QUOTE (hutchy - 18/07/2011 @ 16:22)
I believe it goes something like El Nino – La Nada – La Nina – La Nada – El Nino – La Nada – La Nina and so on. But as all things weather it's not an exact science. A few people are suggesting a La Nina double dip meaning La Nina conditions will continue until 2012.
Have a look here for a graph. This is MEI, which is a variant on the standard ENSO index, but is (I think) a better predictor of how precipitation is affected, since it includes more atmospheric factors. We had more El-Nino's in the last decade so maybe we're set for a run of La-Nina's... No way to know. Last season was the strongest La-Nina in 50 years and lead to some truly awesome conditions in much of North America. Conditions have returned to neutral as of now, but there are some signs that it could return to La-Nina by winter. It's looking 50-50 for neutral or La-Nina. If it's La-Nina it could be anywhere from half as strong to equally as strong as last season. The latter seems doubtful to me though, since we're heading into Autumn already. Still, half as strong would still be a decent La-Nina.
QUOTE (hutchy - 18/07/2011 @ 16:22)
Not being a weather guru though i don't understand the effects on the PDO and Arctic Oscillation etc on these weather patterns. I'd be interested in having them all explained in this context if anyone knows?
I don't think anyone really knows what drives the El-Nino - La Nina cycle. Since it's weather-related it's probably a chaotic system which is too sensitive to predict accurately. There is some persistence though, so once it starts heading up or down they can usually predict where it's going for the next 3-9 months. Some time around Sep-Oct you can start to have some confidence about which way it's going for the winter season. For introductory info on what El-Nino or La-Nina conditions mean for global weather, have a look here. The short version is that El-Nino means less snowfall in most of western North America and La-Nina means more. South-Western and North-Eastern areas of North America are the opposite. But in all cases the variability of snowfall outweighs the effect of El-Nino or La-Nina, so you can't expect all El-Nino seasons to be bad and all La-Nina's to be good. It just increases or decreases your chances. What it means for European snowfall is a lot less clear. I've heard it said that Europe is affected 12-18 months later, but who knows. If it's true Europe could see the benefit of last season's La-Nina this year...