A MODEL OF TROPICAL OCEAN-ATMOSPHERE INTERACTION
Post on 07-Feb-2016
30 Views
Preview:
DESCRIPTION
Transcript
A MODEL OF TROPICAL OCEAN-ATMOSPHERE INTERACTION
Elsa NicklAndreas Münchow
Julian Mc Creary, Jr.
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
HADLEY CIRCULATION
WALKER CIRCULATION
Positive feedback with ocean
MODEL DESCRIPTION:
MODEL OCEAN
Baroclinic mode of a two-layer ocean ~ gravest baroclinic mode of a continuosly stratified ocean
h
Linear equations:
x-mom: ut - yv + px = F + h²uy-mom: vt - yu + py = G + h²v
continuity: pt/c² + ux +vy =0
+
F =x /HG =y /Hh= H + p/g’
Model oceanModel atmosphereAdjustment to equilibriumOscillation conditions
= 2x10-11 m-1s-1
H =100mg’ =0.02 ms-2
h = 104 m2s-1
c=2.5 m/s
Parameters:p=g’ (h-H)
pt= (g’ (h-H)) = g’ h = wc² c²t g’ H t H
MODEL OCEAN
Thermodynamics parametrization:
warm, h>=hcSST cool, h< hc
hc: upwelling along equator and eastern boundary (unspecified)
OCEAN REGION: Tropical Pacific
0 D (10,000km)
EQ (0)
-L (4500km)
L (4500km)
Boundary conditions:
u = v= 0 at sidewallsuy = v = 0 at equator
Solutions for these conditions: Northern Hemisphere (Gent and Semter, 1980)
MODEL ATMOSPHERE
h: strenghtened HCw: well developed WCb: steady Pacific trade winds
Wind field equations (3 patches of zonal wind stress):
xh: 7500 km xw=xb: 5000 km
MODEL OCEAN
Conditions:
D =10,000 km = h =3000 km
HC:
WC:
h
w
Near equilibrium ‘h’ in response to h and w (solutions in Sverdrup balance)
h
w h >100m
h >100m
ADJUSTMENT OF OCEAN MODEL TO EQUILIBRIUM
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 equatorfarther from equator
Equatorial winds: rapid adjustmentt = 4x/c t ~ 6 months
Extra-equatorial winds: gradual adjustmentt = (xy²)/c²For minimum curl region related with h : t~ 4 years
• 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
ADJUSTMENT OF OCEAN MODEL TO EQUILIBRIUM
OSCILLATION CONDITIONS
1. WC positive feedback
hw < H
Initially he < H (SST cold in eastern ocean)WC switches onIf hw < H holds ocean will adjust so he is even
shallower
condition
2. Requires HC, system does not reach equilibrium
hbh < hc < hbw condition
feedback
hbh = equilibrium depth at eastern boundary in response to b and w
hbw = equilibrium depth at eastern boundary in response to b and w
(model can never reach a state of equilibrium)
Initially he > hc (SST warm in eastern ocean)HC switches onhe adjusts to hbh hbh < hc (SST cold in eastern ocean)HC swithces off
Condition hh < hw puts severe limits for oscillation
It is required that h raises the model interface (h smaller) In eastern ocean more than w does
RESULTS
THE MODEL SOUTHERN OSCILLATION
• Presence of a 4-year period oscillation
w on
h off
w off
won
just before w switches on
10 months later
THE MODEL SOUTHERN OSCILLATION
During onset of El Niño event
just before w switches off
2 months later
2 months later
THE MODEL SOUTHERN OSCILLATION
During decay of El Niño eventTHE MODEL SOUTHERN OSCILLATION
2 months later
2 months later
top related