Isopycnal mixing in the North Atlantic oxygen minimum zone revealed by RAFOS floats

Author

Donald William Rudnickas Jr., University of Rhode IslandFollow

Date of Award

2018

Degree Type

Thesis

Degree Name

Master of Science in Oceanography

Specialization

Physical Oceanography

Department

Oceanography

First Advisor

Jaime B. Palter

Abstract

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 oats 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 oats recorded their positions, temperature, pressure, and (at the isopycnal aligned with the O₂ minimum) dissolved oxygen concentration. Using the record of the oat positions at each time interval, we calculate the relative dispersion of pairs of oats. 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 oats in the OMZ area obeyed the canonical 4/3s power scaling that suggests it is representative of two dimensional turbulence. By estimating 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 m²s^-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 O₂ 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.

Recommended Citation

Rudnickas, Donald William Jr., "Isopycnal mixing in the North Atlantic oxygen minimum zone revealed by RAFOS floats" (2018). Open Access Master's Theses. Paper 1269.
https://digitalcommons.uri.edu/theses/1269