Biogeochemical eddy fluxes in the ocean

• Author: Marina Lévy, LOCEAN - IPSL, Universite Pierre et Marie Curie .


• Oral or poster: oral.


• Abstract:
  Over most oceanic regions, it is the supply of nutrients from deeper water to the euphotic layer that determines the level of phytoplankton productivity. The dynamical mechanisms that control this supply occur over a large range of temporal and spatial scales.
  At the basin scale, and for time scales of a year and beyond, the transport of nutrients is controlled by the thermohaline and wind-driven circulations. At mid and high latitudes, convection provides an additional source during winter, over large oceanic regions. At shorter time and space scale, an additional source of nutrients is now recognized, resulting from the vertical velocities associated with mesoscale turbulence.
  The mesoscale flow field, characterized by a horizontal length scale of 10-100 km and by a temporal time scale of a few months, is largely adiabatic and quasi-geostrophic. However the breakdown of geostrophic balance in this regime leads to the development of secondary ageostrophic circulations, at submesoscales 1-10 km, 1-30 days - with relatively large vertical velocities compared to those associated with the mesoscale. Moreover, mesoscale turbulence generates small-scale patchiness in plankton. Due to non-linearities in the biological interactions, this small-scale heterogeneity can create some departure from the mean field approximation, whereby plankton dynamics (such as production or grazing) are evaluated from mean distributions at coarser-scale.
  
  In this presentation, I will discuss the relevance of this "biological Reynolds effect" in the ocean and at the basin scale, where transport processes are thought to control the lead order. This is done in the frame of biogeochemical regimes characteristics of the North Atlantic or North Pacific subpolar and subtropical gyres, with a simple plankton model embedded in a sub-mesoscale permitting ocean circulation model. The relative importance of the transport and biological Reynolds terms is assessed for nutrients, for phytoplankton and for zooplankton.