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

Neill Mackay,Andrew J S Meijers,Marie‐José Messias,Daniel C Jones,Andrew J Watson,James R Ledwell,Alberto C Naveira Garabato,J Alexander Brearley

doi:10.1002/2017jc013536

Abstract

AbstractIn 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.

Highlights

Global climate is strongly influenced by the overturning circulation of the oceans, transporting up to 5 PW of heat meridionally and mediating the exchange of gases between the ocean and atmosphere (Rahmstorf, 2002; Trenberth & Caron, 2001)
Cold dense waters created at high latitudes must return to the surface, and this is achieved in part by adiabatic upwelling along isopycnals in the Southern Ocean, and in part through diapycnal mixing
The path of the tracer during the first 2 years of the experiment goes through two contrasting regions of the Southern Ocean: the SE Pacific where smooth topography leads us to expect relatively weak mixing, and Drake Passage where the interaction of meandering jets with complex topography is expected to produce enhanced mixing rates

Summary

Introduction

Global climate is strongly influenced by the overturning circulation of the oceans, transporting up to 5 PW of heat meridionally and mediating the exchange of gases between the ocean and atmosphere (Rahmstorf, 2002; Trenberth & Caron, 2001). The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), which commenced in 2008, has aimed to characterize the spatial and temporal variability in Southern Ocean mixing and to understand its controlling physical processes. It comprises a large-scale tracer experiment combined with microstructure, finestructure, and mooring-based measurements in a region stretching from the Southeast Pacific through Drake Passage to the Scotia Sea and beyond (Brearley et al, 2013; Ledwell et al, 2011; Sheen et al, 2013; St. Laurent et al, 2012; Watson et al, 2013). The path of the tracer during the first 2 years of the experiment goes through two contrasting regions of the Southern Ocean: the SE Pacific where smooth topography leads us to expect relatively weak mixing, and Drake Passage where the interaction of meandering jets with complex topography is expected to produce enhanced mixing rates (see e.g., Naveira Garabato et al, 2004; Nikurashin & Ferrari, 2010a, 2010b; Scott et al, 2011)

Objectives

Methods

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Discussion

Conclusion