Impact of sub-mesoscale physics on production and subduction of phytoplankton in an oligotrophic regime

Using a protocol of numerical experiments where horizontal resolution is progressively increased, we show that small-scale (or sub-mesoscale) physics has a strong impact on both mesoscale physics and phytoplankton production/subduction. Mesoscale and sub-mesoscale physics result from the nonlinear equilibration of an unstable baroclinic jet. The biogeochemical context is oligotrophy. The explicitly resolved sub-mesoscales, at least smaller than one e fth of the internal Rossby radius of deformation, reinforce the mesoscale eddy e eld and contribute to double the primary production and phytoplankton subduction budgets. This enhancement is due to the reinforced mesoscale physics and is also achieved by the small-scale frontal dynamics. This sub-mesoscale physics is associated with density and vorticity gradients around and between the eddies. It triggers a signie cant small-scale nutrient injection in the surface layers, leading to a phytoplankton e eld mostly dominated by e ne spatial structures. It is believed that, depending on wind forcings, this scenario should work alternately with that of Abraham (1998) which invokes horizontal stirring of nutrient injected at large scales. Results also reveal a strong relationship between new production and negative vorticity, in the absence of wind forcing and during the period of formation of the eddies.

[1]  A. Mariano,et al.  Mesoscale pigment fields in the Gulf Stream: Observations in a meander crest and trough , 1993 .

[2]  D. Stammer.  Global Characteristics of Ocean Variability Estimated from Regional TOPEX/POSEIDON Altimeter Measurements , 1997 .

[3]  B. V. Leer,et al.  Towards the Ultimate Conservative Difference Scheme , 1997 .

[4]  K. Denman,et al.  Phytoplankton patchiness indicates the fluctuation spectrum of mesoscale oceanic structure , 1980, Nature.

[5]  P. Klein,et al.  The mesoscale variability of the sea surface temperature: An analytical and numerical model , 1990 .

[6]  Anthony H. Knap,et al.  Overview of the U.S. JGOFS Bermuda Atlantic Time-series Study and the Hydrostation S program , 1996 .

[7]  F. F. Pérèz,et al.  Large and mesoscale variability of the water masses and the deep chlorophyll maximum in the Azores Front , 2003 .

[8]  R. Pollard,et al.  Vorticity and vertical circulation at an ocean front , 1992 .

[9]  D. Siegel,et al.  Mesoscale Eddies, Satellite Altimetry, and New Production in the Sargasso Sea , 1999 .

[10]  P. Gent,et al.  The Evolution of Balanced, Low-Mode Vortices on the β-Plane , 1986 .

[11]  G. Lapeyre,et al.  Dynamics of the orientation of active and passive scalars in two-dimensional turbulence , 2001 .

[12]  Brian J. Rothschild,et al.  Toward a theory on biological-physical interactions in the world ocean , 1988 .

[13]  James C. McWilliams,et al.  Lagrangian accelerations in geostrophic turbulence , 1998, Journal of Fluid Mechanics.

[14]  Bruno Blanke,et al.  Variability of the Tropical Atlantic Ocean Simulated by a General Circulation Model with Two Different Mixed-Layer Physics , 1993 .

[15]  D. Phinney,et al.  Rotary motions and convection as a means of regulating primary production in warm core rings. [of ocean currents] , 1985 .

[16]  A. Warn-Varnas,et al.  Wind-driven secondary circulation in ocean mesoscale , 1994 .

[17]  P. Klein,et al.  Three-dimensional stirring of thermohaline fronts , 1998 .

[18]  Michio J. Kishi,et al.  Effects of interaction between two warm-core rings on phytoplankton distribution , 1994 .

[19]  A. E. Gill.  Atmosphere-Ocean Dynamics , 1982 .

[20]  G. Madec,et al.  Combined effects of mesoscale processes and atmospheric high-frequency variability on the spring bloom in the MEDOC area , 2000 .

[21]  David Archer,et al.  Modeling the impact of fronts and mesoscale circulation on the nutrient supply and biogeochemistry of the upper ocean , 2000 .

[22]  B. Vanleer,et al.  Towards the ultimate conservative difference scheme. IV. A new approach to numerical convection , 1977 .

[23]  Qin Xu.  Ageostrophic Pseudovorticity and Geostrophic C-Vector Forcing—A New Look at the Q Vector in Three Dimensions , 1992 .

[24]  B. Irwin,et al.  Factors affecting the spatial pattern of the deep chlorophyll maximum in the region of the Azores front , 1985 .

[25]  R. Gall.  Some Non-Quasigeostrophic Effects in Linear Baroclinic Waves , 1977 .

