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Ocean Dynamics

, Volume 67, Issue 8, pp 1027–1046 | Cite as

Nutrient interleaving below the mixed layer of the Kuroshio Extension Front

  • Takeyoshi NagaiEmail author
  • Sophie Clayton
Article
Part of the following topical collections:
  1. Topical Collection on the 48th International Liège Colloquium on Ocean Dynamics, Liège, Belgium, 23-27 May 2016

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.

Keywords

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

Notes

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.

Supplementary material

(MOV 9.86 MB)

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Copyright information

© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  1. 1.Department of Ocean SciencesTokyo University of Marine Science and TechnologyTokyoJapan
  2. 2.Department for Earth, Atmospheric, and Planetary SciencesMassachusetts Institute of TechnologyCambridgeUSA
  3. 3.School of OceanographyUniversity of WashingtonSeattleUSA

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