As summer comes to an end the most common question I have received is about what we should expect for the coming winter. 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 last winter when there was a moderate to strong El Nińo and we were crediting that event with the mild late winter and early spring for much of the Prairies. 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. First, how unusual is it for conditions over the equatorial Pacific to switch so dramatically over a relatively short period? According to several different sources I came across, it is apparently not that unusual and it has occurred several times in the past.
What’s a La Nińa?
Second, just what is a La Nińa and how might it affect our weather? Let’s look at the first part of that question first.
Most of us now know an El Nińo 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 means “the little girl,” and can sometimes be called El Viejo, the anti-El Nińo, or simply, a cold event. A 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 that can persist for several months. Coincidently, these changes are most strongly felt in the winter.
According to several sources, temperature and precipitation effects over the United States and Canada during a La Nińa event are typically weak during the Northern Hemisphere summer and early fall, but strengthen considerably during late fall and winter.
According to the latest La Nińa advisory, nearly all models predict it to continue through early 2011. There is disagreement among the models over the eventual strength of this La Nińa event. Some of the models are predicting 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.