[26]  Bruno Blanke,et al.  Kinematics of the Pacific Equatorial Undercurrent: An Eulerian and Lagrangian Approach from GCM Results , 1997 .

[27]  G. Lapeyre,et al.  Does the tracer gradient vector align with the strain eigenvectors in 2D turbulence , 1999 .

[28]  Edward R. Abraham,et al.  The generation of plankton patchiness by turbulent stirring , 1998, Nature.

[29]  P. Delecluse,et al.  OPA 8.1 Ocean General Circulation Model reference manual , 1998 .

[30]  C. L. Smith,et al.  The impact of mesoscale eddies on plankton dynamics in the upper ocean , 1996 .

[31]  R. Davies-Jones.  The Frontogenetical Forcing of Secondary Circulations. Part I: The Duality and Generalization of the Q Vector , 1991 .

[32]  Andrew J. Watson,et al.  Evidence for slow mixing across the pycnocline from an open-ocean tracer-release experiment , 1993, Nature.

[33]  Andreas Oschlies,et al.  Eddy-induced enhancement of primary production in a model of the North Atlantic Ocean , 1998, Nature.

[34]  G. Halliwell,et al.  Large-scale SST anomalies associated with subtropical fronts in the western North Atlantic during FASINEX , 1989 .

[35]  V. Garçon,et al.  Impact of the North Equatorial Current meandering on a pelagic ecosystem: A modeling approach , 1996 .

[36]  K. Denman.  The variance spectrum of phytoplankton in a turbulent ocean , 1976 .

[37]  B. Jones,et al.  Eddy stirring and phytoplankton patchiness in the subarctic North Atlantic in late summer , 1998 .

[38]  M. Spall.  Frontogenesis, subduction, and cross‐front exchange at upper ocean fronts , 1995 .

[39]  On geometrical alignment properties of two-dimensional forced turbulence , 1999 .

[40]  T. D. Dickey,et al.  Influence of mesoscale eddies on new production in the Sargasso Sea , 1998, Nature.

[41]  Frédéric Hourdin,et al.  The Use of Finite-Volume Methods for Atmospheric Advection of Trace Species. Part I: Test of Various Formulations in a General Circulation Model , 1999 .

[42]  G. Madec,et al.  OPA 8.1 Tracer Model reference manual , 2000 .

[43]  A. Mariotti,et al.  Vortex stripping and the erosion of coherent structures in two‐dimensional flows , 1994 .

[44]  Allan R. Robinson,et al.  Coupled physical and biological modelling of the spring bloom in the North Atlantic (II): three dimensional bloom and post-bloom processes , 1995 .

[45]  P. Boyd,et al.  Importance of stirring in the development of an iron-fertilized phytoplankton bloom , 2000, Nature.

[46]  John D. Woods,et al.  Scale Upwelling and Primary Production , 1988 .

[47]  Adrian P. Martin.  On filament width in oceanic plankton distributions , 2000 .

[48]  Richard G. Williams,et al.  The role of eddies in the isopycnic transfer of nutrients and their impact on biological production , 2000 .

[49]  C. Davis,et al.  Biological effects of Gulf Stream meandering , 1993 .

[50]  Dong-Ping Wang.  Model of frontogenesis: Subduction and upwelling , 1993 .

[51]  W. Blumen.  Inertial Oscillations and Frontogenesis in a Zero Potential Vorticity Model , 2000 .

[52]  R. Rotunno,et al.  A comparison of primitive-equation and semigeostrophic simulations of baroclinic waves , 1991 .

[53]  Audrey Estublier,et al.  Choice of an advection scheme for biogeochemical models , 2001 .

[54]  Allan R. Robinson,et al.  Physical and biological modeling in the Gulf Stream region Part II. Physical and biological processes , 2001 .

[55]  P. Delecluse,et al.  A Three-Dimensional Numerical Study of Deep-Water Formation in the Northwestern Mediterranean Sea , 1991 .

[56]  J. Hart.  A note on nonlinear corrections to the Ekman layer pumping velocity , 2000 .

[57]  K. Denman,et al.  Time scales of pattern evolution from cross‐spectrum analysis of advanced very high resolution radiometer and coastal zone color scanner imagery , 1994 .

[58]  V. Strass,et al.  Chlorophyll patchiness caused by mesoscale upwelling at fronts , 1992 .

[59]  B. V. Leer,et al.  Towards the ultimate conservative difference scheme. IV. A new approach to numerical convection , 1977 .

[60]  Carl Wunsch,et al.  The Vertical Partition of Oceanic Horizontal Kinetic Energy , 1997 .

[61]  S. Spall,et al.  A numerical model of mesoscale frontal instabilities and plankton dynamics — I. Model formulation and initial experiments , 2000 .