Erosion of a surface vortex by a seamount on the beta plane Steven Herbette (PhD-SHOM), Yves Morel (SHOM), Michel Arhan (IFREMER)
Jan 08, 2016
Erosion of a surface vortex by a seamount on the beta plane
Steven Herbette (PhD-SHOM), Yves Morel (SHOM), Michel Arhan (IFREMER)
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Outline of the presentation:
- Tools : equations - potential vorticity anomalie (PVA)
- Presentation of the problem - configuration
- Erosion - one example
- Sensitivity study
- Conclusion
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Tools – Equations :
Shallow Water equations :
Potential Vorticities
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NOT conserved if f = f0 + y H = Htopo(x,y)
PVA = PV – f0/HPVAd = PV – PV(at rest)
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PV
Tools : POTENTIAL VORTICITY “thinking”
= rot (U) important quantity
BUT NOT CONSERVED
PV = (+f) . (= (+f)/h )
is conserved for each particles if adiabatic motion
PV = TRACER
The velocity field can bereconstructed from theknowledge of PVAd (if geostrophicbalance is assumed)
INVERSION PRINCIPLEz
PVAd > 0 => cyclonic
PVAd < 0 => anticyclonic
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Configuration :
f = f0 + yRd = 34 km - 16 km
Top view side view
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FOCUS ON EROSION
(how much of the vortex remains)
COMPARE WITH F-PLANE (WHAT IS NEW)
Problem :
Rfc (t) = h PVA (t) dx dy
h PVA (t=0) dx dy
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Rv = 100 km Q = -1.5 f0 s (Vmax ~ 0.8 m/s)Lf = 100 km (Umax ~ 0.25 m/s)
Result from f-plane (Herbette et al, JPO, 2003)
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PVA 1 PVA 2 PVA 3
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PVA 1 PVA 2 PVA 3
Including BETA :
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Results :
-Same processes still exist (splitting, filamentation),
-Propagation induced by => no pole remains trapped above topo,
-Splitting seems even more vertical (reduced impact on PVA 1),
-Additional PVAd poles emerge because of advection of particles especially in the third layer in our case
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PVA 2 PVA 3 PVAd 3
Effect of the formation of PVAd poles(in the lower layer) :
Erosion Masking (weaker velocity field)
Evolution without topography
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Sensitivity to initial vortex position :
Hypersensitivity
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Minimum Distance reached(opt = 100 km)
Erosion rate(20% for opt. OnF-plane)
Erosion for different seamount positions (along trajectory) :
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Conclusion :
PVAd 3
Circulation layer 3
Flat bottom exp.PVAd and circul.Trapping of fluid parcels
-Same processes still exist (splitting, filamentation),
-Propagation induced by => no pole remains trapped above topo,
-Splitting seems even more vertical (reduced impact on PVA 1),
-Additional PVAd poles emerge because of advection of particles especially in the third layer in our case :
- masks vortex in lower layer (lower erosion rates)
-hypersensitivity