Isopycnal Mixing in the North Atlantic Oxygen Minimum Zone Revealed by RAFOS Floats

• Main Author: Rudnickas, Donald W.
• Format: Text
• Language: English
• Published: DigitalCommons@URI 2018
• Subjects: Physical oceanography|Biogeochemistry; North Atlantic
• Online Access: https://digitalcommons.uri.edu/dissertations/AAI10813182

The Eastern Tropical North Atlantic Oxygen Minimum Zone (OMZ) is a biogeochemically important area in the vicinity of the Cape Verde Islands formed by a combination of biological and physical processes. We use data collected from isopycnal RAFOS floats that were precisely ballasted into two groups and deployed at five locations near the edge of the OMZ. One group was ballasted to drift on the isopycnal where oxygen is at its minimum, and the other group about 300~m deeper. Nearly every six hours for 600 days the floats recorded their positions, temperature, pressure, and (at the isopycnal aligned with the oxygen minimum) dissolved oxygen concentration. Using the record of the float positions at each time interval, we calculate the relative dispersion of pairs of floats. The time derivative of this dispersion provides a diffusivity coefficient that serves to capture the net effect of eddy driven mixing along each isopycnal. With its sluggish mean circulation, the OMZ provided a study area in which this isopycnal mixing is observed with little interference by background advection. The use of Lagrangian subsurface platforms allowed us to investigate the scale dependent nature of two dimensional turbulence. We show that the relative dispersion of the floats in the OMZ area obeyed the canonical 4/3s power scaling that suggests it is representative of two dimensional turbulence. By determining the de-correlation length scale, we determined that the maximum energy containing eddy length scale in the region is approximately 100~km in the zonal direction and 40~km in the meridional. At this length scale, the effective diffusivity is 1400 ± 500 m2 s -1 in the zonal direction and 800 ± 300 m2 s -1 in the meridional. Within our quantification of error, the diffusivities on the two isopycnals are indistinguishable from one another. We compared the estimate of the diffusivity from the paired dispersion with a tracer-based mixing length method. The magnitude of the diffusivity was similar with the two methods, but the dispersion method revealed substantial anisotropy that cannot be diagnosed from the mixing length method. We apply the isopycnal mixing coefficient in a simple model aimed at understanding the steady state oxygen budget in the oxygen minimum zone. This model suggests that the vertical structure of the oxygen minimum zone may be set by the vertical profile of biological respiration and that the lateral structure on both isopycnals is set by a balance between the lateral distribution of biological respiration and the zonal and meridional mixing supply of oxygen.