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Ocean energetics in GCMs: how much energy is transferred from the winds to the thermocline on ENSO...
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Transcript of Ocean energetics in GCMs: how much energy is transferred from the winds to the thermocline on ENSO...
Ocean energetics in GCMs: how much energy is transferred from the winds to
the thermocline on ENSO timescales?
Alexey Fedorov (Yale)Jaci Brown (CSIRO) and Eric Guilyardi (IPSL)
Funded by DOE, NSF, CNRS
Brown J., Fedorov, A.V., and Guilyardi, E., 2010: How well do coupled models replicate ocean energetics relevant to ENSO? Climate Dynamics, in press
Brown J. and Fedorov, A.V., 2010: How much energy is transferred from the
winds to the thermocline on ENSO timescales. J. Climate 23, 1563–1580. Brown J. and Fedorov, A.V., 2008: The mean energy balance in the tropical
ocean, J. Marine Research 66, 1-23. Fedorov, A.V., 2007: Net energy dissipation rates in the tropical ocean and
ENSO dynamics. J.Climate 20, 1099–1108
Fedorov, A.V., Philander, S.G., Harper, S.L., B. Winter, B. and A. Wittenberg, 2003: How predictable is El Niño? Bull. Amer. Meteorol. Soc. 84, 911-919.
Buoyancy Poweracts to displace isopycnals
Wind Power generated by wind stress acting on surface currents
Available Potential Energygenerated when isopycnals are distorted
Kinetic Energy of ocean currents
Atmosphere
Ocean
isopycnals
OCEAN ENERGETICS
Goddard and Philander 2000; Fedorov et al 2003
Questions:
What fraction of power generated by the winds reaches the equatorial thermocline on ENSO timescales?
How much damping occurs for thermocline anomalies?
Can we use the energetics of the tropical ocean to compare different coupled models?
4
A shallow-water model
5
€
E =1
2(g * h2 + Hu2 )∫∫∫ dxdydz; W = uτ dxdy∫∫
€
∂E
∂t= γW −α sE
What are and in GCMs?Fundamental question: How (well) do GCMs describe the
transfer of energy from the winds to the thermocline?
Wind stress
Surface Currents, U (m/s)
€
W = Uτ∫∫ dxdy
U=U(x,y,t) – zonal velocity(x,y,t) – zonal wind stress
6
U and - same
direction
positive wind power
negative wind power
U and - differentdirection
Wind power W is generated when winds work on ocean
currents
MOM4
Buoyancy power B controls vertical
displacements of the thermocline. It is
generated from the conversion of wind power.
€
B = g ρ − ρ *( )wdz dxdy∫∫∫
w – vertical velocityg – gravity(x,y,z,t) – densityz) – reference stratification
7
High APE, La Niña:
Low APE, El Niño:
APE (denoted as E) is generated when isopycnals
are distorted, and is proportional to the
thermocline slope along the equator!
€
E =g
2
ρ − ρ *( )2
δρ *
δz
∫∫∫ dxdydz
(x,y,z,t) – density z) –reference stratification
APE variations are highly anti-correlated with the Nino3
SST, correlation up to -0.9
El Nino of 1997
9
Integration: tropical Pacific (15oN - 15oS,130oE - 80oW, 0-400m)
10
€
dK
dt= W − B − D1
11
E – the APE
K – kinetic energy (negligible, less than 1% of E)
B – buoyancy power (describes the conversion of kinetic into potential energy
W – wind power
D1, D2 – viscous and diffusive dissipations
€
dE
dt= B − D2
Wind Power
Buoyancy Power
APE (~thermocline slope)
SST
Wind Stress
D1
D2
12
Ocean-only, data assimilating, and coupled GCMs
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Data assimilating
models
Ocean models
Coupled models
€
∂K
∂t= W − B − D10
We introduce efficiency =B/W
€
B = γW
That is, only a fraction of wind power W is converted to buoyancy power B
(our assumption)
15
Calculating efficiency
€
B = γW
16
Efficiency versus correlation between Buoyancy and Wind powers
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Characteristic ENSO period versus efficiency
€
dE
dt= B − D2
€
D2 = αE
€
dE
dt= B − αE
(-1 is the APE damping timescale
18
(our assumption)
19
Calculating damping timescale
€
D2 = αE
20
Damping timescale versus correlation between E and D2
21
Summary
Ocean energetics of ENSO is characterized by two physical parameters
- the efficiency of the energy transfer from the winds to the thermocline. Most of coupled GCMs are less efficient (=15-50%) than ocean-only and data assimilating models (=50-60%).
