## Tuesday, August 23, 2011 ... /////

### La Nina conditions seem to be returning

Recall that we had an El Nino in 2009/10 - which helped to make 2010 a warm year. Then we had a La Nina in 2010/11 - which is helping to make 2011 a cooler year. But there's no law that would say that El Ninos and La Ninas have to alternate.

Click to zoom in.

So it actually looks more likely than not right now that there will be another La Nina episode. See the weekly ENSO report by NOAA.

The 2010/11 La Nina anomaly dropped to -1.4 °C - the most extreme average consecutive 3-month reading. Note that 3-month readings have to be below -0.5 °C and one must have at least 5 overlapping 3-month periods like that for the conditions to qualify as a "La Nina episode". Similarly for the plus sign and El Nino.

These anomalies are measured out of the ONI 3.4 index. When the La Nina episode ended and the index reached its local maximum at the end of June 2011, it was slightly positive - like +0.1 °C for a while. Right now, it is -0.7 °C again - so it is in the La Nina territory again. However, I repeat, those conditions have to persist for some months for the La Nina conditions to become another La Nina episode.

A La Nina episode has a cooling impact on the global mean temperature - but this impact is delayed roughly by half a year.

The CFS.v2 model ensemble predicts that the coming La Nina episode will be slightly stronger than the previous one. That could make 2012 an even cooler year than 2011. However, other types of models don't even agree that a new La Nina is more likely than ENSO-neutral conditions in the Fall. This shows that these pretty "macroscopic" properties of the Earth can't be well predicted even for a few months.