El Nino Southern Oscillation (ENSO) 20 April 06 Byoung-Cheol Kim

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

2. Department of Meteorology, U O U Why the ENSO Occurs? http://www.atmos.albany.edu/

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

4. Department of Meteorology, U O U Advanced ENSO Theory: The Delayed Oscillator - -Wakata and Sarachik 1991, Philander et al 1984, and Zebiak and Cane 1987 < Idealized Pacific Ocean basin domain > Assumption : 1. No horizontal density and no temperature variations 2. Only vertical temperature variations (idealized thermocline) Land area Ocean 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 Source : IRI, http://iri.columbia.edu/index.html

5. Department of Meteorology, U O U Perturbing the Ocean < Zonal wind-stress anomaly > • Motionless state • perturbed by turning on an isolated patch of • eastward wind stress near the equator • (kept constant for 30 days) Lat Lon <Zonal wind-stress anomaly (175W)> <wind-stress curl anomaly (175W)> wind-stress - Max : equator wind-stress curl - Negative : SH - Positive : NH Lat Lat • 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, http://iri.columbia.edu/index.html

6. Department of Meteorology, U O U Kelvin and Rossby Waves 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 Kelvin wave Rossby wave Lat Lon <Ocean surface height anomaly (cm)> 180 140W Rossby wave Kelvin wave Kelvin wave : positive - Maximum at equator Rossby wave : negative - Symmetric and minimum at equator cm cm Lat Lat <Ocean surface height anomaly (cm)> Source : IRI, http://iri.columbia.edu/index.html

7. Department of Meteorology, U O U Evolution of Kelvin and Rossby Waves 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 125days 25 days 50 days 175days Lat 75 days 225days 100 days 275days Lon <Time evolution for the idealized experiment> Source : IRI, http://iri.columbia.edu/index.html

8. Department of Meteorology, U O U 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

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

10. Department of Meteorology, U O U 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

11. Questions? Thank you