Warmer summers causing colder winters
Warmer summers in the far Northern Hemisphere are disrupting weather patterns and triggering more severe winter weather in the United States and Europe, a team of scientists say, in a finding that could improve long-range weather forecasts.
Blizzards and extreme cold temperatures in the winters of 2009/10 and 2010/11 caused widespread travel chaos in parts of Europe and the United States, leading some to question whether global warming was real.
Judah Cohen, lead author of a study published on Friday in the journal Environmental Research Letters, and his team found there was a clear trend of strong warming in the Arctic from July to September.
Existing predictions would also expect a warming trend during winter as well. But Cohen and his team found this was not the case for some regions, in a counter-intuitive finding that reveals more about the complexity of the world's climate system than any flaws in the science of global warming.
"For the last two decades, large-scale cooling trends have existed instead across large stretches of eastern North America and northern Eurasia. We argue that this unforeseen trend is probably not due to internal variability alone," the scientists say in the study.
Using temperature, rainfall and snow and ice data, the team found that rising summer temperatures in the Arctic meant the atmosphere could hold more moisture, leading to an increase in autumn snowfall in high-latitude areas.
Analysis of data showed the average snow coverage in Eurasia had increased over the past two decades. This additional snow cover in turn has led to a change in the Arctic Oscillation, the main atmospheric pressure pattern that governs winter weather in the far Northern Hemisphere.
When the oscillation is in a negative phase, high pressure cells over the Arctic push colder air towards the mid-latitudes, triggering colder than usual temperatures and fierce snow storms.
A positive phase tends to bring milder winter weather, such as the case at present in the United States and Europe.
Image credit: J. Wallace, University of Washington