Large-scale alignment of oceanic nitrate and density

• Published in: Journal of geophysical research. Oceans Vol. 118; no. 10; pp. 5322 - 5332
• Main Authors: Omand, Melissa M., Mahadevan, Amala
• Format: Journal Article
• Language: English
• Published: Hoboken, NJ Blackwell Publishing Ltd 01-10-2013; Wiley
• Subjects: Alignment; Biological; Diffusivity; Earth, ocean, space; Eddies; Euphotic zone; Exact sciences and technology; export; External geophysics; Floats; Geophysics; Homogenizing; Internal waves; International trade; isopycnal; large-scale; Marine; mixing; nitrate; Nitrates; Ocean circulation; Oceans; Organic matter; Physics of the oceans; models; Euphotic zone; density; internal waves; Space remote sensing; Profiling float; Satellite observation; Vortex; Turbulent diffusion; Homogenization (mathematics); Uptake; Comparative study
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1002/jgrc.20379

By analyzing global data, we find that over large scales, surfaces of constant nitrate are often better aligned with isopycnals than with isobars, particularly below the euphotic zone. This is unexplained by the movement of isopycnal surfaces in response to eddies and internal waves, and is perhaps surprising given that the biological processes that alter nitrate distributions are largely depth dependent. We provide a theoretical framework for understanding the orientation of isonitrate surfaces in relation to isopycnals. In our model, the nitrate distribution results from the balance between depth‐dependent biological processes (nitrate uptake and remineralization), and the along‐isopycnal homogenization of properties by eddy fluxes (parameterized by eddy diffusivity). Where the along‐isopycnal eddy diffusivity is relatively large, nitrate surfaces are better aligned with isopycnals than isobars. We test our theory by estimating the strength of the eddy diffusivity and biological export production from global satellite data sets and comparing their contributions. Indeed, we find that below the euphotic zone, the mean isonitrate surfaces are oriented along isopycnals where the isopycnal eddy diffusivity is large, and deviate where the biological export of organic matter is relatively strong. Comparison of nitrate data from profiling floats in different regions corroborates the hypothesis by showing variations in the nitrate‐density relationship from one part of the ocean to another.
Key Points
Nitrate surfaces are aligned with isopycnals over large scales.
This large‐scale alignment is not due to isopycnal displacements.
Alignment results from isopycnal mixing dominating over biological processes.