Secondary Upwelling along the shelf break Vincent Rossi, Yves Morel, Vronique Garon

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Secondary Upwelling along the shelf
breakVincent Rossi, Yves Morel, Véronique
Garçon
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Typical summer situation
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Operations during MOUTON2007
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Density
Chl a
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U V
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A  PV thinking  approach of upwelling dynamics
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POTENTIAL VORTICITY thinking
The velocity/stratification field can be
reconstructed from the knowledge of PVA (if
geostrophic balance is assumed) INVERSION
PRINCIPLE
z rot (U) important quantity BUT NOT
CONSERVED PV (zf) . r ( (zf)/h ) is
conserved for each particles if adiabatic
motion PV TRACER
z
d PV gt 0 gt cyclonic d PV lt 0 gt anticyclonic
d PV
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Upwelling development
PV cstt gt z f h/Hrest geostrophy
gt  Kelvin  currents U Sk Uk e
y
tx
- x / Rk
At the coast V(x0) 0 gt U t
gt Uk tk t
Importance of barotropic mode (Ro 500 km)
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OBrien Hurlburt (1972)
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Reference Numerical experiments
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Configuration (2D)
50 m 20 m
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PVA layer 3
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Sensitivity studiesBottom stressShelf
widthmargin slope3D
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Effect of the Bottom stress Cd0
(ref)
Cd 3. 10-3
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Effect of the bottom stress competition between
two processes
Acceleration of cross-shelf circulation gt
Enhances shelf upwelling
Modifies PVA in BBL (layer 3) gt Diminishes PVA
and shelf upwelling
Close to the shelf break, U and 1/h
diminishes gt
gt 0
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PVA in layer 3
Cd3.10-3 Cd 0 (ref)
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Effect of the continental shelf slope
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Effect of the shelf width
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More  realistic  (2 D) experiment (16 layers)
Observations
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• Conclusion
• There exists shelf break upwellings along
  portuguese coasts
• They can be explained by low PV advection from
  off-shore
• by the barotropic circulation developing on an
  extended shelf
• Bottom friction
• equilibrates the wind stress effect and the
  barotropic circulation
• accelerates the cross shore circulation (gt no
  steady state)
• diminishes the PVA (diabatic process) and the
  shelf break upwelling
• 3-D effects play an important role to limit
  velocity (not taken
• Into account here)
• These results can be extended to downwelling winds

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