Volume 12, issue 19 | Copyright

Special issue: KEOPS2: Kerguelen Ocean and Plateau Study 2

Biogeosciences, 12, 5567-5581, 2015
https://doi.org/10.5194/bg-12-5567-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 01 Oct 2015

Research article | 01 Oct 2015

The biogeochemical structuring role of horizontal stirring: Lagrangian perspectives on iron delivery downstream of the Kerguelen Plateau

F. d'Ovidio1, A. Della Penna1,2,3, T. W. Trull4, F. Nencioli5, M.-I. Pujol6, M.-H. Rio6, Y.-H. Park1, C. Cotté1, M. Zhou7, and S. Blain8,9 F. d'Ovidio et al.
  • 1Sorbonne Universités (UPMC, Université Paris 06)-CNRS-IRD-MNHN, LOCEAN Laboratory, 4 Place Jussieu, 75005 Paris, France
  • 2CSIRO-UTAS Quantitative Marine Science Program, IMAS, Private Bag 129, Hobart, Tasmania 7001, Australia
  • 3Université Paris Diderot Cité, Paris, France
  • 4Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, CSIRO Oceans and Atmosphere, Hobart, Tasmania 7001, Australia
  • 5Remote Sensing Group, Plymouth Marine Laboratory, Prospect Place, Plymouth PL1 3DH, UK
  • 6CLS, 8-10 Rue Hermes, Toulouse, France
  • 7School for the Environment, College of Science and Mathematics, University of Massachusetts, Boston, USA
  • 8UMR CNRS 7621, Laboratoire d'Océanographie Microbienne, Observatoire Océanologique, 66650 Banyuls-sur-Mer, France
  • 9Sorbonne Universités (UPMC, Université Paris 06),UMR7621, Laboratoire d'Océanographie Microbienne, Observatoire Océanologique, 66650 Banyuls-sur-Mer, France

Abstract. Field campaigns are instrumental in providing ground truth for understanding and modeling global ocean biogeochemical budgets. A survey however can only inspect a fraction of the global oceans, typically a region hundreds of kilometers wide for a temporal window of the order of (at most) several weeks. This spatiotemporal domain is also the one in which the mesoscale activity induces through horizontal stirring a strong variability in the biogeochemical tracers, with ephemeral, local contrasts which can easily mask the regional and seasonal gradients. Therefore, whenever local in situ measures are used to infer larger-scale budgets, one faces the challenge of identifying the mesoscale structuring effect, if not simply to filter it out. In the case of the KEOPS2 investigation of biogeochemical responses to natural iron fertilization, this problem was tackled by designing an adaptive sampling strategy based on regionally optimized multisatellite products analyzed in real time by specifically designed Lagrangian diagnostics. This strategy identified the different mesoscale and stirring structures present in the region and tracked the dynamical frontiers among them. It also enabled back trajectories for the ship-sampled stations to be estimated, providing important insights into the timing and pathways of iron supply, which were explored further using a model based on first-order iron removal. This context was essential for the interpretation of the field results. The mesoscale circulation-based strategy was also validated post-cruise by comparing the Lagrangian maps derived from satellites with the patterns of more than one hundred drifters, including some adaptively released during KEOPS2 and a subsequent research voyage. The KEOPS2 strategy was adapted to the specific biogeochemical characteristics of the region, but its principles are general and will be useful for future in situ biogeochemical surveys.

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Field campaigns are instrumental in providing ground truth for understanding and modeling global ocean biogeochemical budgets. A survey however can only inspect a fraction of the global oceans, typically a region hundreds of kilometers wide for a temporal window of the order of (at most) several weeks. In this spatiotemporal domain, mesoscale variability can mask climatological contrasts. Here we propose the use of multisatellite-based Lagrangian diagnostics to solve this issue.
Field campaigns are instrumental in providing ground truth for understanding and modeling global...
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