As summer comes to an end the most common question I have received is about what we should expect the coming winter to be like. Long-range forecasting is tough at the best of times, and most forecasts beyond 30 days are usually not more statistically correct than simply doing a coin toss, but – there is always a “but” – there are some general atmospheric circulation patterns that can load the weather coin a little bit.
This year we have a fairly strong La Nińa event developing over the Pacific Ocean. You might remember back to last winter when there was a moderate to strong El Nińo event taking place and we were crediting that event with the mild late winter and early spring weather much of the Prairies experienced. Well, that El Nińo event collapsed in late spring and early summer and conditions over the eastern equatorial Pacific Ocean are now going from warmer than average to colder than average.
This quick change from an El Nińo to a La Nińa event brings up a couple of questions on its own. The first one being, how unusual is it for conditions over the equatorial Pacific to switch so dramatically over a relatively short period of time? According to several different sources I came across, it is apparently not that unusual and has occurred several times in the past. The second big question is, just what is La Nińa and how might it impact our weather? Let’s look at the first part of that question first.
Most of us now know what an El Nińo event is. It is an unusual warming of the central and eastern parts of the equatorial Pacific Ocean. This warming changes the general circulation of the atmosphere over the Pacific. In particular, the trade winds weaken and, in some extreme cases, reverse direction. This can result in large changes in the location of heat and moisture globally, and can give rise to anomalous temperature and precipitation events around the world.
La Nińa is the opposite of an El Nińo event. La Nińa meansthe little girl,and can sometimes be calledEl Viejo, the anti-El Nińo, or simply, a cold event. La Nińa occurs when there is an increase in the strength of the normal pattern of trade wind circulation. Under normal conditions,
these winds move westward, carrying warm surface water to Indonesia and Australia and allowing cooler water to upwell along the South American coast. When a La Nińa event occurs, these trade winds are strengthened, which helps to increase the amount of cooler water along the coast of South and Central America and builds up warmer waters on the western side of the ocean.
This influx of warmer water causes an increase in cloud cover over southeast Asia and results in wetter-than normal conditions for that region during the northern hemisphere winter. So what does this have to do with our weather in Western Canada? These changes in the tropical Pacific are usually accompanied by large changes in the jet stream across the mid-latitudes – our part of the world – that shift the usual location of the jet stream across North America. This shifted jet stream can contribute to large changes in the normal location and strength of storm paths and can result in temperature and precipitation anomalies over North America that can persist for several months. Coincidentally, these changes are most strongly felt in the winter.
MODERATE TO STRONG
According to several sources, temperature and precipitation impacts over the United States and Canada during a La Nińa event are typically weak dur ing the Nor thern Hemi sphere summer and early fall, but strengthen considerably during late fall and winter. According to the latest La Nińa advisory, nearly all models predict La Nińa to continue through early 2011. There is disagreement among the models over the eventual strength of this La Nińa event. Some of the models predict a moderate-to-strong La Nińa, while other models indicate a weaker episode. Since there has been strong cooling over the past several months and there appears to be a positive feedback loop taking place between the atmosphere and the ocean, it seems more likely that a moderate-to-strong episode will occur this winter. Overall, the La Nińa advisory expects conditions to strengthen and last through our Northern Hemisphere winter of 2010-11.
Next issue we’ll take a look at exactly what a strong La Nińa might mean for us here in Western Canada by taking a look back to see what happened the last few times we experienced a La Nińa winter.
Nearlyallmodels predictLaNińato continuethrough early2011.