and Nakamura’s effective diffusivity but defined in a localized and instantaneous sense, have the following advantages: 1) reconcile the theoretical discrepancies between Eulerian-, particle-, and contour-based diffusivities; 2) do not rely on the stationary and homogeneous assumptions of the turbulent ocean and are free from traditional average operators (e.g., Eulerian time-/space or along-contour mean). Our results show that evident discrepancies among these three types of diffusivities do emerge when employing traditional estimates. However, these discrepancies could be significantly mitigated with the adoption of new diagnostic methods, implying that the three types of diffusivities can be effectively reconciled within a global framework. Moreover, finescale mixing structures and transient elevated mixing events due to geostrophic stirring can be clearly identified by the two new diagnostics, in contrast to previous estimates that are spatially and/or temporally smoothed. In particular, it is interesting to note that large values of the new diagnostics usually occur along narrow filaments/fronts associated with mesoscale eddies, and elevated mixing is observed to be located at the periphery of eddies. Our study presents a novel revisit of the global surface mixing induced by geostrophic eddies with an emphasis on irreversibility, and provides new insights into previous questions regarding to different mixing diagnostics in the community.