The Barreirinhas Eddies Conundrum : Why Are These Super Anticyclones at Low Latitudes so Long-lived?
The Barreirinhas Eddies Conundrum : Why Are These Super Anticyclones at Low Latitudes so Long-lived?
Abstract:
The temporal variability of the North Brazil Current (NBC) in the southern hemisphere is not well known. In this work, we investigate the NBC vortical activity off Barreirinhas Bight (Brazil, centered at 2ºS). We analyze quasi-synoptic temperature, salinity and velocity profiles from the Brazilian Navy’s cruise Oceano Norte 1 (June 2001). We also validate the global HYCOM reanalysis Experiment 19.1 for the study region and use the outputs to expand our analyses in both space and time dimensions. The NBC/NBUC jet recirculates and sheds two different types of anticyclones within the Barreirinhas Bight: near-surface eddies comprising the Tropical Water, and pycnocline eddies containing the South Atlantic Central Water.
The near-surface eddy has vertical extent of 100 m and a cross-shore radius of about 50 km. The pycnocline eddy has vertical extent of 150 m and a cross-shore radius of about 90 km.These two eddies occur mainly from March to August, and typically present a 7-day life span. The anticyclones propagate westward colliding into the continental margin while leaving the equatorial border of the bight. With a Rossby and Richardson number of O(1), an aspect ratio of O(10-3) and a Burger number O(10), they are hydrostatic, submesoscale-like, topographically-driven features with vortical dynamics governed by relative vorticity. We also identified an upwelling pattern in the hydrographic data within the near surface anticyclone domain, which suggests eddy-wind interaction.
These super-anticyclones can reach local Ro values of 10 at its center, which should be favorable for growing inertial instabilities. The stabilizing influence of stratification and viscosity are not enough to make them stable.
This work is the first description for the Barreirinhas Eddies, presenting a conundrum on why these unstable ocean anticyclones are so long-lived.
The near-surface eddy has vertical extent of 100 m and a cross-shore radius of about 50 km. The pycnocline eddy has vertical extent of 150 m and a cross-shore radius of about 90 km.These two eddies occur mainly from March to August, and typically present a 7-day life span. The anticyclones propagate westward colliding into the continental margin while leaving the equatorial border of the bight. With a Rossby and Richardson number of O(1), an aspect ratio of O(10-3) and a Burger number O(10), they are hydrostatic, submesoscale-like, topographically-driven features with vortical dynamics governed by relative vorticity. We also identified an upwelling pattern in the hydrographic data within the near surface anticyclone domain, which suggests eddy-wind interaction.
These super-anticyclones can reach local Ro values of 10 at its center, which should be favorable for growing inertial instabilities. The stabilizing influence of stratification and viscosity are not enough to make them stable.
This work is the first description for the Barreirinhas Eddies, presenting a conundrum on why these unstable ocean anticyclones are so long-lived.