Eddy-induced mixed layer shallowing and mixed layer/thermocline exchange

The effects of eddies on the mixed layer and on the transfer of properties between the mixed layer and thermocline are studied in a high-resolution model of an unstable front. A spring scenario is considered, with a mixed layer maintained against surface warming by wind mixing. Tilting of the buoyancy gradients by ageostrophic flow is found to shallow the mixed layer over the front, giving a mixed layer as shallow as 10 m in some places, compared with the equilibrium Monin-Obukhov depth of ?30 m away from the front. Variations of mixed layer depth with time and lateral advection lead to a subduction field that is substantially different from the vertical velocity field. Vertical velocities associated with the growing instabilities push tongues of a ventilation tracer, initially only present within the surface layer, down to depths of up to 100 m. After 24 days a quarter of this ventilation tracer has been deposited in the thermocline. Conversely, the vertical motions pull nutrients from the thermocline up into the mixed layer, giving enhanced surface values along the front. In the basin average the geostrophic eddies give vertical eddy diffusivities for a ventilation tracer of O(3 - 10×10?4m2s?1).

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

[2]  R. Pollard,et al.  Large variations in potential vorticity at small spatial scales in the upper ocean , 1990, Nature.

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

[4]  R. Onken.  Mesoscale Upwelling and Density Finestructure in the Seasonal Thermocline—A Dynamical Model , 1992 .

[5]  David P. Stevens,et al.  A New Tracer Advection Scheme for Bryan and Cox Type Ocean General Circulation Models , 1995 .

[6]  F. F. Pérèz,et al.  Vivaldi 1991 - A study of the formation, circulation and ventilation of Eastern North Atlantic Central Water , 1996 .

[7]  Thomas W. N. Haine,et al.  Gravitational, Symmetric, and Baroclinic Instability of the Ocean Mixed Layer , 1998 .

[8]  M. Follows,et al.  A zonal average model of the stratospheric contributions to the tropospheric ozone budget , 1992 .

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

[10]  D. Halpern,et al.  Variability in the upper ocean during MILE. Part I: The heat and momentum balances , 1981 .

[11]  V. Fiekas,et al.  Mesoscale Instability and Upwelling. Part 1: Observations at the North Atlantic Intergyre Front , 1994 .

[12]  J. Tintoré,et al.  Circulation in the Alboran See as Determined by Quasi-Synoptic Hydrographic Observations. Part II: Mesoscale Ageostrophic Motion Diagnosed through Density Dynamical Assimilation , 1996 .

[13]  R. Weller.  Overview of the Frontal Air‐Sea Interaction Experiment (FASINEX): A study of air‐sea interaction in a region of strong oceanic gradients , 1991 .

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

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

[16]  T. McDougall.  The Relative Roles of Diapycnal and Isopycnal Mixing on Subsurface Water Mass Conversion , 1984 .

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

[18]  V. Strass.  Mesoscale Instability and Upwelling. Part 2: Testing the Diagnostics of Vertical Motion with a Three-Dimensional Ocean Front Model , 1994 .

[19]  Joaquín Tintoré,et al.  Circulation in the Alboran Sea as Determined by Quasi-Synoptic Hydrographic Observations. Part I: Three-Dimensional Structure of the Two Anticyclonic Gyres , 1996 .

[20]  B. Hoskins,et al.  The Mathematical Theory of Frontogenesis , 1982 .

[21]  D. Webb,et al.  An ocean model code for array processor computers , 1996 .

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

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

[24]  B. P. Leonard,et al.  A stable and accurate convective modelling procedure based on quadratic upstream interpolation , 1990 .

[25]  Joaquín Tintoré,et al.  A shelf/slope frontal filament off the northeast Spanish Coast , 1988 .

[26]  G. Madec,et al.  The onset of the Spring Bloom in the MEDOC area: mesoscale spatial variability , 1999 .

[27]  W. Jenkins.  The use of anthropogenic tritium and helium-3 to study subtropical gyre ventilation and circulation , 1988, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

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

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

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