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