Meridional transport of physical and biogeochemical tracers by Southern
Ocean eddies
Abstract
Meridional eddy transport across the Antarctic Circumpolar Current is an
essential component of the global meridional overturning circulation and
the transport of climate relevant tracers. Challenges in comparing model
and observational estimates of the transport arise from varying
methodologies describing ‘eddy’ processes. We reconcile the approach
used in shipboard surveys of eddies, complemented by satellite eddy
tracking, with Reynolds decomposition applied to model outputs. This
allows us to estimate the fraction of total meridional tracer transport
attributed to coherent eddies in a global
0.1$^\circ$ ocean model. The model realistically
simulates observed eddy kinetic energy and three-dimensional
characteristics, particularly in representing an observed cyclonic eddy
near 150 \degrees E, a hotspot for poleward heat flux.
Annual meridional transports due to coherent eddies crossing the
Subantarctic Front are estimated by vertically and radially integrating
the tracer contents of all eddies. Notably, only cyclonic eddies moving
equatorward across the Subantarctic Front contribute to the coherent
eddy transport, with no anticyclonic eddies found to cross the front
poleward in this region. Applying Reynolds decomposition, our study
reveals predominantly poleward meridional transports due to all
transient processes in a standing meander, particularly between the
northern and southern branches of the Subantarctic Front. Coherent,
long-lived eddies tracked from satellite data contribute less than
20\% to transient poleward heat transport, and
equatorward nitrate transport in the model. Furthermore, we demonstrate
that the integrated surface elevation of mesoscale eddies serves as a
reliable proxy for inferring subsurface eddy content.