Ontario's unusually warm winter a mixed blessing

Wednesday, May 2, 2012

Municipalities saved millions on snow removal costs, home heating costs were low and severe weather warnings were rare. But expect more insects and pests this spring and lower water levels in our lakes, ponds and streams

by HENRY HENGEVELD


Most of farmland Ontario has just undergone one of its warmest December-to-February winter seasons on record. Perhaps even more significant, preliminary data released by Environment Canada indicate that much of southern Ontario set new records for the least amount of snowfall during those three months.  Meanwhile, much of Europe has been freezing. What gives?

Much of the mystery behind the sharp contrast between a warm North American winter and a very cold Europe relates to the complex response of the pattern of wind and air mass circulation around the world to several driving mechanisms that influence this circulation.  

One of the key drivers is the behaviour of the tropical Pacific Ocean, particularly as it goes through its semi-regular oscillation between El Niño and La Niña conditions. During El Niños, tropical trade winds are weak, allowing warm surface waters in the tropical Pacific to spread across the ocean from Asia right to the shores of the Americas.

However, during the La Niña phase of the oscillation, strong winds push warm surface waters to the western Pacific, where they pile up near Australia and Asia. This westward flow of surface waters causes an upwelling of cooler waters from the deep Pacific along the shores of the Americas.

The shifts in ocean temperatures have a major influence on the atmospheric pressure patterns above – and hence on the global air circulation. These patterns have now been in a La Niña mode for much of the last 18 months, but are predicted to return to a neutral condition by spring 2012.

Typically, during a La Niña winter season, these wind circulation changes bring colder than normal temperatures to southern Canada, particularly to western regions. That, indeed, was the long-range seasonal forecast issued by Environment Canada as late as last November.  

However, the El Niño-La Niña cycle is not the only driver for atmospheric circulation, and hence our weather. Other important factors include oscillations in atmospheric pressure over the North Atlantic and the Arctic.

By late fall, the computers started to pick up on influences from shifts in these, particularly that of the Arctic Oscillation. A strong high-pressure system over the Arctic caused an increased pressure difference between that region and lower latitudes. This helped strengthen the polar jet stream passing across central North America and push it further north.

This strong wind flow also acted like a dam between the cold Arctic air mass to its north and the warmer air to the south. It prevented the warmer air from the south moving into the Arctic region to mix with the cold air and moderate the intensity, thus isolating the polar region and allowing it to get progressively colder. It also stopped cold polar air outbreaks that frequently give Ontarians periods of intensely cold and stormy winter weather.  

The jet stream "dam" held across North America for most of the winter. As a result, much of Canada was left under the influence of warmer air masses. Preliminary analyses of weather records suggest that, this winter, temperatures were the second warmest on record for most of southern Ontario, exceeded only by the winter of 2001/2.

Much more of the precipitation during the season thus fell as rain. At Toronto Pearson airport, the 35 centimetres of snow that did fall during December, January and February was the least winter season snowfall amount ever recorded there, although only slightly lower than the 38 centimetres that fell in 1997-98. Furthermore, any snow that did fall quickly melted, leaving much of the ground uncovered or with just a thin layer of snow on it.

Not so for most of Europe! The "dam" that protected North America from cold Arctic air mass outbreaks also kept Europe relatively warm and dry during the early part of winter. However, in January, the jet stream began to weaken over Siberia and eventually broke, allowing the super-cooled polar air to spill across most of Europe, northern Asia and into Alaska, a phenomenon sometimes referred to as a "Russian winter."

By the fourth week in January, this cold outbreak had generated heavy snowfalls across Central and Eastern Europe, causing major disruption of transportation and power distribution throughout the region. These extreme conditions continued for three weeks, contributing to more than 600 cold-related human fatalities in the region. Temperatures dropped to as low as -40 C in some eastern European countries. Parts of the Danube River – an important commerce route in Europe – froze over for the first time in 25 years, indicating the intensity and longevity of the cold.

For many Ontarians, the mild winter and low snowfall has been a real boon. Municipalities saved millions on snow removal costs, home heating costs were low and severe weather warnings were rare. However, experts have also raised concerns about how this may impact our natural environment.

For example, during a mild winter, many insects that are normally killed by extreme cold conditions survive and are free to be fruitful and multiply as the spring weather brings warmer temperatures. Hence, mosquitoes, fleas and ticks would have more time to reproduce multiple generations, and could all be out in force far earlier than usual. Plant pests could also flourish and take their toll on emerging crops during the spring season.

The warm conditions also allow plants within the natural environment to bud early, increasing the risk of damage from late winter and spring cold snaps. Finally, unless we received some heavier snowfalls during the early spring season, the lack of snow cover on our fields and in our forests does not bode well for the spring freshet that normally moistens the soils, replenishes the water in our lakes and ponds and fills our streams.    

The latest forecast for the spring season at the time of writing (early March) is for above-normal temperatures for all of Ontario, with near-normal precipitation amounts along the southern Great Lakes and above-normal elsewhere!  BF

Henry Hengeveld is a retired climatologist.