METEO 6030 Earth Climate System El Nino Southern

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

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

Outline v Delayed Oscillator associated with ENSO - Kelvin wave - Rossby wave - Bjerknes hypothesis v Summary 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 Land area Ocean 2. Only vertical temperature variations (idealized thermocline) 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 Department of Meteorology, U O U

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

Kelvin and Rossby Waves Kelvin wave Rossby wave Eastward wind stress forcing Acting to increase depth of warm water layer (equator) mass surplus Dispersing eastward – Kelvin wave (downwelling) Lat *downwelling : deepen thermocline mass-deficit area Lon <Ocean surface height anomaly (cm)> 180 140 W Rossby wave cm Kelvin wave cm Lat <Ocean surface height anomaly (cm)> Propagate westward – Rossby wave (upwelling) *upwelling : shallow thermocline Kelvin wave : positive - Maximum at equator Rossby wave : negative - Symmetric and minimum at equator Source : IRI, http: //iri. columbia. edu/index. html Department of Meteorology, U O U

Evolution of Kelvin and Rossby Waves Rossby wave Lat Kelvin wave 25 days 125 days 50 days 175 days 75 days 225 days 100 days 275 days Lon <Time evolution for the idealized experiment> Wave signals propagating at different speeds Kelvin wave (70 days) traveling eastward (idealized case : 2. 9 m/s) extending from 120°East to 80°West Rossby wave (210 days) propagating westward traveling westward at 1/3 speed of the Kelvin wave Source : IRI, http: //iri. columbia. edu/index. html 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 thermocline • Equatorial Kelvin waves travel 3 times faster than the equatorial Rossby waves Department of Meteorology, U O U

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

Summary Ø Delayed Oscillator associated with ENSO § Equatorial ocean waves show a mechanism to reverse phase of perturbations to 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

Questions? Thank you