Friday, July 1, 2011

What's to Blame for Wild Weather? "La Nada"

A couple different opinions.
From NASA:
Such sheets of fire, such bursts of horrid thunder,
Such groans of roaring wind and rain, I never
Remember to have heard; man's nature cannot carry
The affliction nor the fear
from Shakespeare's Tragedy of King Lear
June 21, 2011: Record snowfall, killer tornadoes, devastating floods: There’s no doubt about it. Since Dec. 2010, the weather in the USA has been positively wild. But why?

Some recent news reports have attributed the phenomenon to an extreme "La Niña," a band of cold water stretching across the Pacific Ocean with global repercussions for climate and weather. But NASA climatologist Bill Patzert names a different suspect: "La Nada."

"La Niña was strong in December," he says. "But back in January it pulled a disappearing act and left us with nothing – La Nada – to constrain the jet stream. Like an unruly teenager, the jet stream took advantage of the newfound freedom--and the results were disastrous."

La Niña and El Niño are opposite extremes of a great Pacific oscillation. Every 2 to 7 years, surface waters across the equatorial Pacific warm up (El Niño) and then they cool down again (La Niña). Each condition has its own distinct effects on weather.

The winter of 2010 began with La Niña conditions taking hold. A "normal" La Niña would have pushed the jet stream northward, pushing cold arctic air (one of the ingredients of severe weather) away from the lower US. But this La Niña petered out quickly, and no El Niño rose up to replace it. The jet stream was free to misbehave.

"By mid-January 2011, La Niña weakened rapidly and by mid-February it was 'adios La Niña,' allowing the jet stream to meander wildly around the US. Consequently the weather pattern became dominated by strong outbreaks of frigid polar air, producing blizzards across the West, Upper Midwest, and northeast US."1
The situation lingered into spring -- and things got ugly. Russell Schneider, Director of the NOAA-NWS Storm Prediction Center, explains:

"First, very strong winds out of the south carrying warm, moist air from the Gulf of Mexico met cold jet stream winds racing in from the west. Stacking these two air masses on top of each other created the degree of instability that fuels intense thunderstorms."

Extreme contrasts in wind speeds and directions of the upper and lower atmosphere transformed ordinary thunderstorms into long-lived rotating supercells capable of producing violent tornadoes.2
In Patzert's words, "The jet stream -- on steroids -- acted as an atmospheric mix master, causing tornadoes to explode across Dixie and Tornado Alleys, and even into Massachusetts." ...MORE

From Scientific American 

Global Warming and the Science of Extreme Weather
How rising temperatures change weather and produce fiercer, more frequent storms. Second of a three-part series
Editor's note: This article is the second of a three-part series by John Carey. Part 1, posted on June 28, is "Storm Warning: Extreme Weather Is a Product of Climate Change".

Extreme floods, prolonged droughts, searing heat waves, massive rainstorms and the like don't just seem like they've become the new normal in the last few years—they have become more common, according to data collected by reinsurance company Munich Re (see Part 1 of this series). But has this increase resulted from human-caused climate change or just from natural climatic variations? After all, recorded floods and droughts go back to the earliest days of mankind, before coal, oil and natural gas made the modern industrial world possible.

Until recently scientists had only been able to say that more extreme weather is "consistent" with climate change caused by greenhouse gases that humans are emitting into the atmosphere. Now, however, they can begin to say that the odds of having extreme weather have increased because of human-caused atmospheric changes—and that many individual events would not have happened in the same way without global warming. The reason: The signal of climate change is finally emerging from the "noise"—the huge amount of natural variability in weather.

Scientists compare the normal variation in weather with rolls of the dice. Adding greenhouse gases to the atmosphere loads the dice, increasing odds of such extreme weather events. It's not just that the weather dice are altered, however. As Steve Sherwood, co-director of the Climate Change Research Center at the University of New South Wales in Australia, puts it, "it is more like painting an extra spot on each face of one of the dice, so that it goes from 2 to 7 instead of 1 to 6. This increases the odds of rolling 11 or 12, but also makes it possible to roll 13."

Why? Basic physics is at work: The planet has already warmed roughly 1 degree Celsius since preindustrial times, thanks to CO2and other greenhouse gases emitted into the atmosphere. And for every 1-degree C (1.8 degrees Fahrenheit) rise in temperature, the amount of moisture that the atmosphere can contain rises by 7 percent, explains Peter Stott, head of climate monitoring and attribution at the U.K. Met Office's Hadley Center for Climate Change. "That's quite dramatic," he says. In some places, the increase has been much larger. Data gathered by Gene Takle, professor of meteorology at Iowa State University in Ames, show a 13 percent rise in summer moisture over the past 50 years in the state capital, Des Moines.

The physics of too much rain
The increased moisture in the atmosphere inevitably means more rain. That's obvious. But not just any kind of rain, the climate models predict. Because of the large-scale energy balance of the planet, "the upshot is that overall rainfall increases only 2 to 3 percent per degree of warming, whereas extreme rainfall increases 6 to 7 percent," Stott says. The reason again comes from physics. Rain happens when the atmosphere cools enough for water vapor to condense into liquid. "However, because of the increasing amount of greenhouse gases in the troposphere, the radiative cooling is less efficient, as less radiation can escape to space," Stott explains. "Therefore the global precipitation increases less, at about 2 to 3 percent per degree of warming." But because of the extra moisture, when precipitation does occur (in both rain and snow), it's more likely to be in bigger events.

Iowa is one of many places that fits the pattern. Takle documented a three- to seven-fold increase in high rainfall events in the state, including the 500-year Mississippi River flood in 1993, the 2008 Cedar Rapids flood as well as the 500-year event in 2010 in Ames, which inundated the Hilton Coliseum basketball court in eight feet (2.5 meters) of water . "We can't say with confidence that the 2010 Ames flood was caused by climate change, but we can say that the dice are loaded to bring more of these events," Takle says....MORE