The upper limb of the Atlantic meridional overturning circulation draws waters with negative potential vorticity from the Southern Hemisphere into the Northern Hemisphere. The North Brazil Current is one of the cross-equatorial pathways in which this occurs: upon crossing the equator, fluid parcels must modify their potential vorticity to render them stable to symmetric instability and to merge smoothly with the ocean interior. In this work a linear stability analysis is performed on an idealized western boundary current, dynamically similar to the North Brazil Current, to identify features that are indicative of symmetric instability. Simple two-dimensional numerical models are used to verify the results of the stability analysis. The two-dimensional models and linear stability theory show that symmetric instability in meridional flows does not change when the nontraditional component of the Coriolis force is included, unlike in zonal flows. Idealized three-dimensional numerical models show anticyclonic barotropic eddies being spun off as the western boundary current crosses the equator. These eddies become symmetrically unstable a few degrees north of the equator, and their PV is set to zero through the action of the instability. The instability is found to have a clear fingerprint in the spatial Fourier transform of the vertical kinetic energy. An analysis of the water mass formation rates suggest that symmetric instability has a minimal effect on water mass transformation in the model calculations; however, this may be the result of unresolved dynamics, such as secondary Kelvin–Helmholtz instabilities, which are important in diabatic transformation.
The Atlantic meridional overturning circulation includes an ocean current that transports heat, carbon, and other climatically important tracers from the Southern Hemisphere into the Northern Hemisphere. Theoretical considerations suggest that this current may become unstable through the so-called “symmetric instability” upon crossing the equator. In this study, a hierarchy of models is used to investigate how symmetric instability might manifest itself if excited in cross-equatorial flows. We find that when the instability is excited, it generates stacked overturning cells which reorganize the current to make it neutrally stable to symmetric instability. We hypothesize this process could be occurring in the ocean off the coast of Brazil.
Supplemental information related to this paper is available at the Journals Online website: https://doi.org/10.1175/JPO-D-20-0273.s1.
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