A MODEL OF TROPICAL OCEAN-ATMOSPHERE INTERACTION

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A MODEL OF TROPICAL OCEAN-ATMOSPHERE INTERACTION
Julian Mc Creary, Jr.
Elsa Nickl Andreas Münchow
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• OBJECTIVE
• A coupled ocean-atmosphere model is used to
  simulate long time scales systems like the
  Southern Oscillation (SO)
• HYPOTHESIS
• The interaction ocean-atmosphere forms a coupled
  system with scale of 2-9 years
• Atmospheric models rapid adjustment to a SST
  change
• Ocean models react radiating baroclinic Rossby
  waves
• The model takes account the atmosphere-ocean
  interaction suggested by Bjerknes (1966) for the
  Tropical Pacific
• Hadley and Walker circulation

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HADLEY CIRCULATION
WALKER CIRCULATION
Positive feedback with ocean
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Model ocean Model atmosphere Adjustment to
equilibrium Oscillation conditions
MODEL DESCRIPTION
MODEL OCEAN
h
Baroclinic mode of a two-layer ocean gravest
baroclinic mode of a continuosly stratified ocean
?
? ??
Linear equations x-mom ut - ?yv px F
?h?²u y-mom vt - ?yu py G
?h?²v continuity pt/c² ux vy 0
F ?x /H G ?y /H h H p/g

• 2x10-11 m-1s-1
• H 100m
• g 0.02 ms-2
• ?h 104 m2s-1
• c2.5 m/s

Parameters
pg (h-H)
pt ? (g (h-H)) g ?h ?w c² c²?t
g H ?t H
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MODEL OCEAN
Thermodynamics parametrization
warm, hgthc SST cool, hlt hc
hc upwelling along equator and eastern
boundary (unspecified)
OCEAN REGION Tropical Pacific
L (4500km)
EQ (0)
-L (4500km)
0
D (10,000km)
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MODEL OCEAN
Boundary conditions u v 0 at sidewalls uy
v 0 at equator
Solutions for these conditions Northern
Hemisphere (Gent and Semter, 1980)
MODEL ATMOSPHERE
Wind field equations (3 patches of zonal wind
stress)
?h strenghtened HC
?w well developed WC
?b steady Pacific trade winds
xh 7500 km
xwxb 5000 km
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Conditions
D 10,000 km ? ?h 3000 km
HC
?h
WC
?w
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ADJUSTMENT OF OCEAN MODEL TO EQUILIBRIUM
?h
h gt100m
?w
h gt100m
Near equilibrium h in response to ?h and ?w
(solutions in Sverdrup balance)
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ADJUSTMENT OF OCEAN MODEL TO EQUILIBRIUM

• Rossby and Kelvin waves radiate from patch. The
  response of ocean to wind is basinwide
• Kelvin c
• Rossby c/3
• c c²/(? y²)

At equator
farther from equator
Equatorial winds rapid adjustment t 4x/c
t 6 months Extra-equatorial winds gradual
adjustment t (?xy²)/c² For minimum curl region
related with ?h t 4 years
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ADJUSTMENT OF OCEAN MODEL TO EQUILIBRIUM

• Sverdrup balance good approximation for
  equilibrium state

p constant to be determined
p is related to h h H p/g h H p/g
h equilibrium thickness at eastern boundary
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OSCILLATION CONDITIONS

• WC positive feedback
• hw lt H
• Initially he lt H (SST cold in eastern ocean)
• WC switches on
• If hw lt H holds ocean will adjust so he is even
  shallower

condition
feedback
2. Requires HC, system does not reach
equilibrium hbh lt hc lt hbw
condition
(model can never reach a state of equilibrium)
hbh equilibrium depth at eastern boundary in
response to ?b and ?w
hbw equilibrium depth at eastern boundary in
response to ?b and ?w
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Initially he gt hc (SST warm in eastern ocean) HC
switches on he adjusts to hbh hbh lt hc (SST
cold in eastern ocean) HC swithces off
Condition hh lt hw puts severe limits for
oscillation It is required that ?h raises the
model interface (h smaller) In eastern ocean
more than ?w does
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RESULTS
THE MODEL SOUTHERN OSCILLATION

• Presence of a 4-year period oscillation

?w on
?w off
?h off
?won
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THE MODEL SOUTHERN OSCILLATION
just before ?w switches on
10 months later
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THE MODEL SOUTHERN OSCILLATION
During onset of El Niño event
just before ?w switches off
2 months later
2 months later
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THE MODEL SOUTHERN OSCILLATION
During decay of El Niño event
2 months later
2 months later