Understanding the role of ocean eddies in setting the climate of the Earth

December 20, 2016

Ocean mesoscale eddies are the weather of the ocean. These swirling motions are ubiquitous in the world's ocean and act as vehicles for the transport and mixing of heat, carbon and momentum. While unquestionably important, a rigorous quantification of the role of mesoscale eddies in setting the climate of the Earth remains elusive. This work makes significant progress toward accurately measuring the force that mesoscale eddies exert on mean, climatological ocean flow. When viewed over long periods of time, the ocean fluid is neither accelerating nor decelerating, so the "a" in F=ma is zero and Newton's Second Law reduces a statement that the sum of all forces acting on the fluid parcel must be zero, i.e. sum(F)=0. This work closes the force balance in an eddy-rich re-entrant channel similar to that of the Southern Ocean. While it may seem like a trivial exercise to close the force balance of a fluid system, separating out the role of eddy motions in an unambiguous manner was only recently accomplished through the derivation of the Thickness-Weighted Averaged (TWA) equations by Prof. William Young at Scripps. We diagnose the force-balance in our eddy-rich simulation through analysis of the TWA equations. Across the depth of the ocean in a region analogous to the Antarctic Circumpolar Current (ACC), we find that mesoscale eddies play a leading role in the force balance. In fact, the very structure of the ACC and the entire Southern Ocean depends on the existence of mesoscale eddies to move the atmosphere-driven wind stress from the ocean surface to the ocean floor. Also, it has long been recognized that the ocean has a surface layer where diabatic processes dominate and an underlying, interior region that is adiabatic. This work has clarified how and where these two regions are connected. The vertical motion induced by mesoscale eddies brings, in an alternating manner, warm (light) and cold (dense) waters to the ocean's surface. At a given location, the range of ocean water densities that reach the ocean surface produce what we termed the "ventilation-defined surface layer." This layer separates the class of waters that are influenced by diabatic processes from the underlying adiabatic zone. We find that the force and buoyancy budget within the ventilation-defined surface layer is self-contained and dynamically isolated from the adiabatic interior. In summary, this work has taken a significant step toward the construction of robust conceptual model to interpret and quantify the role of ocean mesoscale eddies in the Earth's climate system.

 

This work is published as an Open Access article the Journal of Physical Oceanography. The article is freely available here: DOI: 10.1175/JPO-D-16-0096.1

 

Ringler, Todd and Juan A. Saenz and Phillip J. Wolfram and Luke Van Roekel (2016): A thickness-weighted average perspective of force balance in an idealized circumpolar current, Journal of Physical Oceanography, DOI: 10.1175/JPO-D-16-0096.1

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