Is ENSO a cycle or a series of events? William S. Kessler NOAA / - PDF document

Is ENSO a cycle or a series of events? William S. Kessler NOAA / Pacific Marine Environmental Laboratory, Seattle Early ideas (Wyrtki) saw El Ni�o as an isolated disturbance. The advent of simple and intermediate coupled models s u g g e s t e d a c y c l i c p h e n o m e n o n , w i t h e a c h phase leading to the next in a self-sustained oscillation. W i l l u s e t h e J i n ( 9 7 ) r e c h a r g e o s c i l l a t o r a s a paradigm for this type of behavior (the delayed oscillator would do as well). Examine observed ENSO evolution since 1980: Does each phase provide the impetus to advance to the next phase? ( W i l l f i n d a f u n d a m e n t a l a s y m m e t r y a n d b r e a k i n t h e c y c l e ) . The results suggest that El Ni�os are event-like, requiring a n initiating impulse not contained in the dynamics of the cycle itself.

Schematic of the Jin (1997) Recharge-Discharge mode: (After Meinen and McPhaden (2000))

Conclude: Wyrtki was right! (well, not quite ....) O b s e r v a t i o n s s u g g e s t t h a t a n E l N i � o e v e n t l e a v es the system i n a c o o l s t a t e w i t h s l i g h t l y i n c r e a s e d w a r m w a t e r v o l u m e , a n d this state can persist for years, losing memory of previous conditions. ENSO is not a self-sustained oscillation. T h e e v o l u t i o n o f E N S O s i n c e 1 9 8 0 a p p e a r s c o n s i s t e n t w i t h t h e idea that the basic state is stable or nearly so, and warm events are produced by (stochastic?) forcing external to the cycle itself. All figures from this talk are available at: http://www.pmel.noaa.gov/~kessler/ � Latest talk (Kessler, 2002: Geophys. Res. Lett. , 29(23), 2125.)

Extra slides .....