Enso theory How does the phase of enso reverse?

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)