An intriguing new twist turns up in the relationship between weather and climate.
Even if short stretches of extreme weather resemble overall climate trends, they are often unrelated.
Climate is defined as average weather of a particular region over, say, a 30-year period.
Scientists now argue that certain types of extreme weather can actually influence climate directly.
The great Russian heat wave of 2010 -- the region's worst in 1,000 years -- was a terrible bout of extreme weather. A new study suggests it may also be a harbinger of changing climate, but not in the way you might think.
The weather phenomenon responsible for the raging fires and thousands of deaths in Moscow does not relate directly to global warming. Instead, researchers say in the current issue of Science, multiple episodes of these short-term weather anomalies, which occur over the North Atlantic, can force changes in ocean circulation that alter the region's climate for decades.
In the normal weather pattern over the North Atlantic, the jet stream carries air from North America across the Atlantic to Europe (pictured above).
The underlying cause of the 2010 Russian disaster was an atmospheric phenomenon known as blocking, in which large, stationary weather systems block the normal westerly flow of storm tracks traveling on the jet stream. These obstructions can steer rain-bearing cyclones off their usual path, leaving Europe and northwest Asia scorched and parched.
Oceanographer Sirpa Häkkinen of NASA Goddard Space Flight Center and colleagues describe in Science a more long-lived consequence of blocking: These large-scale obstructions also divert the usual airflow to the North and South, which results in a dramatic reversal of rotating air masses air either side of the jet stream (pictured above).
The researchers suggest that this reversal wreaks havoc with wind patterns in a way that shrinks the ocean current to the North of the jet stream's normal path, making room for warm, saline water from the subtropics to move to higher latitudes.
"You add up these things over years and decades, and they can really make a difference," Häkkinen told Discovery News.
When the block dissipates, and the jet stream gets going again, it passes over warmer seawater, and thus carries more warmth toward Europe. Over the longer term, the existence of warmer waters up North can also slow overturning of the ocean's circulation patterns, which can mean warmer climate for the region overall.
This new idea stems from an uncanny correlation between clusters of frequent blocking episodes and a series of warming and cooling episodes of North Atlantic seawater, often referred to as Atlantic Multidecadal Variability.
Scientists have long blamed these temperature changes on the ocean's overturning circulation alone. In this new scenario, the warm spells are greatly enhanced by the outcomes of atmospheric blocking events, many of which last less than a week.
"People knew that warm Atlantic waters relate to this weather pattern, but they had never really explained it before," Häkkinen said.
In a related article, Tim Woolings, a climate scientist Walker Institute for Climate System Research at the University of Reading in the U.K., writes that Häkkinen's study is "part of an exciting resurgence of interest in ocean-atmosphere interaction."
Woolings told Discovery News that the strength of the new study is "its focus on short timescale weather events, which can happen anywhere over the northern North Atlantic, rather than the traditional approach of looking at fixed, long-term patterns."