Circulation of the thermocline salinity maximum waters off the Northern Brazil as inferred from in situ measurements and numerical results

Abstract. High resolution hydrographic observations of temperature and salinity are used to analyse the subsurface circulation along the coast of North Brazil, off the Amazon mouth, between 2° S and 6° N. Observations are presented from four cruises carried out in different periods of the year (March–May 1995, May–June 1999, July–August 2001 and October–November 1997). Numerical model outputs complement the results of the shipboard measurements, and are used to complete the descriptions of mesoscale circulation. The Salinity Maximum Waters are here analyzed, principally in order to describe the penetration of waters originating in the Southern Hemisphere toward the Northern Hemisphere through the North Brazil Current (NBC)/North Brazil Undercurrent (NBUC). Our results show that, if the Equatorial Undercurrent (EUC) is fed by Northern Atlantic Waters, this contribution may only occur in the ocean interior, east of the western boundary around 100 m depth. Modeling results indicate a southward penetration of the Western Boundary Undercurrent (WBUC) below the thermocline, along the North Brazilian coast into the EUC or the North Equatorial Undercurrent (NEUC) (around 48° W–3° N). The WBUC in the region does not flow more south than 3° N. The northern waters are diverted eastward either by the NBC retroflection or by the northern edge of the associated clockwise rings. The existence of subsurface mesoscale rings associated to the NBC retroflection is evidenced, without any signature in the surface layer, so confirming earlier numerical model outputs. These subsurface anticyclones, linked to the NBC/NBUC retroflection into the North Equatorial Undercurrent and the EUC, contribute to the transport of South Atlantic high salinity water into the Northern Hemisphere.

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Symmetric Instability in Cross-Equatorial Western Boundary Currents

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0273.1 ◽

2021 ◽

Vol 51 (6) ◽

pp. 2049-2067

Author(s):

Fraser W. Goldsworth ◽

David P. Marshall ◽

Helen L. Johnson

Keyword(s):

Water Mass ◽

Numerical Models ◽

Western Boundary ◽

Two Dimensional ◽

Boundary Current ◽

The North ◽

Brazil Current ◽

North Brazil ◽

North Brazil Current ◽

Symmetric Instability

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

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.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.

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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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Impact of mesoscale eddies on salinity and CO2 ocean parameters in the western tropical Atlantic in February 2020

10.5194/egusphere-egu21-1253 ◽

2021 ◽

Author(s):

Léa Olivier ◽

Jacqueline Boutin ◽

Nathalie Lefèvre ◽

Gilles Reverdin ◽

Peter Landschützer ◽

...

Keyword(s):

Dissolved Inorganic Carbon ◽

Salinity Gradient ◽

Sea Surface Salinity ◽

Amazon River ◽

Tropical Atlantic ◽

Surface Salinity ◽

The North ◽

Brazil Current ◽

North Brazil

Large oceanic eddies are formed by the retroflection of the North Brazil Current (NBC) near 8&#176;N in the western tropical Atlantic. The EUREC4A-OA/Atomic cruise took place in January - February 2020, and extensively documented two NBC rings. The NBC flows northward across the Equator and pass the mouth of the Amazon River, entraining fresh and nutrient-rich water along its nearshore edge. From December to March, the Amazon river discharge is low but a freshwater filament stirred by a NBC ring was nevertheless observed. The strong salinity gradient can be used to delineate the NBC ring during its initial phase and its westward propagation. Using satellite sea surface salinity and ocean color associated to in-situ measurements of salinity, temperature, dissolved inorganic carbon, alkalinity and fugacity of CO2 we characterize the salinity and biogeochemical signature of NBC rings.

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Preliminary results of drift-bottle releases and recoveries in the Western Tropical Atlantic

Boletim do Instituto Oceanográfico ◽

10.1590/s0373-55241967000100002 ◽

1967 ◽

Vol 16 (1) ◽

pp. 13-22 ◽

Cited By ~ 5

Author(s):

Ellen F. Luedemann

Keyword(s):

Surface Current ◽

Northeastern Brazil ◽

Brazilian Coast ◽

Northern Coast ◽

The North ◽

Strong Current ◽

North Brazil ◽

Equatorial Current ◽

The Caribbean ◽

Area North

In February-March 1963, during the Equalant Operation for the ICITA program, 476 drift-bottles were released at the equatorial waters off the northern Brazilian coast (01ºS to 09ºN and 043ºW to 053ºW). Thirty-five, bottles were recovered (7,4%). Those were classed in five groups according to their different velocity ranges and areas of recovery (Trinidad Island; Lesser Antilles; Caribbean and Florida area; north Brazil; northeastern Brazil). The recoveries of bottles released within 300 nm off the northern and northeastern coast of Brazil confirmed a strong current along the north and northeast coast in northwestern direction (South Equatorial Current and branches). In late February-early March the bottles showed maximum velocities of this current (3,6 knots). The region off the northern coast of Brazil, between 05ºN and 09ºN up to 050ºW seems to have been under the influence of an eastward component of the surface current in this same period, while at late March this influence appears to have been weaker. In the region from Trinidad Island to Yucatan Peninsula the current seems to be stronger on the northeastern side of the Caribbean Sea.

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