winter is coming: a dynamical systems approach to better...
TRANSCRIPT
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Winter Is Coming: A Dynamical SystemsApproach to Better El Nino Predictions
Andrew Roberts
Esther Widiasih, Chris Jones, Axel Timmerman
October 14, 2014
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Outline
• What is ENSO?
• Why do we care?
• Conceptual ENSO Models
• Dynamical Systems
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The Data
3/26
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2014 PredictionsAugust:
September: No ENSO this yearOctober 9: El Nin0 watch back on. Chances back over 65 % (NOAA).
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Physical Cartoon
From: NOAA 5/26
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Jin’s Model - 1997
Recharge Oscillator Model:
dTE
dt= RTE + γhW − en(hW + bTE )3
dhWdt
= −rhW − αbTE .
Variables:
• TE temperature anomaly in E Pacific
• hW thermocline depth anomaly in W Pacific
Processes:
• Relaxation to mean state
• Upwelling
• Thermocline adjustment to wind-stress
6/26
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Results
7/26
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Supercharged Recharge Oscillator
dT1
dt= −α(T1 − Tr )− δµ(T2 − T1)2
dT2
dt= −α(T2 − Tr ) + ζµ(T2 − T1)[T2 − Tsub(T1,T2, h1)]
dh1dt
= −r[h1 −
bLµ
2β(T2 − T1)
]Modifications from Recharge Oscillator model:
• Variables are no longer anomalies
• Subscript 1 indicates West, 2 indicates East
• Includes advection
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Simulation vs. Data
From: Timmerman, Jin, and Abshagen – 2003 9/26
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MMOs
• Timmerman, Jin, Abshagen (2003): Shilnikov-type mechansim(saddle-focus)
• Other methods require multiple time-scales!
Change of variables:
• S = T2 − T1
• T = T1 − Tr
• h = h1 − K
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New System
dS
dt= −αS + δµS2 + ζµS [S + T + Cf (S ,T , h)]
dT
dt= −αT − δµS2
dh
dt= −r
(h − K +
bLµ
2βS
)We want to analyze as a fast/slow system → need to non-dimensionalize!
11/26
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Dimensionless System
x ′ = ε(x2 − ax) + x
[x + y − nz + d − c
(x − x3
3
)]y ′ = −ε(ay + x2)
z ′ = m(k − z − x
2
)• x ∼ S
• y ∼ T
• z ∼ h
• ε = δ/ζ
• c = bLµ(Tr−Tr0)2h∗β
• a = αbLδβh∗
• m = rbLζβh∗
• n, k, d come from f
12/26
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Dimensionless System
x ′ = ε(x2 − ax) + x
[x + y − nz + d − c
(x − x3
3
)]y ′ = −ε(ay + x2)
z ′ = m(k − z − x
2
)• If ε is small (how small?), we have a fast/slow system.
• If m is also small, we have 1-fast, 2-slow OR 3 time-scale system.
• If m = O(1), we have a 2-fast, 1-slow system.
εx = ε(x2 − ax) + x
[x + y − nz + d − c
(x − x3
3
)]y = −(ay + x2)
εz = m(k − z − x
2
)13/26
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Fast and Slow Dynamics
If we take the limit as ε→ 0, the two versions become: (1) The layerproblem
x ′ = x
[x + y − nz + d − c
(x − x3
3
)]z ′ = m
(k − z − x
2
)y ′ = 0
Or (2) The reduced problem
0 = x
[x + y − nz + d − c
(x − x3
3
)]0 = m
(k − z − x
2
)y = −(ay + x2)
14/26
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The Critical Manifold (ε = 0)
Figure: a = 0.4, c = 4, d = 3.69, k = 1,m = 0.26, n = 2.69
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The Critical Manifold (ε = 0)
Figure: a = 0.4, c = 4, d = 3.69, k = 1,m = 0.26, n = 2.69
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The Critical Manifold (ε = 0)
Figure: a = 0.4, c = 4, d = 3.69, k = 1,m = 0.26, n = 2.69
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The Critical Manifold (ε = 0)
Figure: a = 0.4, c = 4, d = 3.69, k = 1,m = 0.26, n = 2.69
18/26
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The Critical Manifold (ε = 0)
Figure: a = 0.4, c = 4, d = 3.69, k = 1,m = 0.26, n = 2.69
19/26
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The Critical Manifold (ε = 0)
Figure: a = 0.4, c = 4, d = 3.69, k = 1,m = 0.26, n = 2.69
20/26
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The Critical Manifold (ε = 0)
Figure: a = 0.4, c = 4, d = 3.69, k = 1,m = 0.26, n = 2.69
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The Critical Manifold (ε = 0)
Figure: a = 0.4, c = 4, d = 3.69, k = 1,m = 0.26, n = 2.69
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MMO Orbit: ε = 0.001
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MMO Orbit: ε = 0.1
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Re-dimensionalized Model Output
0 20 40 60 80 100 120 140 160 180 20022
23
24
25
26
27
28Cubic Approximation−Dimensionalized
T1
T2
0 20 40 60 80 100 120 140 160 180 20060
70
80
90
100
h
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Consequences in the US
From: NOAA26/26