Observations of near-inertial oscillations along the Brazilian continental shelf break

Abstract

Near-inertial oscillations (NIO) are intermittent motions with a frequency close to the inertial frequency and represent an important fraction of the energy to the currents in the upper ocean. Based on hourly velocity records at the shelf break covering the top 50 m of the water column in 5 locations and one additional location at Cabo Frio upwelling system (22° S) covering the top 200 m of water column with hourly velocity and temperature measurements, the aim of the present study is to characterize NIO along the Brazilian continental shelf break (16° S–31.5° S) describing the spatial variability of inertial energy in the mixed layer and the role of the NIO in the high-frequency hydrodynamic. The sites between 31.5° S and 28.5° S were those that revealed more near-inertial energy and higher relative importance of the near-inertial band for the variance of the currents (29% and 31%, respectively). This is associated with the proximity between the inertial frequency and the sea breeze frequency in this region, which delimits an inertial resonance area for the NIO around 30° S. The near-inertial currents between 31.5° S and 23.5° S range between 30 and 50 cm s−1 and decrease toward the locations at lower latitudes, where the range is between 5 and 25 cm s−1. Vertical shear of the order of 10−3 s−1 was reached during NIO events between 31.5° S and 23.5° S in the upper layer of the ocean (mixed layer, seasonal thermocline, and upper permanent thermocline) and can play an important role in the vertical mixing along the Brazilian continental shelf break. The NIO events analyzed at Cabo Frio upwelling system exhibited a mean duration of around 7.6 days, upward vertical phase velocity of the order of 10−1 cm s−1, vertical wavelengths of the order of 102 m, and vertical downward group velocity of the order of 10−2 cm s−1. Nevertheless, the magnitude of the vertical propagation of energy and duration of the events demonstrate the importance of NIO as a source of kinetic energy to the ocean interior.