- the APE damping rate. Most of coupled GCMs produce shorter damping timescales (-1=0.4-1 year) than ocean-only and data assimilating models (-1=1-1.2 year).
The two parameters can be used as metrics for evaluating dissipative properties of the models and other model properties
22
Implications
How models describe quantitatively the transfer of energy (and momentum) from the winds down the water column is important
Efficiency 30% vs. 60% mattersEnergy e-folding damping 4 months vs. 1 year matters
Error compensation in GCMs
23
24
Decadal Variability
Wind Power Available Potential Energy
• Shift in late 1970s in Wind power and resulting APE
• Interannual variability consistent.
25
Wind Power
Buoyancy Power
APE (~thermocline slope)
SST
Wind Stress
26
Coupled models generate lower efficiency and stronger damping than ocean-only or data-assimilating models!
27Annual-mean zonal averages between160oE to 90oW
28
K – kinetic energy (small)
B – buoyancy power;
P – the rate of work of ageostrophic pressure;
AM – advection of K away from the tropics
DM – turbulent viscous dissipation; kMV, kMH – viscosities
B – buoyancy powerQ – damping by surface heat fluxesA – advection of APE away from the tropicsD – turbulent diffusive dissipation; kMV, kMH – diffusivities 31
32
33
34
Wind Power
Buoyancy Power
APE (~thermocline slope)
SST
Wind Stress
Dissipation1
Dissipation2
35
36
Win
d P
owe
r (T
W)
Buoyancy Power (TW)
Efficiency of Wind Power to Buoyancy Power Transfer
W
B 37
38
APE damping timescales -1:
ocean models and data assimilations: -1 = 1 year
coupled models: -1 = 0.5 -1 years
Correlation (between D2 and E)
39
€
dρ *
dz
Wind Power
Buoyancy Power
APE (Thermocline slope)
Sea Surface Temperatures
Wind Stress
The energetics of the tropical ocean:
D1
D2
40 E is highly anti-correlated with the SST
in the eastern equatorial Pacific (r=-0.9)!
E is highly anti-correlated with the SST in the eastern equatorial Pacific (r=-0.9)! High APE means La Niña; Low APE means El Niño
41
El Nino 1997
K – kinetic energy (small)
B – buoyancy power;
P – the rate of work of ageostrophic pressure;
AM – advection of K away from the tropics
DM – turbulent viscous dissipation; kMV, kMH – viscosities
K – kinetic energy (small)
B – buoyancy power;
P – the rate of work of ageostrophic pressure;
AM – advection of K away from the tropics
DM – turbulent viscous dissipation; kMV, kMH – viscosities
€
˜ ρ = ρ − ρ *
B – buoyancy powerQ – damping by surface heat fluxesA – advection of APE away from the tropicsD – turbulent diffusive dissipation; kMV, kMH – diffusivities 44
B – buoyancy powerQ – damping by surface heat fluxesA – advection of APE away from the tropicsD – turbulent diffusive dissipation; kMV, kMH – diffusivities 45
€
˜ ρ = ρ − ρ *
€
dρ *
dz
46
Calculating damping rates
Available Potential Energy
Dis
sipa
tion
An
om
aly
E 47
D2
48
Correlation (between D2 and E)