Warmer summers may be causing colder winters
Published Sunday, January 15, 2012 12:11PM PST
International scientists have some bad news for those wondering when global warming will kick in during Canadian winters.
Researchers suggest that rising temperatures during the other three seasons are actually cooling off winters in North America -- all because of snowfall in Siberia and an atmospheric pressure pattern in high latitudes called the Arctic Oscillation.
"When you have more snow cover in October across Eurasia, you have this negative phase of the Arctic Oscillation," said lead author Judah Cohen of Atmospheric and Environmental Research, a weather research company, whose paper was published Thursday in Environmental Research Letters.
"When you have a negative Arctic Oscillation, southern Canada, the eastern U.S. and western Europe tend to have colder winters."
Cohen and his co-authors began by asking themselves why winter temperatures in the northern hemisphere aren't going up as quickly as in the spring, summer and fall.
A look at weather and climate records showed that Arctic sea ice is forming later and disappearing earlier -- and that the northern atmosphere is gradually getting warmer. At the same time, Siberia is getting heavier and heavier snowfalls.
Cohen theorized the two were linked, that more ice-free water and warmer air in the Arctic leads to more moisture in the northern atmosphere. In the cold temperatures of northern Eurasia, that moisture falls as snow.
Records show the Siberian snowpack has increased significantly over the last 25 years.
"In October, you start building up this cold dome of air," Cohen explained. "If you have a lot of snow, it reflects much of the sunlight back into space and cools the lower part of the atmosphere and you start to build cold, dense air. You get this dome of cold air that we associate with the Siberian High."
Previous studies have shown that snow in Siberia affects the strength of the Arctic Oscillation, which affects North America.
When that oscillation is strong, it creates powerful east-west winds that block cold polar air from drifting south. But when the oscillation is weak, more of that air starts moving north-south, pulling the Siberian High downwards.
"I like to think of Siberia as a refrigerator for the entire northern hemisphere," Cohen said.
"If you have less snow, it's like keeping the refrigerator door closed. The cold air stays locked up in the Arctic. But if the snow cover is much more expansive, it's like opening up the refrigerator door -- the cold air spills out into the kitchen."
Cohen said his group's work explains why North American winter temperatures and Siberian snowfalls aren't doing what climate models predict they should.
"The model projections for the other three seasons turned out very good. The one season they're not doing so well is winter.
"I think there's a good explanation here."
That may be great for climate scientists. It's not so cheery for those enduring another season of discontent.
Cohen said the pattern of warm springs, summers and falls followed by cold winters is likely to persist -- at least until climate change advances to the point where November precipitation in Siberia falls as rain, not snow.