Underwater glider observations of the Brazil Current offshore Cabo Frio, Southeastern Brazil
Underwater glider observations of the Brazil Current offshore Cabo Frio, Southeastern Brazil
Abstract:
The Brazil Current system off Cabo Frio (latitude 23°S) was continuously monitored by underwater gliders between March 2013 and July 2015. The gliders navigation followed 100-km long sections normal to the shelf break. The mean time to complete a section length was 5 days, which was considered synoptic for the purpose of the study, and allowed the computation on a weekly basis. We analyzed the seasonal pattern of the geostrophic current, mass transport and water masses occurrence in the region. At least one transect per month was considered valid as a normal section of the Brazil Current system down to 1000 m depth. Temperature and salinity profiles from the CT-sail installed on the gliders were used to compute relative geostrophic velocity profiles. Instead of assuming an arbitrary level of no motion, absolute geostrophic velocities were estimated referencing the relative profiles to the surface geostrophic velocities derived from satellite altimetry data. An intensification of the southward flow during spring and summer was observed with maximum cross-section velocities frequently higher than 0.3 m/s. Autumn and winter maximum cross-section velocities reached 0.2 m/s on average. The characteristic shear depth between the southward flow of the Brazil Current and the northward flow of the Intermediate Current varied around 300 to 500 meters with seasonal behavior (deeper in autumn/winter, shallower during spring/summer). A cyclic behaviour of the South Atlantic Central Water was also observed, with both top and bottom rising during spring/summer and submerging in autumn/winter, keeping an average thickness of 400 m. The Cabo Frio eddy was present in different moments throughout the observed period with impact over the transport in the region. For the first time a synoptic data series longer than 2 years was produced using autonomous vehicles to study the Brazil Current. Early data obtained by traditional ship-based observations are sparse in space and time. The present dataset shows details and possible patterns that require further investigation.
