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Abstract Past studies of dispersion with float‐pairs have indicated that they may remain close together for much longer when they equilibrate on the same isopycnal, presumably due to the reduced influence of vertical shear. To examine this question more closely, we use a set of 13 and 15 float pair combinations that equilibrated within 0.1 °C (∼σθ = 0.01 kg m−3) of each other on two density surfaces in the main thermocline in a Lagrangian dispersion study. Their average rate of separation after launch was 0.0021 ± 0.0014 ms−1(∼5.5 km after 30 days). Relative dispersion is accurately expressed by <D2> = 4•106exp (t/10.8) m2from start to about 30 days. Relative diffusivity (K) versus separation dropped well below the classical 4/3rds power law settling out at about 2–3 m2s−1for separations less than ∼6 km, far lower than results from other float studies, but in accord with dye dispersion estimates.
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Isopycnal Mixing in the North Atlantic Oxygen Minimum Zone Revealed by RAFOS Floats
Abstract The 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 O2and 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 O2gradient is the leading supply term of oxygen to the OMZ.
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- Award ID(s):
- 1736985
- PAR ID:
- 10459709
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 124
- Issue:
- 9
- ISSN:
- 2169-9275
- Page Range / eLocation ID:
- p. 6478-6497
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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