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

Donald Rudnickas,David Hebert,H Thomas Rossby,Jaime Palter

doi:10.1029/2019jc015148

Donald Rudnickas, David Hebert + Show 2 more

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https://doi.org/10.1029/2019jc015148

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Abstract

AbstractThe Eastern Tropical North Atlantic Oxygen Minimum Zone (OMZ) is a biogeochemically important area. The low oxygen in this region is thought to be maintained by a balance between the slow mixing supply of O2 and its removal by respiration. We use data from 90 isopycnal RAFOS floats to characterize the mixing coefficients responsible for the supply of oxygen to the region. One group 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. Float pairs deployed at shared locations but across the two target densities reveal that the influence of vertically sheared currents is to accelerate the dispersion by 10–15% relative to true isopycnal dispersion. Relative dispersion of the floats in the OMZ area obeyed the canonical four‐thirds power scaling, representative of two‐dimensional turbulence. At the length scale of the maximum energy‐containing eddy (approximately 100 km), the effective diffusivity is 1,400±500 m2/s in the zonal direction and 800±300 m2/s in the meridional. Within the uncertainty, the diffusivities on the two isopycnals are indistinguishable from one another. An idealized model suggests that meridional mixing across the large‐scale O2 gradient is the leading supply term of oxygen to the OMZ.

Highlights

Between the energetic equatorial currents, the anticyclonic subtropical gyre, and the highly productive coastal upwelling zone along the west coast of Africa, lies the Eastern Tropical North Atlantic (ETNA) Oxygen Minimum Zone (OMZ)
CHAPTER 3 Results and Discussion Snapshots of the float trajectories over the first 200 days are shown in Figure 7 and the results of our calculation of isopycnal diffusivity from relative dispersion and mixing length methods are provided in detail below
Isopycnal diffusivity in the ETNA OMZ was calculated from two-point dispersion of the Lagrangian RAFOS floats as a result of Equation 3 in which the diffusivity, K, is taken as the time derivative of the squared ensemble mean separation distance, D2, using a forward difference between consecutive time-steps

Summary

Introduction

Between the energetic equatorial currents, the anticyclonic subtropical gyre, and the highly productive coastal upwelling zone along the west coast of Africa, lies the Eastern Tropical North Atlantic (ETNA) Oxygen Minimum Zone (OMZ). Upwelling along the coast and in the OMZ leads to biologically productive surface waters and results in the export of particulate organic carbon through the water column. Remineralization of these particulates below the euphotic zone consumes oxygen. There is no direct advective connection with a well-oxygenated mixed layer, and a low O2 layer persists beneath the surface (Luyten et al, 1983). The core is hypoxic (dissolved oxygen concentrations between 0.4 and 2 ml l−1) and is comprised of a mixture of South Atlantic Central Water and Antarctic Intermediate Water, with a mean water mass age (i.e. time since being at the ocean surface) of over 125 years (Stramma et al, 2016; Gnanadesikan et al, 2013; Brandt et al, 2015)

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