La Niña portends a cool winter and spring for Ontario

Monday, November 1, 2010

Projected weather patterns for the coming seasons appear to reflect the intensifying but complex influence of this Pacific Ocean phenomenon. But don't bet the farm on it!

by HENRY HENGEVELD

It's been unusually warm across the Great Lakes basin during the first eight months of 2010 – the ninth warmest winter, the warmest spring and the fourth warmest summer on record. Now, according to the prognoses recently issued by both American and Canadian weather services, the Ontario climate pendulum is about to swing back to below normal temperatures. 

One of the key reasons for this expected shift in Ontario weather is the Pacific Ocean phenomenon commonly known as the El Niño/Southern Oscillation (ENSO). The Southern Oscillation is an irregular seesaw of pressure patterns over the southern Pacific Ocean that alters the tropical trade winds from a strong westerly flow to a weak one – and back again.

The phase of the oscillation when the tropical westerlies are strong is called the "La Niña" phase, while the weak wind part of the cycle is referred to as "El Niño." A full ENSO cycle can take as little as two years or can last as long as seven years. 

During the La Niña phase, the strong trade winds push surface waters of the tropical Pacific Ocean from east to west, creating a large pool of warm waters on the Australian/Asian side. The build-up of water in the west in turn generates a return current at deeper levels of the Pacific Ocean. When the return part of the conveyor belt once again reaches the eastern Pacific along the western shores of central and South America, it sends nutrient rich cool waters from the deeper levels of the Pacific Ocean back to the surface.

In contrast, the weak winds during the El Niño phase of the oscillation allow the Pacific Ocean surface water conveyor belt process to slow down and even stop. The pool of warm water which has piled up along the Asian side of the Pacific Ocean during the La Niña phase then spreads across the entire tropical ocean, and the upwelling on the east side ceases. 

Understandably, the ENSO phenomenon has a large impact on the year-to-year variations of weather experienced in Central and South America. It also affects the pattern of atmospheric pressure elsewhere around the world, changing the dominant atmospheric wind flows and the position of weather fronts and storm tracks. In fact, experts now acknowledge that ENSO behaviour is one of the most important factors in the seasonal and inter-annual weather patterns, including that across North America. 

The good news is that oceanographers have, over the past two decades, learned a lot about how the ENSO behaves. They have developed a complex index that can indicate the changes in phase from month to month. Such changes have been reconstructed back to 1950 and can now be projected a number of months into the future with some confidence. The ENSO projections, in turn, contribute to improved seasonal weather forecasting. 

According to the agencies that monitor the ENSO behaviour index, this system was in a moderate El Niño mode from May-June 2009 until March-April 2010. Added to other forces that have been pushing global temperatures upward, it helped bring average global temperatures to record levels for several months in early 2010. In fact, global data analysts suggest that 2010 may well become the warmest year ever in recorded climate history. 

However, in an unusually rapid reversal, the ENSO index abruptly moved into a weak La Niña mode by June-July of 2010. Two months later, it had already intensified to full-blown La Niña conditions. Researchers indicate that this has been one of the most rapid shifts in the ENSO index during the past 60 years and predict that moderate to strong La Niña conditions will continue into at least the early months of 2011. 

As they do at the beginning of each new season, Environment Canada forecasters issued an update in early September on their seasonal temperature and precipitation projections. The advanced weather forecasting models used to develop these projections factor in various phenomena that influence month-to-month and seasonal weather variability, including the behaviour of the ENSO.

The September update suggests that the autumn of 2010 will continue to be unusually warm over all of Eastern Canada, including Ontario, and that most of Southern Canada will be wetter than normal. On the other hand, the western regions of Canada, particularly Alberta and British Columbia, are expected to begin to revert to unusually cool conditions that are often associated with La Niñas. 

Environment Canada researchers have also developed another type of seasonal forecasting tool that uses historical relationships between weather patterns of the past 12 months with those for the next 12 months. Projections for autumn 2010 using this statistical tool largely agree with those derived from the dynamic weather forecasting models, thus adding credibility to the above prognosis.

Forecasters have also stuck their necks out with predictions for the coming winter, spring and summer seasons. Projected weather for all three seasons appears to reflect the intensifying but complex influence of La Niña. Over Ontario, for example, average temperatures during the coming winter (December to February) are expected to be colder than normal. Regions along the lower Great Lakes are also likely to experience normal to below normal precipitation levels, in contrast to above normal levels projected for the upper lakes.

Likewise, spring 2011 (March through May) is expected to be unusually cool, with much of the Great Lakes basin likely to receive normal to more than normal precipitation amounts. Finally, while northern and western regions of Ontario are projected to experience normal to warmer than normal conditions during the summer of 2011, southern and eastern portions appear likely to continue the cold anomalies of the preceding seasons. Summer precipitation levels are projected to be below normal across the entire province.

There is one important caveat. The forecasters involved in making these projections note that, for most of Canada, they have relatively low confidence in them – in fact, not much better than chance. 

Despite the important role of La Niña during the coming seasons, it would still be prudent not to bet the farm on these forecasts! BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada