Enso theory How does the phase of enso reverse?

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ENSO - Theory

  • How does the phase of ENSO reverse?

  • What triggers an El Nino event?

Power Spectrum

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?

Delayed Oscillator Theory 2

Delayed Oscillator Theory 3

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

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 trigger.

Onset of the 1997/98 El Nino

Summary of El Nino onset

  • Need high heat content in equatorial Pacific

  • Triggering by wind fluctuations (e.g. WWBs) over central/western Pacific

  • Growth through Bjerknes positive feedback mechanism

Theories for ENSO Oscillations

  • Delayed Oscillator (e.g. Battisti and Hirst, 1989; Suarez and Schopf, 1988

  • Recharge/discharge theory (Jin, 1997)

  • Western Pacific Oscillator (e.g. Weisberg, R. H., and C. Wang, 1997)

  • Advective-Reflective Oscillator (e.g. Picaut et al, 1997)

  • Unified Oscillator (Wang, 2001 J Clim)

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