Nutrient interleaving below the mixed layer of the Kuroshio Extension Front

Abstract

Nitrate interleaving structures were observed below the mixed layer during a cruise to the Kuroshio Extension in October 2009. In this paper, we investigate the formation mechanisms for these vertical nitrate anomalies, which may be an important source of nitrate to the oligotrphoc surface waters south of the Kuroshio Extension Front. We found that nitrate concentrations below the main stream of the Kuroshio Extension were elevated compared to the ambient water of the same density (σ 𝜃 = 23.5–25). This appears to be analogous to the “nutrient stream” below the mixed layer, associated with the Gulf Stream. Strong turbulence was observed above the vertical nitrate anomaly, and we found that this can drive a large vertical turbulent nitrate flux \(>\mathcal {O}\) (1 mmol N m−2 day−1). A realistic, high-resolution (2 km) numerical simulation reproduces the observed Kuroshio nutrient stream and nitrate interleaving structures, with similar lateral and vertical scales. The model results suggest that the nitrate interleaving structures are first generated at the western side of the meander crest on the south side of the Kuroshio Extension, where the southern tip of the mixed layer front is under frontogenesis. Lagrangian analyses reveal that the vertical shear of geostrophic and subinertial ageostrophic flow below the mixed layer tilts the existing along-isopycnal nitrate gradient of the Kuroshio nutrient stream to form nitrate interleaving structures. This study suggests that the multi-scale combination of (i) the lateral stirring of the Kuroshio nutrient stream by developed mixed layer fronts during fall to winter, (ii) the associated tilting of along-isopycnal nitrate gradient of the nutrient stream by subinertial shear, which forms vertical interleaving structures, and (iii) the strong turbulent diffusion above them, may provide a route to supply nutrients to oligotrophic surface waters on the south side of the Kuroshio Extension.

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Acknowledgements

We thank Capt. Ukekura of R/V Natsushima (JAMSTEC) and JAMSTEC for the cruise opportunity, Japan Meteorological Agency for R/V Kofu-maru data, M. Aiba for useful discussion and support from MIT-Hayashi Seed Fund. TN thanks support from OMIX (MEXT KAKENHI Grant Number JP16H01590) and SKED(funded by MEXT). SC thanks Gordon and Betty Moore Foundation.

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Correspondence to Takeyoshi Nagai.

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This article is part of the Topical Collection on the 48th International Liège Colloquium on Ocean Dynamics, Liège, Belgium, 23-27 May 2016

TN thanks JSPS (KAKENHI 20710002, 24684036, 16H01590 (OMIX)), “The Study of Kuroshio Ecosystem Dynamics for Sustainable Fisheries (SKED)” funded by MEXT. SC and TN acknowledge the support of the MIT-Hayashi Seed Fund. SC was supported by the Gordon and Betty Moore Foundation Microbiology Initiative and the Moore/Sloan Data Science and Washington Research Foundation Innovation in Data Science Postdoctoral Fellowship.

Responsible Editor: Simon Ruiz

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Nagai, T., Clayton, S. Nutrient interleaving below the mixed layer of the Kuroshio Extension Front. Ocean Dynamics 67, 1027–1046 (2017). https://doi.org/10.1007/s10236-017-1070-3

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Keywords

  • Nitrate interleaving
  • Lateral stirring
  • Geostrophic and ageostrophic shear
  • Near-inertial waves
  • The Kuroshio Extension
  • Nutrient stream
  • Turbulent vertical nitrate flux