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

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 DESCRIPTION: MODEL OCEAN Baroclinic mode of a two-layer ocean ~ gravest baroclinic mode of a continuosly stratified ocean  h Linear equations: x-mom: u t -  yv + p x = F + h  ²u y-mom: v t -  yu + p y = G + h  ²v continuity: p t /c² + u x +v y =0  +  F =  x /H G =  y /H h= H + p/g’ Model ocean Model atmosphere Adjustment to equilibrium Oscillation conditions  = 2x10 -11 m -1 s -1 H =100m g’ =0.02 ms -2 h = 10 4 m 2 s -1 c=2.5 m/s Parameters: p=g’ (h-H)p t =  (g’ (h-H)) = g’  h =  w c² c²  t g’ H  t H

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

6. Boundary conditions: u = v= 0 at sidewalls u y = v = 0 at equator Solutions for these conditions: Northern Hemisphere (Gent and Semter, 1980) MODEL ATMOSPHERE  h: strenghtened HC  w: well developed WC  b: steady Pacific trade winds Wind field equations (3 patches of zonal wind stress): x h: 7500 kmx w =x b: 50 00 km MODEL OCEAN

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

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

9. 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. Sverdrup balance: good approximation for equilibrium state p: constant to be determined p is related to h: h= H + p/g’ h: equilibrium thickness at eastern boundary ADJUSTMENT OF OCEAN MODEL TO EQUILIBRIUM

11. OSCILLATION CONDITIONS 1.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 2. Requires HC, system does not reach equilibrium h bh < h c < h bw condition feedback h bh = equilibrium depth at eastern boundary in response to  b and  w h bw = equilibrium depth at eastern boundary in response to  b and  w (model can never reach a state of equilibrium)

12. Initially he > hc (SST warm in eastern ocean) HC switches on he adjusts to h bh h bh < hc (SST cold in eastern ocean) HC swithces off Condition h h < h w 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  h off  w off  w on

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

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

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