Update 9/20/10 8:30 p.m.: Please check out John Fleck’s excellent post about the connection between La Niña and flow in the Colorado River. His reporting shows there is not much of a correlation. (Also see my update with comments from Klaus Wolter below.)
By one measure, the tropical Pacific Ocean has plunged into La Niña conditions at a record pace over the past three months. The result: as of September 3, this La Niña is the second strongest on record for this time of year.
If moderate to strong La Niña conditions persist, as scientists expect, they should alter weather patterns around the world, including over the United States. Projecting specific impacts in the future is obviously a difficult proposition. But one scenario is that already dry conditions in the Southwest could worsen, further depleting lakes Mead and Powell on the Colorado River.
UPDATE 9/20/10 4 p.m. And some impacts may have already turned up. According to Wolter, La Niña typically is associated with dry and windy conditions in late summer and fall in Colorado, with a heightened risk of wildfire — and that has already happened.
In fact, a particularly intense wind storm on Labor Day weekend along Colorado’s Front Range was the strongest he had seen at that time of year for over two decades. That was the day that the Fourmile Canyon fire, the most destructive wildfire in Colorado history, started. Driven by the winds through dry forests, it quickly spread to more than 6,000 acres and destroyed 169 homes.
At ESRL, Wolter monitors the El Niño/Southern Oscillation, in which conditions in the tropical Pacific seesaw between warmer and cooler than normal sea surface temperatures. La Niña is the cool phase of the seesaw, and it began to take hold earlier this year.
NOAA’s official ENSO advisory, published monthly, focuses on a a number of variables, including sea surface temperature. The most recent advisory states simply that “it is likely that the peak strength of [of the current La Niña] will be at least moderate . . . to strong.”
Wolter uses a more detailed approach than the one employed in the official NOAA advisory. Called the “Multivariate ENSO Index,” or MEI, it takes into account more variables, such as winds, cloudiness, ocean and air temperatures, and atmospheric pressure. When La Niña conditions take hold, the index declines into negative values. This time, the MEI “has dropped just about as fast as it can,” Wolter says.
One has to go back to 1955 to find stronger La Niña conditions for this time of year in the MEI record, and back to September-October 1975 for lower MEI values at any time of year.
UPDATE 9/20/10 4 p.m.: Wolter says NOAA has been a little slow to acknowledge the speed of this La Niña’s development, and its depth. “NOAA has just been a bit too conservative in calling this one ‘moderate to strong’, he says. “My guess is that they will change their tune by next month’s update, or the month after that.”
Climatologist Bill Patzert of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., agrees that ”this La Niña has strengthened for the past four months, is strong now and is still building.” Quoted in a JPL press release, he worries about the impact on the southwestern United States and California:
“After more than a decade of mostly dry years on the Colorado River watershed and in the American Southwest, and only one normal rain year in the past five years in Southern California, water supplies are dangerously low . . . This La Niña could deepen the drought in the already parched Southwest and could also worsen conditions that have fueled Southern California’s recent deadly wildfires.”
La Niña typically brings enhanced chances of below-average precipitation in the Southwest (along with above average precipitation in the Pacific Northwest).
UPDATE 10/20/10 9:40 a.m: Here is part of Klaus Wolter’s response to some questions I sent him after I posted the original article. I’ll post all of his relevant comments as time allows. For now, here is what he says about the potential impact of the developing La Niña on Colorado and areas further south.
Big impact is dry late summer/fall weather, already being observed. There is also a higher risk of wildfires due to more windstorms as we get into early winter (the Labor Day windstorm west of Boulder was the strongest I have witnessed since 1988 for this time of year). As I wrote above, the prospects for areas to our south are fairly grim this winter, while November through February can be quite snowy with La Niña in our mountains (recall how 2007-8 had lots of snow during the winter months, except for November). I will probably come up with a more detailed forecast later this fall, stay tuned.
Lakes Powell and Mead along the Colorado River have been severely impacted by a decade of dry conditions. Lake Powell is currently at 63 percent of capacity. Lake Mead is in even worse shape: It’s at just 40% of capacity, and still dropping. To find a lower level for the lake, you have to go
way back to 1936 1956, shortly after Mead began to fill.
Given the strengthening La Niña, a rebound doesn’t seem terribly likely. And that may be putting it mildly.