Behind this summer's wild, tragic weather

By , Staff writer of The Christian Science Monitor

Your instincts about the weather are probably right: It has been a weird summer worldwide.

It's not just the egg-cooking temperatures in Europe, which, tragically, have been blamed for as many as 3,000 deaths in France alone.

While Parisians have been able to caramelize crème brûlée on the street and turned fountains into splash pools, residents of America's Northeast have looked, mostly in vain, for rain-free weekends at the beach. In the Southwest, persistent triple-digit temperatures wouldn't just fry eggs; it would char them asphalt-black, without mesquite flavor. From Denver to Delhi, it's been the kind of summer where, rain or shine, the place to be, it seems, is indoors. Even climatologists, who have seen or studied it all, haven't seen this pattern in such a strong and persistent form.

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The big question is: Why? Some usual suspects - El Niño, La Niña, and global warming - don't seem to explain the unusual conditions.

Clearly, they say, irregular bends in the jet stream are involved. But no clear-cut reason for that pattern has emerged. In their hunt for answers, some scientists, are looking as far away as the monsoons of South Asia for clues to Miami's unseasonal rains and Madrid's outdoor oven.

"The inherent instability of the atmosphere by itself can produce a persistent pattern," such as the one that has dominated over North America and Europe this summer, says Angie Seth, an associate research scientist at the International Institute for Climate Prediction at Columbia University's Earth Institute.

One thing is clear: The year so far has been a record-breaker in a number of regions, according to the UN's World Meteorological Organization. In a recent, brief tally for the year so far, the organization notes that while old weather records topple every year somewhere around the globe, the number of extreme weather events is rising.

Indeed, the summer's weather has given people plenty to talk about. To some extent, researchers say, the conversations stem from short memories. In America's Northeast, for example, the winter snows and wet spring and summer have broken a two-year drought. Thus compared with the past two years, snowfall and rainfall this year look even more intense than they might otherwise.

Still, this season's pattern, which appeared first in late April and May, "has been really unusual" in its persistence and strength, says Vernon Kousky, a research meteorologist at the National Oceanic and Atmospheric Administration's Climate Prediction Center.

The jet stream, a high altitude river of air that flows from west to east and typically marks the boundary between cold polar air and warmer air to the south, meandered northward in the west and dipped back south over the east.

At the jet stream's altitude, this drew warm air to northward over western Canada, while bringing cool air down along the East Coast. There, the cool air aloft, combined with the warm, moist air at the surface, destabilized the atmosphere and led to a long string rainy days, he explains. Typically for this time of year, the jet stream's "ridge" over the west should have been sitting over the continent's heartland, while the "trough" over the East Coast should have been offshore.

Europe's heatwave, he says, came as this amplified pattern set up conditions very similar to those in the western US."

Conditions globally are similar to those of the late 1940s and early 50s, when Europe experienced another unusual heat wave, says Joe Bastardi, a forecaster with Accuweather.

One big puzzle atmospheric scientists are trying to solve about this summer's weather involves the larger features of atmospheric circulation that drive regional patterns.

The first place they tend to look is to the tropical western Pacific. This is where El Niño or La Niña - triggered by the accumulation of unusually warm or cold water in the western tropical Pacific - emerge to alter and accentuate atmospheric circulation patterns. But tropical oceans are relatively neutral now.

Thus, Seth and other researchers are beginning to look at features whose effects would otherwise be swamped by a strong El Niño or La Niña. A possibility that intrigues her involves the Indian monsoons. "That's the big kicker now in the global atmospheric energy system," Seth says. This season, monsoons are intense. Cloud formation associated with that rainy season releases large amounts of latent heat into the atmosphere. Researchers in Britain have proposed that around the edges of the monsoon-affected region, air is subsiding, or sinking. This tends to dry it out. This, they posit, could affect circulation patterns as far away as Europe.

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