Abstract
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.
Highlights
The North Brazil Current (NBC) starts as a subsurface-intensified jet called the North Brazil Undercurrent (NBUC; Silveira et al, 1994; Stramma et al, 1995)
We observe that when the current is more intense in the model (May, Figure 4C), the NBC remains offshore and does not enter the Barreirinhas Bight. This seems to favor the formation of the Barreirinhas Eddies, which we identify from a positive relative vorticity patch associated with eastward velocities near the boundary of the Barreirinhas Bight
Large mesoscale anticyclones are shed from this necking-off process of the current. They propagate, bordering the shelf break toward the Caribbean; the shedding is modulated by 50-day Rossby waves and 30-day Yanai waves (Johns et al, 1998). This process occurs further downstream, and we do not see any relation between the variability we observed for the Barreirinhas Eddies and the variability of the NBC retroflection
Summary
The North Brazil Current (NBC) starts as a subsurface-intensified jet called the North Brazil Undercurrent (NBUC; Silveira et al, 1994; Stramma et al, 1995). Ranging from intermediate levels to the surface, the NBUC—fully formed north of 15◦S—transports 25 Sv (Schott et al, 1995) From this latitude toward the equator, this current receives contributions from shallower SEC branches and undergoes a series of pycnoclinic and subpycnoclinic retroflections. Constrained by the inertial instability; only a few studies describe them using observational data (e.g., D’Asaro, 1988; Chavanne et al, 2010; Bosse et al, 2016) Prior to these observations, no information regarding the submesoscale variability has been reported for the NBC domain or for any western-boundary current in the equatorial region. We gather all the available in situ data to first describe the Barreirinhas eddies, and to investigate whether and how frequently anticyclones are formed in this region; we employ a global ocean model reanalysis to place our analyses in context, spatially and temporally as well as to address its zeroth-order dynamics
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