The ENSO : El Niño and the Southern Oscillation

J.P. Céron (Météo-France) and

R. Washington (Oxford University)

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.

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

ATMOSPHERE

Equation ofWater Vapour

ThermodynamicEquation

Radiation

Equation ofMotion

OCEAN

Salt Equation

Ice Equation

ThermodynamicEquation

Equation ofMotion

Precipitation

Evaporation Heat Surface Stress

Radiative transfer

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.

The importance of wind forcing

The importance of Solar forcing and evaporation

fluxes

The relationship between the SST forcing

and the clouds

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 …)

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

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

The ENSO (1)

El Niño 97

Surface Temperature

Dynamical Height

Thermocline depth

The ENSO (2) Surface Oceanic currents

The ENSO (2)

The ENSO (2)

The ENSO (2)

The ENSO (3) The deep Oceanic structure and its evolution

The ENSO (3) The deep Oceanic structure and its evolution

The ENSO (3) The deep Oceanic structure and its evolution

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

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

The ENSO (5) Southern Oscillation description

Tahiti

Darwin

The ENSO (5) Southern Oscillation

description

SEP 97

NOV 88

The ENSO (6) Influence over the Pacific El Niño year Normal year La Niña year

winter spring

The ENSO (6)

Influence over the Pacific

The ENSO (6)

Influence over the Pacific

The ENSO (6)

Influence over the Pacific

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

The ENSO (8) The mid-latitude influence

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

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

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

The ENSO (10) Los Niños y Las Niñas

Time evolution comparison of the 7 strongest Niños

The ENSO (10) Los Niños y Las Niñas

Time evolution comparison of the 7 strongest Niñas

The ENSO The predictability

The E N S O

The ENSO

The ENSO

The ENSO

The ENSO

The ENSO Influence over MJO

The decadal climatic variability The decadal Pacific oscillation

Resume the ENSO The variability of the

Ocean/Atmosphere system

The coupled modelisation