Would you like someone or something to blame for the hail that savaged your garden, the incessant tornado warnings that scare the wits out of your kids and the swollen rivers that ravaged Iowa and now threaten further south?
An explanation wouldn’t begin to restore flood-soaked river cities — or even, more trivially, repair the hostas in your yard. It would give some small measure of satisfaction, though, if we could understand why the weather is so wild this year.
But a full accounting is as elusive as a cloud-free day.
It is tempting to look to climate change because that is a dominant concern for these times. And global-warming experts predict that some regions will get more rain while others may dry up. But scientists still are sorting through those scenarios.
Meanwhile, they caution against drawing conclusions from any single year, even one as weird as this one has been.
Blame ‘the little girl’
Instead, many scientists say at least part of the blame should go to the weather phenomenon called La Niña, Spanish for the little girl. The World Meteorological Organization said (PDF) in February that we were in a La Niña phase and could expect cool temperatures, storms and heavy rain during at least the first half of this year.
A La Niña event occurs when easterly winds cool sea surface temperatures along the equator in the Pacific Ocean, according to NASA’s Goddard Space Flight Center.
These winds force cooler deep water up to the surface of the ocean.
“With this La Niña, the sea-surface temperatures are about two degrees colder than normal in the eastern Pacific and that’s a pretty significant difference,” said David Adamec of the Space Flight Center.
“I know it doesn’t sound like much,” he said. “But remember this is water that probably covers an area the size of the United States. It’s like you put this big air conditioner out there — and the atmosphere is going to feel it.”
This “air conditioner” influences weather in a vast region.
Cloud growth slows
“The cool water temperatures of a La Niña slow down cloud growth overhead, causing changes to the rainfall patterns from South America to Indonesia,” NASA said.
Among other results, the changes affect the strength and location of the jet stream — the strong winds that guide weather patterns over the United States. The Northwest generally experiences cooler, wetter weather during a La Niña. On the Great Plains, residents normally see a colder than normal winter. Southern states typically get less rainfall.
A La Niña event is the reverse of an El Niño, where the ocean surface warms. The name El Niño — Spanish for “the little boy” — refers to the Christ child because the event usually becomes apparent around Christmas time.
When “the little girl” visits, she usually stays about a year.
The current La Niña started taking shape in mid-2007. Initially it was expected to last through March. But in February, the World Meteorological Organization said, “the likelihood of La Niña conditions remains heightened through the second quarter and, at a lower level of confidence, into the first part of the third quarter of 2008.”
La Niña probably had something to do, at least indirectly, with Iowa’s floods. A good share of Iowa’s soil, especially in the state’s eastern reaches, was saturated after heavier than usual snowfall La Niña caused last winter. When the rains came, there was no sponge to soak up the moisture, so it rushed to the rivers.
La Niña also helps explain why this is shaping up to be the deadliest year for tornados since 1998, the Associated Press reported.
La Niña very likely was a factor in tornados that tore through Tennessee, Arkansas and Kentucky in February, the AP said.
But “the little girl” doesn’t fully explain recent stormy weather. And scientists can’t explain it either.
Puzzled scientists say that the Midwest seems to have been stuck for weeks in a rut between hot, sticky air on the East Coast and cool air parked in the West, the AP said. When those systems collide, the Midwest is hit by tornados, thunder storms and heavy rain.
While scientists can forecast hurricane seasons, predicting their land-bound cousins is much harder, research meteorologist Harold Brooks at the National Severe Storms Lab in Norman, Okla., told the AP.
Crucial triggers are local
Tornadoes, like hurricanes, rely on large-scale weather phenomena, but the crucial triggers are extremely local weather conditions, Brooks said.
On top of that, tornadoes have a “Goldilocks” issue. To make a tornado, the conditions have to be just right. Too much or too little of one ingredient and there is no tornado. For example, wind shear — when upper and lower winds are at different speeds or coming from different directions — is crucial to create a funnel cloud. Too little and there is no spin. Too much and the tornado falls apart.
Meanwhile, there is reason for hope that an end is in sight.
La Niña is tired out, according to Iowa State University’s extension service.
The measure of atmospheric pressure that defines a La Niña event has fallen recently, the service said. And the baffling cold and wet weather pattern is ending too.
“The two especially troublesome components of the winter and spring in the Corn Belt this year appear to be coming to an end,” the service reported. “Although the end is not clearly in sight for the La Niña or for the cold-wet conditions, both are moving in the direction of ending.”
Sharon Schmickle writes about foreign affairs and science. She can be reached at sschmickle [at] minnpost [dot] com.