The ENSO : El Ni ñ o and the Southern Oscillation

1. Fundamentals of Seasonnal forecasting The ENSO : El Niño and the Southern Oscillation J.P. Céron (Météo-France) and R. Washington (Oxford University)

2. The fundamentals (1) • Mean circulation in Tropical regions deeply influenced by large scale organised convection (Hadley-Walker divergente circulations), • Energetic exchanges mainly driven by the mean circulation in tropical regions, • Mean circulation mainly driven by the evolution of boundary conditions (particularly SST but also continentale surface conditions), • Feedbacks and coupling processes between the atmospheric circulation and the boundary conditions at the surface.

3. The fundamentals (2) • Ocean/atmosphere • Surface fluxes (sensible, latent, momentum) • Ocean/atmosphere interactions • Convergences/divergences + wind stress (coupled dynamic) • Ekmann Transport • Coupling processes • Oceanic circulation • Surface and sub-surface currents • Upwelling, downwelling

4. ATMOSPHERE Thermodynamic Equation Equation of Water Vapour Equation of Motion Radiation Evaporation Heat Surface Stress Precipitation Radiative transfer Salt Equation Thermodynamic Equation Equation of Motion Ice Equation OCEAN

7. Ekman Transport • Wind friction effect at the surface (wind stress), • Action of the Coriolis force on the oceanic fluide, • Resulting Ekman transport, right to the wind in the Northern hemisphere and left in the Southern hemisphere, • Coupling between vertical and horizontal motions through the continuity equation.

13. The importance of wind forcing

14. The importance of Solar forcing and evaporation fluxes

15. The relationship between the SST forcing and the clouds

16. Historical chronology • Phase opposition in the surface pressure pointed out at Sydney and Buenos Aires (Hildebrandsson – 1897) • Southern Oscillation (P,T,RR) – G. Walker (~1920) • Relationship between Pacific SST and SO (Bjerkness – 1966, 1969 – discovering the Teleconnexion notion), • « Build-up » from Wyrtki (1975-1979) • Niño composite and seasonnal phase lock – Rasmusson and Carpenter (1982), • Ocean/atmosphere coupled models (1980 ….), • Theoritical developpments (1980 …)

17. Theoritical Developpments • « Build-up » from Wyrtki • Driving of ocean conditions by atmosphere (trade wind relaxation – warm SST redistribution from West to East). • Delayed Oscillator • Rossby waves reflexion in Kelvin waves (West boundary) conducting to a deepening of the thermocline. Kelvin waves reflexion (East boundary). Alternately warm («downwelling» kelvin wave) and cold anomalies («upwelling» kelvin wave). • Coupled Instabilities • SST - thermocline relationship (positive coupling process associated with a surface wind convergence), • Low frequency coupled Mode (periods 3 to 4 years and 6 month), • «SST mode» - SST mean gradient zonal advection by the current disturbances – Temperature vertical mean gradient advection by vertical motion disturbances – modulated vertical advection by the Mean Equatorial Upwelling. • Salt Barrier role – shifting of the Warmpool’s Eastern boundary in relationship with the SST gradient zonal advection

18. The ENSO • Ocean/Atmosphere coupling system, • ENSO main variability source of the climatic system at interannual timescales, • « El Niño/La Niña » oceanic behaviour in the Pacific, both in surface and sub-surface, • Caracteristic periods from 3 to 7 years and tendancy to alternate « El Niño/ La Niña » events. • Planetary influence through general circulation modifications. Teleconnections. • Oceanic behaviour partly predictable (Starting up causes of the process?) • Interactions ENSO/PDO, ENSO/MJO, MJO/Synoptic

19. The ENSO (1) El Niño 97 Surface Temperature Dynamical Height Thermocline depth

20. The ENSO (2) • Surface Oceanic currents

21. The ENSO (2)

22. The ENSO (2)

23. The ENSO (2)

24. The ENSO (3) • The deep Oceanic structure and its evolution

25. The ENSO (3) • The deep Oceanic structure and its evolution

26. The ENSO (3) • The deep Oceanic structure and its evolution

27. The ENSO (4) • The Ocean/Atmosphere coupling process In the Western Pacific (156°E) In the Central Pacific (155°O)

28. The ENSO (4) • The Ocean/Atmosphere coupling process In the Central Pacific (155°O) In the Eastern Pacific (95°O)

29. The ENSO (5) • Southern Oscillation description Darwin Tahiti

30. The ENSO (5) • Southern Oscillation description

35. SEP 97

36. NOV 88

37. The ENSO (6) • Influence over the Pacific El Niño year Normal year La Niña year winter spring

38. The ENSO (6) • Influence over the Pacific

39. The ENSO (6) • Influence over the Pacific

40. The ENSO (6) • Influence over the Pacific

41. The ENSO (7) • The planetary influence of El Niño (left) and La Niña (right)

42. The ENSO (8) • The mid-latitude influence

43. The ENSO (9) • El Niño influence over Africa (Boreal winter)

44. The ENSO (9) • El Niño influence over Africa (Boreal summer)

45. The ENSO (9) • La Niña influence over Africa (Boreal summer)

46. The ENSO (10) Time evolution comparison of the 7 strongest Niños • Los Niños y Las Niñas

47. The ENSO (10) Time evolution comparison of the 7 strongest Niñas • Los Niños y Las Niñas

48. The ENSO • The predictability

49. The E N S O

50. The ENSO