dynamics of the dense water formation in the northwestern
TRANSCRIPT
GMMC 20-22 Juin 2017 Ifremer Brest [email protected] 1
Dynamics of the Dense Water Formation in the NorthWestern Mediterranean during the
HyMeX/ASICS Experiment: A PV-Perspective
H. Giordani and C. Lebeaupin-Brossier
GMMC 20-22 Juin 2017 Ifremer Brest [email protected] 2
Month-average surfaceBuoyancy flux -400W/m2
Deep Mixed-Layer Depths 2500 m
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Gliders, Profiling floats, Marisondes, Cruises…Courtesy, P. Testor
01/07/2012 -> 07/10/201330 Glider deployments ~ 13,000 profiles (0-1000m)6 CTD cruises ~ 400 profiles (0-bottom)27 Argo ~ 1,500 profiles (0-1000m or 0-2000m)10 Marisondes and surface drifters
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ASICS-MED : Oceanographic Component of the HyMeX Program
� Two Scientific questions:
1. How the strong wind events interact with the mesoscale & submesoscale oceanic structures ?
2. What are the processes of dense water formation in these mesoscale structures ?
�
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From Marshall & Shott, 1999
Fb
What Dynamics in frontal region ?
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Ocean model: NEMO-WMED36 (Lebeaupin-Brossier et al., 2015)
code: NEMO v3_2 [Madec, 2008] SIMED [Arsouze et al., 2012]
grid: 760 x 480 x 50 z-levelsΔx=1/36° ORCA grid (from 2.2 to 2.5km) bathymetry: v10 Mercator-LEGOSΔt=240s
2 open-boundaries: Alboran Sea (East of the Strait of Gibraltar) and Sicily Channel
river runoff: climatology [Beuvier et al., 2010]
atmospheric surface forcing: hourly AROME-WMED forecasts [Fourrié et al., 2015]
initial and boundary conditions: Mercator Analysis
Simulation: Sept 1 to March 15
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Dynamics & Forcings of DWF in Frontal region ?
DWF > 29.0 kg/m3 Vol Buoyancy (Tziperman, 1986)
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Generation of low-PV water critical to the formation of dense water
Dense Waters characterized by Low PV
q=(ζ+ f )N 2⏟qv
−f (∂U g
∂ z )2
⏟qh
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Generation of low-PV water critical to the formation of dense water
Dense Waters characterized by Low PV
q=(ζ+ f )N 2⏟qv
−f (∂U g
∂ z )2
⏟qh
GMMC 20-22 Juin 2017 Ifremer Brest [email protected] 10
Generation of low-PV water critical to the formation of dense water
Dense Waters characterized by Low PV
q=(ζ+ f )N 2⏟qv
−f (∂U g
∂ z )2
⏟qh
Role of the Wind in PV-Destruction in Frontal region ?
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Destruction of Potential Vorticity :Implications for the Formation of Deep Water
PV destruction: Wind oriented down-front Buoyancy loss from the ocean to the atmosphere PV-Flux Divergence changes the Stratification
k
qUJ
z
BfJ
kzz
UfJ
Adv
Diab
gFric
)(
PV-FluxJt
q .
Haynes and McIntyre, 1987
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PV Destruction
Dense Water Formation
Feb 2-4 Feb 6-14 Feb 23-26
PV budget on the volume bounded by > 29.0 kg/m3
Feb 2-4 Feb 6-14 Feb 23-26
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PV Budget DWF
N2 Budget
Connection PVdestruction - DWF
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PV budget in the Frontal Zone
JFlux DWF
JDiab JFric
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A
B
Vertical PV-Advection in the Frontal Section [A-B]
2-4 Feb. 6-14 Feb. 23-26 Feb.
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Western Frontal Section February
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Sentinel-2ABarents
MARS-MENORΔx=400 m(P. Garreau)
Cold IntrusionsSymmetric Instability
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Sentinel-2ABarents
MARS-MENORΔx=400 m(P. Garreau)
Cold IntrusionsSymmetric Instability
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Ug
Wind
Uag Uag
kzz
UfJ g
Fric
kz
BfJDiab
)(
Conceptual Scheme
Vertical cell and turbulence act as a PV pump drawing high PV from the pycnocline to limit the frictional and diabatic PV-destruction at the surface (Giordani et al., 2017)
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PV in TIW (Giordani et al., 2015)
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Intra-Thermocline bolus/eddy of low-PV
J fric=− f∂ U g
∂ z∂ τ∂ zk
Uag
frontal zone
MLD
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Intra-Thermocline bolus/eddy of low-PV
kzz
UfJ g
fric
agU
frontal zone
MLD
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NAC & EDW Formation – Mazé et al., 2011, 2013
Bolus of low-PV
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NAC – EDW Mazé et al., 2011, 2013
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Kuroshio – STMW Rainville et al.
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Kuroshio – STMW Rainville et al.
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Conceptual Scheme
Jf,Jb q<0 ageostrophic circulation & turbulence
Eddy
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2
2
.5000
.500
mWy
bM
mWF
e
Batm
Subduction
y
bMFF e
Batm
Beff
Conditions favorable à la destruction de EPV par la tension de surface: conséquences énergétiques des « cold-air outbreaks »
dans les grands systèmes frontaux(Joyce et al., 2010)
Destruction de PV
⊙vent & courant
eM
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EnergeticsEquivalent Buoyancy Flux
(Giordani et al., 2017)
op
diab
gpfric
RBg
CEBF
z
U
g
CEBF
10
0
Friction
Diabatic
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EnergeticsEBFfric EBFdiab
Buoyancy Flux
• Appropriate x to resolve energy trapping in fronts and eddies
• Parameterize EBF in climate models ?
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Sentinel-1ABaltique
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Sentinel-2ABarents
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SST gradients at small-scale are as energetic as large scales(Held et al., 95; Lapeyre & Klein, 06; Tulloch & Smith, 09)
Lapeyre & Klein, 06
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Vertical Velocity – February 7
NEMO EKMAN
WQ-FLX
Strain field associated with the flux-structures interactions induces intense vertical motion
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Conclusions
ASICS-MED: alternative approach based on the PV-flux to estimate the processes of DWF along the cyclonic gyre in the WestMed
PV-Budget : Turbulence and ageostrophic cell act as PV pump drawing high PV water from the pycnocline to limit the frictional and buoyancy PV-destruction at the surface. Horizontal Advection also limits the PV-destruction at the surface
Frictional and diabatic EBF result from wind and submesoscale interactions (Scale selection in OA coupling) and involve KOLOSSAL energy exchanges at the surface
OA coupling is not limited to surface fluxes
How to restore the dynamics of the oceanic deserts in models ? -> NH-Model CROCO