On the propagation and decay of North Brazil Current rings

Abstract

Near the western boundary of the tropical North Atlantic, where the North Brazil Current (NBC) retroflects into the North Equatorial Countercurrent, large anticyclonic rings are shed. After separating from the retroflection region, the so‐called NBC rings travel northwestward along the Brazilian coast, until they reach the island chain of the Lesser Antilles and disintegrate. These rings contribute substantially to the upper limb return flow of the Atlantic Meridional Overturning Circulation by carrying South Atlantic Water into the northern subtropical gyre. Their relevance for the northward transport of South Atlantic Water depends on the frequency of their generation as well as on their horizontal and vertical structure. The ring shedding and propagation and the complex interaction of the rings with the Lesser Antilles are investigated in the Family of Linked Atlantic Model Experiments (FLAME) model. The ring properties simulated in FLAME reach the upper limit of the observed rings in diameter and agree with recent observations on seasonal variability, which indicates a maximum shedding during the first half of the year. When the rings reach the shallow topography of the Lesser Antilles, they are trapped by the island triangle of St. Lucia, Barbados and Tobago and interact with the island chain. The model provides a resolution that is capable of resolving the complex topographic conditions at the islands and illuminates various possible fates for the water contained in the rings. It also reproduces laboratory experiments that indicate that both cyclones and anticyclones are formed after a ring passes through a topographic gap. Trajectories of artificial floats, which were inserted into the modeled velocity field, are used to investigate the pathways of the ring cores and their fate after they encounter the Lesser Antilles. The majority of the floats entered the Caribbean, while the northward Atlantic pathway was found to be of minor importance. No prominent pathway was found east of Barbados, where a ring could avoid the interaction with the islands and migrate toward the northern Lesser Antilles undisturbed.

The impact of upwelling on the intensification of anticyclonic ocean eddies in the Caribbean Sea

“…The modeled surface current in Ekman100 enters the Caribbean Sea through the southern passages of the Lesser Antilles ( Fig. 3a), which is similar to what is seen in observations (Johns et al, 2002). The flow through the Lesser Antilles is highly variable and depends on variations of the flow upstream and the wind forcing (Johns et al, 2002).…”

Vertical structure of eddies and Rossby waves, and their effect on the Atlantic meridional overturning circulation at 26.5°N

“…The speed increase observed around 40–50°W (Figure ) may be attributable to the deepening of the thermocline [ Chelton et al ., ] or to topographic interactions with the MAR [ Tailleux and McWilliams , ]. There is also the potential for eddies shed from the North Brazil current to propagate northward to the RAPID latitude [ Jochumsen et al ., ]. These additional sources of variability as well as atmospheric forcing over the basin interior may degrade the coherence between anomalies generated at the eastern boundary from propagating all the way to the west.…”

On the suitability of North Brazil Current transport estimates for monitoring basin‐scale AMOC changes

“…After crossing the equator, the NBC transport diminishes, because it seasonally retroflects into the zonal equatorial current system [ Johns et al , ]. At the retroflection at 5°–8°N, anticyclonic eddies are shed, whose northwestward migration contributes to a mean tropical to subtropical cross‐gyre transport and represents a major part of the upper limb of the AMOC in this regime [ Barnier et al , ; Johns et al , ; Jochumsen et al , ]. A comprehensive collection of NBC measurements in the western tropical South Atlantic is provided by Schott et al [], including results of repeated shipboard sections at 5°S and 11°S and of a moored array deployed near 11°S between 2000 and 2004.…”