Weather: Will El Niño mean a warmer winter for Ontario?
Thursday, December 3, 2009
Normally, the return of El Niño in the Pacific heralds a warmer than normal winter ahead, and that's what models were projecting. Now a revised forecast is indicating that this will not be the case
by HENRY HENGEVELD
In late September, the American Earth Systems Research Laboratory in Colorado confirmed that El Niño was back. If past El Niño events are any indications, this may mean more unusual weather for Ontario this winter.
El Niños are linked to a slow sloshing back and forth of atmospheric pressure patterns in the Southern Hemisphere, known as the Southern Oscillation. Every two to seven years, this oscillation enters a "warm" phase that causes the tropical trade winds to weaken. The weaker eastward-blowing winds allow warm surface waters in the tropical Pacific, which are normally pushed to the western Pacific, to flow back toward the east. As a result, water temperatures in the tropical eastern Pacific become abnormally warm, heating the atmosphere above.
These events have recurred with regularity for centuries. Since these warm waters off South America frequently arrive around Christmas time, coastal people in the region began to refer to it in Spanish as El Niño, or "The Boy Child." Its sister, La Niña, occurs during the cold phases of the Southern Oscillation, when trade winds become stronger than normal, warm water pooling in the western Pacific becomes enhanced, and the eastern Pacific cools.
The increased heating of the tropical atmosphere over the central and eastern Pacific during El Niño events affects atmospheric circulation features, such as the jet streams in the subtropics and in the temperate latitudes. In winter seasons, it also results in abnormally vigorous low-pressure systems in mid-latitudes of the eastern Pacific that tend to pump unusually warm air into Western Canada, Alaska and the extreme northern portion of the rest of the United States.
Mid-latitude storms and frontal systems also follow paths significantly different from normal, resulting in persistent temperature and precipitation anomalies in much of North America. Storms also tend to be more vigorous in the Gulf of Mexico and along the southeast coast of the United States, resulting in wetter than normal conditions in that region. As the season progresses, these warmer-than-normal conditions usually push into Central and Eastern Canada as well. In contrast, the High Arctic turns abnormally cold.
To keep track of El Niño-La Niña behaviour, researchers monitor changes in a number of key features of the tropical Pacific Ocean, including sea level pressures, wind patterns, sea surface and air temperatures and cloud cover. Changes are entered into complex equations to generate indexes of El Niño behaviour.
A number of such indexes are now in use by different research teams around the world. During July and August of 2009, the index used by the Colorado group (known as the 'Multivariate ENSO Index', or MEI) significantly exceeded the threshold for El Niño conditions. Supporting evidence included surface waters of the equatorial Pacific that had been at least 0.5 C warmer than average since June (with some regions one degree warmer by end of September), and above normal heat content of the upper 300 metres of oceans in this region since April.
By the end of September, most models predicted that these El Niño conditions would continue through the Northern Hemisphere winter of 2009-10. While the models disagreed on the eventual strength of the new El Niño, a majority projected at least a moderate strength El Niño (eastern and central tropical Pacific anomalies greater than plus 1.0 C) during November-December-January of 2009-10.
El Niños consistently bring warm winters to Western Canada. Hence, the projection for an El Niño winter in 2010 is not good news for Olympic organizers in B.C. However, the implications for Ontario, which is further downstream from the source of the tropical Pacific anomaly, are less certain. This is partly due to the concurrent effects of Atlantic Ocean anomalies such as the North Atlantic Oscillation. These can enhance the El Niño influence in Eastern Canada during some winters, and more than offset it in others.
For example, in the moderate to strong El Niño events of 1982-3 and 2002-3, average winter temperatures across the Great Lakes Basin were more than two degrees above normal. On the other hand, during the major El Niño winters of 1957-58 and 1968-69, winter temperatures in the region were significantly below normal. It was also the combined effect of a strong El Nino and an anomalous high pressure system over the North Atlantic Ocean that contributed to the infamous ice storm which ravaged eastern Ontario in 1998.
Weather models used for seasonal forecasting try to sort out these different influences and project what the net effect for each region of the country will be. In early September, the Canadian models projected that, during this winter (December through February) and next spring (March through May), almost all of Southern Canada, including southern Ontario, would remain significantly warmer. Southern Ontario would also be dryer than normal.
However, the revised forecast for November through January, issued in early October, suggested that most of Southern Canada, with the exception of the upper Great Lakes, would be colder than normal. Not surprisingly, the confidence level given to both forecasts was low. BF
Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada.
