Symmetric Instability in Cross-Equatorial Western Boundary Currents

AbstractThe 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.

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Symmetric (inertial) instability in cross-equatorial western boundary currents

10.5194/egusphere-egu21-1159 ◽

2021 ◽

Author(s):

Fraser Goldsworth ◽

David Marshall ◽

Helen Johnson

Keyword(s):

Potential Vorticity ◽

Three Dimensional ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Three Dimensional Model ◽

The North ◽

Brazil Current ◽

North Brazil ◽

North Brazil Current ◽

Inertial Instability

<p>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. It is known that upon crossing the equator fluid parcels within this current must modify their potential vorticity, to render them stable to symmetric (inertial) instability and to merge smoothly with the ocean interior.</p><p>A hierarchy of models predict the excitement of inertial instability in cross-equatorial flows dynamically similar to the North Brazil Current. A linear stability analysis of a barotropic flow is able to predict the structure and growth rate of the instability. A two-dimensional numerical model verifies these predictions and shows how the instability is able to stabilise unstable potential vorticity configurations. A simplified three-dimensional model demonstrates how large anti-cyclonic rings spun up at the equator entrain waters with negative PV, before the rings themselves become inertially unstable. The high-resolution, observationally constrained, MITgcm LLC4320 model is probed for signs of this instability process.</p>

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Why Does the North Brazil Current Regularly Shed Rings but the Brazil Current Does Not?

Journal of Physical Oceanography ◽

10.1175/2009jpo4246.1 ◽

2010 ◽

Vol 40 (2) ◽

pp. 354-367 ◽

Cited By ~ 5

Author(s):

Volodymyr Zharkov ◽

Doron Nof

Keyword(s):

Momentum Flux ◽

Atlantic Coast ◽

Propagation Rate ◽

Western Boundary ◽

Western Atlantic ◽

The North ◽

Brazil Current ◽

North Brazil ◽

North Brazil Current ◽

The Difference

Abstract Both the North Brazil Current (NBC) and the Brazil Current (BC) are western boundary currents (WBCs) that separate from the western Atlantic coast. The NBC retroflects and sheds several rings per year (at the retroflection region), whereas the BC rarely sheds rings near its separation point. Traditionally, the difference between these two WBCs has been attributed to the Malvinas Current (MC), whose momentum flux opposes the poleward momentum flux of the BC, thus preventing rings shedding at the point where the current leaves the coast. Even in the absence of the MC, rings from the separating BC would have never been regularly generated because of the relatively large slant of the coastline relative to the zonal direction. Using the recently proposed theory of Zharkov and Nof, it is demonstrated that the large inclination of the coastline between 20° and 45°S (approximately 50°) lies within the regime that does not allow the BC a continuous shedding of rings. In contrast, the inclination of the coastline between 5° and 8°N is sufficiently small to allow the NBC a continuous and smooth shedding of rings. The importance of the coastline inclination comes about through a ring β-induced westward propagation rate. In the small inclination case, the alongshore migration is fast, allowing the newly formed rings to quickly escape from their generation zone (i.e., before they are recaptured by the newly born rings generated behind). In contrast, in the high inclination case, the alongshore speed is so small that the rings spend a long time in the generation area and, consequently, are usually recaptured by the new rings generated just behind them. The authors argue, paradoxically, that the rings occasionally shed by the BC are probably due to the MC that advects the rings away from the generation area, preventing their recapture by the current behind them. Although no new analytical solutions are presented, the authors elaborate on the application of the recapturing condition to the NBC and BC and show new numerical simulations for both the NBC and the BC.

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The Barreirinhas Eddies: Stable Energetic Anticyclones in the Near-Equatorial South Atlantic

Frontiers in Marine Science ◽

10.3389/fmars.2021.617011 ◽

2021 ◽

Vol 8 ◽

Author(s):

Iury T. Simoes-Sousa ◽

Ilson Carlos A. Silveira ◽

Amit Tandon ◽

Glenn R. Flierl ◽

Cesar H. A. Ribeiro ◽

...

Keyword(s):

Numerical Models ◽

Joint Effect ◽

Rossby Number ◽

The North ◽

Brazil Current ◽

Starting Point ◽

North Brazil ◽

North Brazil Current ◽

Different Depths ◽

Strong Winds

We explore the Barreirinhas Eddies, submesoscale vortices generated by the North Brazil Current (NBC) off the Barreirinhas Bight (Brazil, centered at 1.75°S), using vessel-mounted and moored ADCP data, and a Global HYCOM reanalysis. These double-stacked anticyclones with incredibly high Rossby Number O (10)] occur independently at different depths (high Burger number). Anticyclones with Rossby number greater than unity are unstable according to inviscid linear theory, and hence these submesoscale features are not easily observable at mid latitudes. At these low latitudes, they last about a week, allowing characterization by oceanographic surveys. Our analyses suggest this increased stability is due to the joint effect of strong winds, stratification, proximity to the equator, and topography. Heretofore hypothesized via analytical studies and seen in numerical models, our study confirms this stabilization process in observations, and is also a starting point for the description of the submesoscale dynamics in the NBC domain.

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