1. Three cases: (1) La Nina event in 1989 (2) A strong El Nino in 1998 (3) A moderate El Nino in 1987 Three fields: (a) Surface temperatures and anomalies (b) Sea level pressure and anomalies (c) Precipitable water and anomalies

2. Colder than average

4. Lower than average pressure

6. Drier than average

20. Not all events have same amplitude…best to think of an analogy between El Nino and La Nina and summer and winter: there are things common to each winter to make it a useful concept, but each one is different owing to different weather, different amplitudes of temperature extremes, etc. El Ninos, particularly strong ones, are generally confined to a single 12 month interval. La Ninas may persist for a few consecutive years, so there the oscillation is not linear in nature.

22. NOAA/NCEP/CPC

26. Westerly Wind Bursts (like from an MJO) cause an ocean response

27. Delayed Oscillator Theory Initially, Kelvin is downwelling (bringing warmer SSTs), and Rossby is upwelling Rossby is much slower than Kelvin At reflection, wave type shift Key: The reflected Rossby from maritime continent becomes an UPWELLING Kelvin “Seeds of its own demise” If this were the only thing happening, would get a very regular oscillation

28. A real example

29. There must be more… Clearly there must be other companion mechanisms, with input from atmosphere, to get us to 3-5 (2-7) year irregular cycles – The original feedback theory – Recharge/discharge theory – Heat builds up in the equatorial region, discharged eastward and poleward during El Niño Lots of other theories (text), and active research

30. Monitoring 3.4 is frequently used because of signal of both convection and SST 1&2 warm first!

31. SOI = Tahiti - Darwin (normalized)

32. Niño Region SST Departures ( o C) Recent Evolution The latest weekly SST departures are: Niño 4 -1.2ºC Niño 3.4 -1.6ºC Niño 3 -1.8ºC Niño 1+2 -2.0ºC

33. SOI and Nino 3-4 index

34. Jet stream configur- ations

35. Global Effects

36. Mean number of days per season (November - March 1948 through 1993) in which precipitation exceeded 0.50 inches for Neutral years (top). Lower left map is the difference in this quantity between El Niño years and Neutral years. Lower right map is the difference in this quantity between La Niña years and Neutral years. North American Effects