A MODEL OF TROPICAL OCEAN-ATMOSPHERE INTERACTION

1. A MODEL OF TROPICAL OCEAN-ATMOSPHERE INTERACTION Julian Mc Creary, Jr. Elsa Nickl Andreas Münchow

2. 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

3. HADLEY CIRCULATION WALKER CIRCULATION Positive feedback with ocean

4. 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 c=2.5 m/s Parameters: p=g’ (h-H) pt=  (g’ (h-H)) = g’ h = w c² c²t g’ H t H

5. MODEL OCEAN Thermodynamics parametrization: warm, h>=hc SST cool, h< hc hc: upwelling along equator and eastern boundary (unspecified) OCEAN REGION: Tropical Pacific L (4500km) EQ (0) -L (4500km) 0 D (10,000km)

6. 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 xw=xb: 5000 km

7. Conditions: D =10,000 km  = h =3000 km HC: h WC: w

8. ADJUSTMENT OF OCEAN MODEL TO EQUILIBRIUM h h >100m w h >100m Near equilibrium ‘h’ in response to h andw (solutions in Sverdrup balance)

9. 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

10. 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

11. OSCILLATION CONDITIONS • WC positive feedback • hw < H • Initially he < H (SST cold in eastern ocean) • WC switches on • If hw < H holds ocean will adjust so he is even shallower condition feedback 2. Requires HC, system does not reach equilibrium hbh < hc < 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

12. Initially he > hc (SST warm in eastern ocean) • HC switches on • he 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

13. RESULTS THE MODEL SOUTHERN OSCILLATION • Presence of a 4-year period oscillation w on w off h off won

14. THE MODEL SOUTHERN OSCILLATION just before w switches on 10 months later

15. THE MODEL SOUTHERN OSCILLATION During onset of El Niño event just before w switches off 2 months later 2 months later

16. THE MODEL SOUTHERN OSCILLATION During decay of El Niño event 2 months later 2 months later