Biological and physical impacts of ageostrophic frontal circulations driven by confluent flow and vertical mixing

Subduction, upwelling, and phytoplankton blooms are commonly observed features at oceanic fronts. This study isolates the role of vertical mixing for enhanced production and water mass subduction near fronts, considering the time-developing problem with a Semi-Geostrophic circulation model coupled to a planktonic ecosystem model. Our model results show that vertical mixing in the surface boundary layer strongly modifies the time evolution of the front and of its associated biology. Ageostrophic flows caused by the combined effects of confluence and vertical mixing enhance primary production on the less dense side and increase water mass subduction on the dense side of the front. Confluence alone results in the intensification of the front by the same advective response, while the phytoplankton bloom on the less dense side does not arise without vertical mixing. Vertical mixing alone slumps the front near the surface and provides weak subduction on the dense side and uplift of the isopycnals at the center of the front. We find that it is possible to sustain an isolated phytoplankton patch above the domed isopycnals at the center of the front with the nutrients supplied by the secondary circulations arising due to vertical mixing. These results suggest that the phytoplankton bloom and patches found on the less dense side of fronts in many field observations are likely caused by fine-scale along-isopycnal upwelling of nutrients forced by adiabatic confluence in the meander trough of fronts and further pumping and entrainment of nutrients by the secondary circulation due to vertical mixing. Isolated patches observed at the center of the front in many frontal surveys could be caused by secondary flows due to vertical mixing.

[1]  P. Franks,et al.  Phytoplankton patches at fronts : A model of formation and response to wind events , 1997 .

[2]  Patrice Klein,et al.  Impact of sub-mesoscale physics on production and subduction of phytoplankton in an oligotrophic regime , 2001 .

[3]  Brian J. Rothschild,et al.  Biological-physical interactions in the sea , 2002 .

[4]  R. Samelson.  Linear instability of a mixed‐layer front , 1993 .

[5]  R. Samelson,et al.  Evolution of the instability of a mixed-layer front , 1995 .

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

[7]  John A. Barth,et al.  Direct Observations of Along-Isopycnal Upwelling and Diapycnal Velocity at a Shelfbreak Front* , 2004 .

[8]  J. W. Zhang,et al.  Eddy-induced mixed layer shallowing and mixed layer/thermocline exchange , 2000 .

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

[10]  M. Spall.  Baroclinic Jets in Confluent Flow , 1997 .

[11]  D. B. Olson,et al.  Biophysical Dynamics of Ocean Fronts, In: Biological-Physical Interactions, in the Sea, A.R. Robinson, J.J. McCarthy and B. Rothschild, (eds.) , 2002 .

[12]  D. Smeed,et al.  Potential Vorticity and Vertical Velocity at the Iceland-Færœs Front , 1996 .

[13]  Francis P. Bretherton,et al.  Atmospheric Frontogenesis Models: Mathematical Formulation and Solution , 1972 .

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

[15]  Numerical Study on the Oyashio Water Pathways in the Kuroshio–Oyashio Confluence* , 2004 .

[16]  D. Olson,et al.  Distributions of pigments and primary production in a Gulf Stream meander , 1993 .

[17]  R. Houghton,et al.  Phytoplankton growth at the shelf-break front in the Middle Atlantic, Bight , 1990 .

[18]  D. Rudnick,et al.  Intensive surveys of the Azores Front 1. Tracers and dynamics , 1996 .

[19]  W. Large,et al.  Oceanic vertical mixing: a review and a model with a nonlocal boundary layer parameterization , 1994 .

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

[21]  D. Rudnick,et al.  Two‐dimensional ageostrophic secondary circulation at ocean fronts due to vertical mixing and large‐scale deformation , 2005 .

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

[23]  C. Garrett,et al.  Dynamical aspects of shallow sea fronts , 1981, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[24]  A. Robinson,et al.  Eddy-induced nutrient supply and new production in the Sargasso Sea , 1997 .

[25]  Gurvan Madec,et al.  Impacts of sub-mesoscale physics on phytoplankton production and, subduction , 2001 .

[26]  B. Hodges,et al.  Simple models of steady deep maxima in chlorophyll and biomass , 2004 .

[27]  Scott C. Doney,et al.  Eddy‐driven sources and sinks of nutrients in the upper ocean: Results from a 0.1° resolution model of the North Atlantic , 2003 .

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

[29]  M. Gregg,et al.  Signatures of Mixing from the Bermuda Slope, the Sargasso Sea and the Gulf Stream , 1980 .

[30]  J. Moum,et al.  Enhancement of fronts by vertical mixing , 1990 .

[31]  Craig M. Lee,et al.  Intensification of ocean fronts by down-front winds , 2005 .

[32]  J. Allen,et al.  Mesoscale Subduction at the Antarctic Polar Front Driven by Baroclinic Instability , 2001 .

[33]  Pigment and primary production distributions in a Gulf Stream meander , 1993 .

[34]  R. Houghton.  Lagrangian flow at the foot of a shelfbreak front using a dye tracer injected into the bottom boundary layer , 1997 .

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

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

[37]  Michel Rixen,et al.  Diatom carbon export enhanced by silicate upwelling in the northeast Atlantic , 2005, Nature.

[38]  L. A. Anderson,et al.  Physical and biological modeling in the Gulf Stream region:: I. Data assimilation methodology , 2000 .

[39]  L. Thompson.  Ekman layers and two‐dimensional frontogenesis in the upper ocean , 2000 .

[40]  J. A. Elliott,et al.  Vertical temperature gradient structure across the Gulf Stream , 1977 .

[41]  Y. Shimizu,et al.  Distribution and Modification of North Pacific Intermediate Water in the Kuroshio-Oyashio Interfrontal Zone , 1996 .

[42]  Joaquín Tintoré,et al.  Mesoscale subduction at the Almeria-Oran front: Part 1: Ageostrophic flow , 2001 .

[43]  L. Prieur,et al.  “Almofront-1” (April–May 1991): an interdisciplinary study of the Almeria-Oran geostrophic front, SW Mediterranean Sea , 1994 .

[44]  D. Rudnick.  Intensive surveys of the Azores Front: 2. Inferring the geostrophic and vertical velocity fields , 1996 .

[45]  J. Ryan,et al.  Chlorophyll enhancement and mixing associated with meanders of the shelf break front in the Mid-Atlantic Bight , 1999 .

[46]  Sophie Fielding,et al.  Mesoscale subduction at the Almeria-Oran front. Part 2: biophysical interactions. , 2001 .

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

[48]  R. Pickart.  Bottom Boundary Layer Structure and Detachment in the Shelfbreak Jet of the Middle Atlantic Bight , 2000 .

[49]  J. Barth,et al.  Diagnosis of the Three-Dimensional Circulation in Mesoscale Features with Large Rossby Number , 2000 .

[50]  J. Ryan,et al.  Enhanced chlorophyll at the shelfbreak of the Mid‐Atlantic Bight and Georges Bank during the spring transition , 1999 .

[51]  H. Leach.  The diagnosis of synoptic-scale vertical motion in the seasonal thermocline , 1987 .

[52]  Amit Tandon,et al.  An analysis of mechanisms for submesoscale vertical motion at ocean fronts , 2006 .

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

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

[55]  A. Mahadevan,et al.  Mesoscale variability of sea surface pCO2: What does it respond to? , 2004 .