Lagrangian isopycnal RAFOS floats reveal mixing rates and dynamics in the Eastern Tropical North Atlantic Oxygen Minimum Zone

We use data from 90 isopycnal Lagrangian floats to explore the dynamics responsible for supplying oxygen to the Atlantic Eastern Tropical Oxygen Minimum Zone. One group of these floats was ballasted to drift on the isopycnal where oxygen is at its minimum, and the other about 300 m deeper. Using the record of the float positions at each 6-hr interval, we calculate the relative dispersion of pairs of floats. The time derivative of this dispersion provides a diffusivity coefficient that captures the net effect of eddy driven mixing along each isopycnal. Our large number of float pairs, launched in a region of sluggish mean circulation, allows us to carefully assess the role of vertical shear, which accelerated the relative dispersion by 10-15\% relative to true isopycnal dispersion. Relative dispersion of the floats in the OMZ area obeyed the canonical $4/3^{rds}$ power scaling, representative of two dimensional turbulence. At the length scale of the maximum energy containing eddy (approximately 100 km), the effective diffusivity is 1400 $\pm$ 500 m${}^2$ s${}^{-1}$ in the zonal direction and 800 $\pm$ 300 m${}^2$ s${}^{-1}$ in the meridional. The strong anisotropy of the mixing coefficients appeared to be linked with zonally-elongated eddies. Within the uncertainty, the diffusivities on the two isopycnals are indistinguishable from one another. The diffusivities estimated from the Lagrangian floats were used in an idealized, steady state model, and suggest that the dominant balance on the oxygen budget is one between the consumption of oxygen through respiration and its supply via meridional mixing.