El Nino Southern Oscillation (ENSO) 20 April 06 Byoung-Cheol Kim METEO 6030 Earth Climate System

Department of Meteorology, U O U Why the ENSO Occurs?

Outline  Delayed Oscillator associated with ENSO - Kelvin wave - Rossby wave - Bjerknes hypothesis  Summary Department of Meteorology, U O U

Advanced ENSO Theory: The Delayed Oscillator - Assumption : Department of Meteorology, U O U 3. No horizontal pressure variations 4. Only vertical pressure variations (balance between changes pressure gradient force and the gravity, dependent only on density) Evolution of disturbances key processes associated with delayed oscillator theory Land area Ocean -Wakata and Sarachik 1991, Philander et al 1984, and Zebiak and Cane 1987 Source : IRI, 1. No horizontal density and no temperature variations 2. Only vertical temperature variations (idealized thermocline)

Perturbing the Ocean Department of Meteorology, U O U Motionless state perturbed by turning on an isolated patch of eastward wind stress near the equator (kept constant for 30 days) wind-stress - Max : equator wind-stress curl - Negative : SH - Positive : NH Response of a wind-stress forcing : influenced by the Coriolis Force Near-surface ocean : balance between wind stress forcing and the Coriolis Resulting in net transport : to the right of the direction of wind stress (NH) Source : IRI, Lon Lat

Kelvin and Rossby Waves Department of Meteorology, U O U Eastward wind stress forcing Acting to increase depth of warm water layer (equator) mass surplus Dispersing eastward – Kelvin wave (downwelling) * downwelling : deepen thermocline mass-deficit area Propagate westward – Rossby wave (upwelling) *upwelling : shallow thermocline Rossby wave Kelvin wave W Source : IRI, Kelvin wave : positive - Maximum at equator Rossby wave : negative - Symmetric and minimum at equator Kelvin waveRossby wave Lat Lon Lat cm

Evolution of Kelvin and Rossby Waves Department of Meteorology, U O U Wave signals propagating at different speeds Kelvin wave (70days) traveling eastward (idealized case : 2.9 m/s) extending from 120°East to 80°West Rossby wave (210days) propagating westward traveling westward at 1/3 speed of the Kelvin wave Rossby wave Kelvin wave Source : IRI, Lon Lat 25 days 50 days 75 days 100 days 125days 175days 225days 275days

Equatorial Waves Equatorial waves in the ocean play an important role in the onset and variability of ENSO Two types: –Kelvin waves (propagate eastward along the equator and along coasts) –Rossby waves (long waves propagate westward) The relevant waves are baroclinic: internal to the ocean, propagating along the density contrast of the thermocline Equatorial Kelvin waves travel 3 times faster than the equatorial Rossby waves Department of Meteorology, U O U

Equator North Warm Deepening thermocline Western anomaly (Weak trade wind) 180 o Weakened upwelling Warming (East) Weaker trade Weakend upwelling Less entrainment cooling (eastern Basin ) Bjerknes hypothesis associated with ENSO + feedback Down  Bjerknes first to recognize the coupling between changes in the oceanic and atmospheric circulations during ENSO

Summary  Delayed Oscillator associated with ENSO  Equatorial ocean waves show a mechanism to reverse phase of perturbations to the thermocline depth  Without further wind forcing waves finally decay  Thermocline depth perturbations influence SST in the upwelling regions of central / eastern equatorial Pacific => coupling to atmosphere  Equatorial waves and Bjerknes feedback (Positive) can generate a self-sustaining oscillation Department of Meteorology, U O U

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