Lecture 5 The El Nino Southen Oscillation 讲座5 El尼诺南方涛动.pptVIP

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Lecture 5 The El Nino Southen Oscillation 讲座5 El尼诺南方涛动.ppt

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Lecture 5 The El Nino Southen Oscillation 讲座5 El尼诺南方涛动

* Section 2.2.1 (ii) ENSO - Theory How does the phase of ENSO reverse? What triggers an El Nino event? Power Spectrum What determines the 4-year periodicity? Equatorial Waves Equatorial waves in the ocean are believed to play an important role in the onset and variability of ENSO Two types: Kelvin waves (propagate eastward along the equator and also 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 fastest equatorial Rossby waves How does the phase of ENSO reverse? Battisti and Hirst, 1989; Suarez and Schopf, 1988 Thermocline depth Delayed Oscillator Theory Westerly winds force downwelling on Equator and upwelling to North and South = Excites Kelvin and Rossby waves Figures from IRI: /climate/ENSO/theory/ Delayed Oscillator Theory 2 25 days 100 days 50 days 75 days Upwelling Rossby waves Downwelling Kelvin wave Delayed Oscillator Theory 3 125 days 275 days 175 days 225 days Phase has reversed Delayed Oscillator Theory 4 Equatorial ocean waves offer a mechanism to reverse the phase of perturbations to the thermocline depth Without further wind forcing waves eventually decay Thermocline depth perturbations influence SST in the upwelling regions of central / eastern equatorial Pacific = coupling to atmosphere Bjerknes feedback + equatorial waves can generate a self-sustaining oscillation discharging discharged recharged Recharge/Discharge theory (Jin, 1997) Prior to El Nino heat content in equatorial region builds up During El Nino heat is “discharged” eastward and polewards Source: A. Timmermann Equatorial temperature sections What triggers El Nino? Still not fully understood High heat content (deep thermocline) in the equatorial region necessary but not sufficient condition “Westerly wind bursts” (few days duration) associated with the Madden Julian Oscillation may act as one trig