Embed Size (px)
Citation preview
On the Dynamics of Oceanic GravityCurrents
Achim Wirth
LEGI / CNRS
LEGOS, Dec 6, 2011
Ocean Circulation
Gyre“weather”
Overturning“climat”
Ocean Dynamics by Scale
-k
energy injection
(107m)−1 (105m)−1 (10m)−1 (10−2m)−1
energy dissip
Ocean Dynamics by Scale
-k
energy injection
(107m)−1
inverse energy cascade
(105m)−1
bc/bt inst.
geostrophy QG turbulence ? ? ? ? ? strat. rot. turb.
(10m)−1
turb. 3D
(10−2m)−1
energy dissip
? ? ? ? ? ? direct energy cascade
Ocean Dynamics by Scale
-k
energy injection
(107m)−1
inverse energy cascade
(105m)−1
bc/bt inst.
geostrophy QG turbulence ? ? ? ? ? strat. rot. turb.
(10m)−1
turb. 3D
(10−2m)−1
energy dissip
? ? ? ? ? ? direct energy cascade
geophys. fluid dyn. turbulence
Ocean Dynamics by Scale
-k
energy injection
(107m)−1
inverse energy cascade
(105m)−1
bc/bt inst.
geostrophy QG turbulence ? ? ? ? ? strat. rot. turb.
(10m)−1
turb. 3D
(10−2m)−1
energy dissip
? ? ? ? ? ? direct energy cascade
convectiongravity currentinteract topographyfrontfilamentint. waves..........
geophys. fluid dyn. turbulence
small scale processes
Ocean Dynamics by Scale
-k
energy injection
(107m)−1
inverse energy cascade
(105m)−1
bc/bt inst.
geostrophy QG turbulence ? ? ? ? ? strat. rot. turb.
(10m)−1
turb. 3D
(10−2m)−1
energy dissip
? ? ? ? ? ? direct energy cascade
convectiongravity currentinteract topographyfrontfilamentint. waves..........
geophys. fluid dyn. turbulence
small scale processes
Gravity Current
••
•• •
•
••
Geostrophy
������α
��
��
��
��
��
��
���
�� F ′
g
F ′
c
������������������������������
-x
α
⊗-
?
���9���:
Fg
Fc
F ′
gF ′
cF ′
c
u =g′
ftan α
Forced Geostrophy
vein
friction layer
Ekman layer
Forced Geostrophie
veine
friction layer
Ekman layer
Forced Geostrophy
vein
friction layer
Ekman layer
Heat Equation
∂th = −∂xUEk =δ
2∂xvgeo =
δg′
2f∂xxh = ∂x (κH∂xh)
24h 60h
Friction... determines the dynamics of oceanic gravity currents.
The frictional processes can not be explicitely representedin today’s (and tomorrow’s) ocean models
τ + cD|u|
linear Rayleigh friction (τ ) quadratic drag law (cD)
Initial Conditions
Temp Vg
Reference Exp. (2D)
Z σ
Grid σ
Convective Adjustment and Classic (2D)
2+4+3 Levels (2D)
Down-slope transport
2.5D 3D
Conclusions A
◮ The vertical resolution is key to correctly represent oceanicgravity currents.
◮ A few σ (< 6) levels in the bottom layer are sufficient.
◮ The refinement of the vertical resoltuion at the bottom ismore important than at the surface.
◮ These results are NOT restricted to gravity currents.
How do we find REAL bottom-friction
◮ Optimist : Study non-hydorstatic PBL-dynamics.◮ Pessimist : Use data-assimilation to determine friction
parameters from obs. (that we do not have).
Non-hydrostatic simulation :
HAROMOD
Coherent Structures
Coherent Structures
Coherent Structures
?roughness of the ocean floor ?
variability of roughness ?
multiscale roughness (bio) ?
roughness type “k” vrs. “d” ?
orientation of roughness elements ?
suspension of sediments ?
tidal currents ?
waves ?
retroaction of currents on roughness ?
And : “The matter is far from beeing understood” Jiménez, Ann.Rev. Fluid Mech. (2004).
Friction... determines the dynamics of oceanic gravity currents.
The frictional processes can not be explicitely representedin today’s (and tomorrow’s) ocean models
τ + cD|u|
linear Rayleigh friction (τ ) quadratic drag law (cD)
Data Assimilation :Estimation of parameters and friction laws :detetction of transition from linear to quadratic law.
0 500 1000 1500 2000 2500Re
0
5
10
15
C_D
(10
^-4)
c̃D = cD +τ
|u|
cD
Conclusions
Convection
Gravity Current
Conclusions & Perspectives
Convection
Gravity CurrentDWBC
?m
ixin
g?
mixed layer, air-sea interaction
