Vertical Mixing in the Atlantic Equatorial Undercurrent

Abstract

With the objective to study vertical mixing in the Atlantic Equatorial Undercurrent (EUC), a total of 14 microstructure vertical profiles, down approximately to 400 m, were done with a free-fall microstructure profiler (turboMAP-L). The data set was collected from 11 to 18 April 2010, during the first part of MOC2-Ecuatorial cruise (project MOC2), carried out with R/V Hespérides. A total of 47 conductivity- temperature-depth (CTD) casts, down to 1500 m or the sea floor, and velocity data acquired with the 75-kHz vessel-mounted Acoustic Doppler Current Profiler (ADCP) using bins of 8 m until about 500 m depth, were done in the equatorial region (Claret et al, 2012). In most cases, the EUC maximum velocity was near 70 m. We found high values of mean turbulent kinetic energy dissipation rates, on 5x10−7 m2 s−3 between 20 and 60 m, low values on 4x10−9 m2 s−3 between 60 and 120 m through the EUC core, and moderate values on 6x10−8 m2 s−3 just below the core. We also found differences between western and eastern stations along EUC. A western station on 38.5ºW shows a maximum velocity on 0.64 m s−1 with an irregular vertical salinity profile below the core with various steps, while an eastern station on 33ºW shows a maximum velocity on 0.96 m s−1 with a vertical salinity profile less irregular. The estimations of density ratio, Rρ, and gradient Richardson number, Ri, suggest that the sources of the vertical mixing are the vertical shear above and just below the EUC and salt fingers below it. Parameterizing the vertical diffusivity as a function of Ri and Rρ jointly with observations of ꞓ, we can estimate the vertical fluxes of different properties through the EUC in order to know the role of the different vertical mixing processes (Wang and Müller, 2002; Zaron and Moum, 2009; Claret et al, 2012).