John Grymes | 5/4/2006 11:41:06 PM
Extremely dry weather in recent months across the southern Louisiana parishes has many farmers concerned about a repeat of the mega-drought conditions of 1998-2001. And their fears are not unfounded, according to LSU AgCenter weather specialist and extension climatologist Jay Grymes.
For some areas of southern Louisiana, March proved to be the driest March on record, and April continued that trend. In fact, 2006 rainfall through April was less than 50 percent of normal for much of South Louisiana, and at least part of the blame can be placed on the development of La Niña.
"La Niña is most easily identified by the development of cooler-than-normal sea-surface temperatures in the central and eastern equatorial Pacific Ocean," Grymes says, adding, "We view it as the 'alter-ego' of El Niño, when water temperatures in the same region of the Pacific are above average."
El Niño and La Niña are part of a large-scale, irregular cycle in the Pacific Ocean called the El Niño/Southern Oscillation. ENSO has been linked to changing weather patterns over sections of the United States, including the Gulf of Mexico where ENSO's greatest impact can be seen in changing rainfall patterns during the winter and spring. El Niño events have gained the greatest national attention in recent decades and are known to produce "wet" winter-springs for the Bayou State, resulting in enhanced flood threats at that time of year. But La Niñas are proving to be just as important for Louisiana, as evidenced by the state's devastating drought of 1998-2001, which was attributed in part to a prolonged La Niña cycle.
El Niño (the "little boy") and La Niña (the "little girl") events tend to flip-flop over time, with each phase of the ENSO cycle recurring every two to six years, on average. The irregular cycle is further compounded by the fact that the ENSO phases may last as little eight to 12 months but occasionally may persist for periods of up to three years.
"During La Niñas, winter and spring storm tracks tend to be diverted farther north than usual. At the same time, we tend to see fewer winter storm systems developing in the northwestern Gulf. The lack of well-developed winter rainmakers can leave much of Louisiana, especially the southern third of the state, short on winter and spring rains," Grymes says. "A look at La Niñas over the past 60 years suggests that as many as 8-in-10 La Niña events result in dry winters and springs for the southernmost parishes."
Dryness is a growing concern for south Louisiana agriculture and forestry. Current soil-moisture levels appear to be running at less than 25 percent of capacity at a time of year when soilwater content is typically at an annual maximum.
"As we head into the warmer months, the moisture that vegetation would normally obtain from the deeper soils is simply not going to be there," Grymes notes. "Without significant rains in the next few weeks, even those fields, pastures, timberstands and natural landscapes that are doing OK today will be suffering serious moisture stress."
And a return to "normal" rainfall may not be sufficient to bring a quick end to the drought. "What we need is a prolonged run of wet weather to get the southern parishes ready for the summer," Grymes says, adding, "Unfortunately, that does not seem to be in the cards."
Current long-range outlooks suggest that drier-than-normal weather may persist into the early summer or beyond. Projections by the National Weather Service’s Climate Prediction Center indicate only a 3-in-10 chance for above-normal rainfall over the next three months for South Louisiana.
"We also must recognize that the recent run of drought weather is compounded by what proved to be a very dry year in 2005," Grymes says. Rainfall was below normal statewide last year, with southern parishes reporting annual totals from 10 inches to nearly 20 inches below average.
"And that's even with the three hurricanes – Cindy, Katrina and Rita," Grymes recalls, noting, "In fact, were it not for those tropical rains, some southern parishes would have experienced below-normal rainfall for each of the past 12 months."
But La Niña may continue to play a role in south Louisiana long after she releases her stranglehold on winter-spring rains.
The consensus opinion among tropical-weather experts is that 2006 will be another active year in terms of storm counts, and La Niña is tagged as a probable contributor. History shows that hurricane activity in the Atlantic Basin tends to be increased when La Niña persists into the summer and fall months.
Although the future of the current La Niña is somewhat uncertain, most indicators suggest that the current La Niña event should remain intact to the start of the upcoming hurricane season, June 1. Thus, hurricane forecasters, including the pre-eminent tropical prognosticator, Dr. William Gray, and his Colorado State University forecast team are citing La Niña as one of the reasons for expecting an above-average season again this year.
"A couple of things to remember," notes Grymes. "First, hurricane forecasters are not anticipating a repeat of last year's record setting pace of 27 names storms, but just about everyone expects storm counts to climb back into the teens this season. Second, keep in mind that 10 of the past 11 seasons have been above-normal, and that alone is reason enough to be wary."
Add the likely increase in storm frequencies should La Niña persist into the fall to the recent decade-long trend of above-normal storm counts, and the threat of landfall somewhere in the Gulf becomes a near certainty, the climatologist predicts.
"That does not mean that Louisiana is a definite target this hurricane season, but the past five years have been the busiest five years on record for the Bayou State in terms of landfalls, punctuated by last year's devastation. All indications point to a heightened threat for a Louisiana landfall this season," Grymes says.
On the Internet: LSU AgCenter: www.lsuagcenter.com
Source: Jay Grymes (225) 578-6870, or JGrymes@agcenter.lsu.edu