Deep water island wakes

Deep water island wakes
Baroclinic case
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1
Model configuration
ROMS – Regional Ocean Modeling System
Analytical prescribed conditions
http://www.myroms.org/
Hyperbolic function incoming flow:
um1 −surface current
um2−bottom current
h s−shear layer thickness
hd −central layer thickness
Upstream density:
 −1/2 density difference hc −thermoclinecentral depth
ht −thermoclinethickness
Surface elevation:
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Geostrophic balance
2
Baseline experiment
x=180km; y=80km
➔
Island => x=45; y=40
➔
D=20 km
➔
Hmax=k=500m
➔
Coriolis=K=10^-4
➔
Eddy viscosity=0
➔
Umax=0.2; Umin=0.1 m/s
➔
Ro=0.1
➔
N~2Coriolis
➔
Rd=20km
➔Bu~1
➔Ri=0.25
➔
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Baseline experiment
Eddies life cycles:
Asymmetry @ N-S edge
➔
S => wavy cyclonic eddy pattern
➔
N => small extrema + noise
➔
Anticyclonic instability => centrifugal
➔
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Baseline experiment
Weak flow @ depth
➔
10m
Wake 2D @ 300m
➔
Cyclonic vort. max @ 10m => 0-150m
➔
100m
Anticyclonic vort. Max 130m
➔
200m
300m
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Wake vertical structure
9 day average => full A-C cycle
(a) South (left) wake
deflection
Assymetry C-A
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Island vorticity generation
Volume integrated vorticity (from vertical vorticity equation):
VG => steady in time
VG => ~symmetric C-A
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Wake instability
Types of instabilities:
lateral shear
asymmetry C-A => centrifugal instability
PE => baroclinic instability (L>>Rd)
Eddy evolution:
f
Centrifugal instability => Changes +/- f
ening
rapid weak
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Wake instability
Volume integrated
mean Ke
eddy Ke > 1
mean Pe
eddy Ke
>1
=> barotropic instability
=> baroclinic instability
Vertical integrated KE & PE conversions:
Max barotropic conversion @ L (max KE)
Max. (-ive) baroclinic conversion
=>Dominant Cyclonic region
max KE > max PE => barotropic conversion!
Eddy
gen
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9
Re study
Implicit viscosity
Re=20
Re=800
Re=20 symmetric eddies
Re~100 detachment
Re=30
Re=1600
Re=30
Re=3200
Re=40
Re=100
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Re=6400
Re=inf.
10
Re study
Lateral boundary layer thickness
−
Power law Re
Island
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Bu study
2
 
Rd
Bu=
L
 
NH
Bu=
f
N=

−g ∂ 
0 ∂ z
Baroclinic instability theory:
L>>Rd => Eddy size >>Rd
Weakens anticyclonic eddies
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Ro/Bu study
Symmetry
Asymmetry
Re=400
Anticyclones
dominate
Centrifugal instability
Re=∞
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Cyclones
dominate
13
Southern California Bight study
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Model configuration
ROMS – Regional Ocean Modeling System
http://www.myroms.org/
ROMS setup:
Numerical experiments:
dx=dy=1km
40 z levels
Orlanski =>tangential flow
Flather=>normal flow
To=>nudging 1996-2003 ROMS
MM5 wind forcing (54/18/6/2km)
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Island induced vorticity
Decrease eddy activities
2
28.9% decrease in enstrophy 
Upwelling event
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Channel Island wakes
Island - gaps
Large current shear
Eddy shedding
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Velocity profile
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San Nicolas Island wake
Wake asymmetry
Centrifugal instability
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19
Santa Catalina Island wake
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