Sea surface structure of North Brazil Current rings derived from shipboard and moored acoustic Doppler current profiler observations

Abstract

[1] In the western tropical Atlantic, the North Brazil Current retroflection periodically sheds large anticyclonic rings, which then propagate northwestward. Between 1998 and 2000, the North Brazil Current Rings Experiment sampled a large number of these rings by shipboard and moored acoustic Doppler current profiler. Ten of the sampled rings are analyzed in this study, focusing on their sea surface dynamic properties. The rings exhibit a radial structure consisting of two regimes, an "inner" core region in near solid body rotation and an "outer" ring regime with an approximately exponentially decaying structure. The observations show a sharp change in vorticity at the regime transition and the presence of a strong opposite vorticity shield bounding the inner solid body core. We show that Gaussian models, commonly used to represent the surface expression of these and other rings, are adequate for determining the sea surface height anomaly but tend to poorly estimate other properties such as the maximum swirl velocity. Therefore, we propose a new two-part model as a better approximation of the rings' radial structure. According to the cyclogeostrophic balance approximation, the sea surface height distribution across the inner ring has a parabolic shape, while the outer ring has an exponential structure similar to the velocity field. Interestingly, many of the observed rings have an intensity very close to the theoretical limit for anticyclones at these latitudes, which is believed to be due to inertial instability.Citation: Castelão, G. P., and W. E. Johns (2011), Sea surface structure of North Brazil Current rings derived from shipboard and moored acoustic Doppler current profiler observations,

Cyclonic activity in the eastern Gulf of Mexico: Characterization from along-track altimetry and in situ drifter trajectories

“…Solid-body rotation has been observed for other ocean eddies, although it is not the only profile possible. This characteristic has been observed for Meddies (Paillet et al 2002), for mesoscale eddies in the North Equatorial Pacific Ocean (Flament et al, 2001) and in the Labrador Sea (Lilly and Rhines, 2002), or for North Brazil Current anticyclonic rings (Castelão and Johns, 2011).…”

The influence of salinity on tropical Atlantic instability waves

“…Variability in these regions is not associated with TIWs in the traditional sense, but it does involve the instability of the flow field such as the variability associated with the retroflected NBC and the NECC. The variability of the currents in this region is often discussed in the context of eddies and rings [e.g., Johns et al, 1990;Johns et al, 1998;Castelão and Johns, 2011]. Whether the variability in the northeast and southwest Atlantic are better described as eddies or Rossby waves [e.g., Polito and Sato, 2003] are beyond the scope of this study.…”

Nonlinear Analysis of Perturbed Rotating Whirlpools in the Ocean and Atmosphere

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“…Considering the link between whirlpools and climate change and our limited understanding of climate-related ocean processes, our ability to measure the changes that are taking place is conspicuously inadequate ( [42]). The discovery of two giant whirlpools (approximately 400 kilometers in diameter) in the Atlantic Ocean, off the coast of Guyana and Suriname, as shown in Figure 1 became a sensational discovery because this part of the ocean has been studied thoroughly, and no one expected anything like that to appear in the area (see [5] and [6]). Today this is an open question where the whirlpools came from and why do those whirlpools exist for such a long time?…”

“…Two whirlpools that are approximately 400 kilometers in diameter in the Atlantic Ocean, off the coast of Guyana and Suriname. This image is available in the online in [5].…”