Diapycnal Mixing in the Southern Ocean Diagnosed Using the DIMESTracer and Realistic Velocity Fields

In this work, we use realistic isopycnal velocities with a 3‐D eddy diffusivity to advect and diffuse a tracer in the Antarctic Circumpolar Current, beginning in the Southeast Pacific and progressing through Drake Passage. We prescribe a diapycnal diffusivity which takes one value in the SE Pacific west of 67°W and another value in Drake Passage east of that longitude, and optimize the diffusivities using a cost function to give a best fit to experimental data from the DIMES (Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) tracer, released near the boundary between the Upper and Lower Circumpolar Deep Water. We find that diapycnal diffusivity is enhanced 20‐fold in Drake Passage compared with the SE Pacific, consistent with previous estimates obtained using a simpler advection‐diffusion model with constant, but different, zonal velocities east and west of 67°W. Our result shows that diapycnal mixing in the ACC plays a significant role in transferring buoyancy within the Meridional Overturning Circulation. Realistic velocity fields used to advect a model tracer in the Southern Ocean produce lateral tracer distributions similar to an observed tracerDiapycnal diffusivities diagnosed from a best fit of the model tracer distribution to observations are 20 times larger in Drake Passage than in the Southeast PacificResults from models using two different velocity field products are consistent with one another, and with the previous result of Watson et al. (2013)