Clues to Climate Mysteries

New ideas about climate variability in the Northern Hemisphere are attracting international attention to the Arctic Oscillation —the phenomenon that describes how changes in atmospheric surface pressure in the Northern Hemisphere affect mid-latitude weather patterns. The ideas, which stem from research conducted at Environment Canada's Canadian Centre for Climate Modelling and Analysis (CCCma) in Victoria, British Columbia, may alter the way climate variability and change are studied in the future.

Once described in Science magazine as the “master switch” for climate, the Arctic Oscillation has an even greater reach than the El Niño Southern Oscillation, which is associated with the warming and cooling of the tropical Pacific Ocean. When the Arctic Oscillation is in what is called its positive phase, low pressure dominates the Arctic and high pressure prevails in the mid-latitudes of the planet. This causes warmer than usual temperatures over Europe and Asia, and cooler than usual readings over northern Canada. It usually takes somewhere between a few days and several months for the Oscillation to flip to its negative phase, which brings high pressure over the Arctic, low pressure over the mid-latitudes, and a resultant reversal in temperatures.

Of great interest to scientists is the fact that the Oscillation has been stuck in its positive phase over recent decades — something many point to as yet another symptom of human-induced changes in the climate system. In 1999, the CCCma demonstrated that increased levels of carbon dioxide may partly account for the more frequent appearance of the positive phase of the Arctic Oscillation. Since then, scientists around the world have undertaken a large amount of research on the Oscillation, and the phenomenon was the subject of a special session at a recent meeting of the 10 000-member American Geophysical Union.

In the meantime, Environment Canada scientists have taken their research a step further. Based on analyses using a new, more powerful statistical technique, they now suggest that climate variability in the Northern Hemisphere is best interpreted in terms of a small set of climate regimes, or states. The atmosphere resides for extended periods of time in a given regime and then moves relatively quickly to another. From this perspective, global warming is seen as an increased tendency toward one regime (a relatively warm one, for instance) over another.

Using the CCCma's sophisticated computer model of the earth's atmosphere and oceans, scientists have identified two dominant climate regimes. The first, more persistent regime is characterized by large-scale changes in surface pressure centred over Eurasia. The second, more episodic regime is centred over the north Atlantic and associated with dramatically altered storm tracks, with storms being deflected into the Arctic instead of northern Europe, as is usually the case.

When the model is run with significantly increased carbon dioxide levels, as are expected in the coming decades, the second regime mysteriously vanishes, while the first, relatively warm regime continues on even more powerfully than before. Recent studies published by Environment Canada indicate that the real atmosphere may already be behaving in much the same way. These findings shed new light on the climatic conditions Canada and other countries may face in the future, and reinforce the need to take firm steps to control greenhouse gas emissions on a global scale.

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