Weather forecasting is both art and science. But the science – especially long-range forecasting for south Louisiana in the summer – makes prediction difficult.
For the past few years, south Louisiana has had blocks of fairly dry weather in the summer, especially when compared to the 1980s and 1990s, according to Jay Grymes, LSU AgCenter climatologist. He expects this pattern of warmer and somewhat drier summers to continue for south Louisiana.
“This pattern is a shift from what farmers had become accustomed to,” he said. “But these current weather patterns are more typical of the longer-term pattern.”
Many years during the 1980s and the early 1990s were unusually wet and were followed by what Grymes called a “mega drought” in 1998-2000.
“The tendency for wet years has ratcheted down since the ‘80s and ‘90s,” he said. “The 1980s and 1990s saw a run of years ranking among the wettest of the past century. And after nearly a two-decade run of wet years, I think most South Louisiana farmers had geared themselves to thinking wet was the new norm.
“Then comes the drought of the late 1990s – quite likely one of the three most intense droughts of the past 100 years for South Louisiana,” he said. “Not only was this a major drought event, but I sense that the run of wetter years before the drought – and the expectation of wet during the summer months – actually amplified the severity of the drought in terms of its impact on South Louisiana agriculture, aquaculture and forestry.”
Drier summers, compared to the 1980s and 1990s, will likely be the pattern for South Louisiana for the next few years, Grymes said.
Weather patterns in the sugarcane-growing regions of Louisiana are influenced by water temperatures in the equatorial region of the Pacific Ocean during the winter and spring seasons. Changes in the water temperatures in the Pacific produce shifts in the behavior of the atmosphere above the ocean.
Ultimately, these atmospheric responses to changing ocean temperatures can produce downstream changes in weather patterns over the Gulf Coast region because the atmosphere transports weather systems from west to east.
Patterns of higher and lower equatorial Pacific water temperatures have been dubbed El Niño and La Niña. And the ocean atmosphere phenomena have been collectively labeled ENSO: El Niño – Southern Oscillation. The term ENSO refers to both the role of Pacific water temperatures (El Niño) and the atmospheric see-saw (Southern Oscillation) that occur in the ocean climate process, Grymes said.
El Niño is defined by warmer-than-normal sea-surface temperatures in the eastern equatorial Pacific Ocean, he said. Historically, El Niño results in wetter-than-normal winters and springs for South Louisiana. La Niña is the opposite – when sea surface temperatures in the eastern equatorial Pacific Ocean are cooler than normal. Winter-spring dryness is likely when La Niña is in place.
When equatorial Pacific water temperatures are near normal, it’s called La Nada – what Grymes calls the “who knows?” range. Neither an El Niño nor a La Niña is present. This happens about half the time.
Grymes says a weak-to-moderate El Niño developed in the late summer and fall of 2006 and produced above-normal rains in the winter of 2007. But El Niño faded rapidly, and much of 2007 was marked by La Nada. Then, a weak La Niña developed in the fall of 2007 for a drier-than-normal 2007-08 winter.
The current La Niña is fading, and forecasters expect La Nada to be in effect for the late summer and fall, Grymes said.
“Even with ENSO signals, weather is variable year to year,” he said. “We can see short dry periods, short wet periods and, of course, near-normal periods.
“And our forecasting skill in summer is dismal,” he said. “There just aren’t any good long-range indicators for this time of year.”
Grymes sees no unusual pattern of dry weather on the horizon for sugarcane growers and expects the remainder of 2008 to be pretty typical.
“Looking ahead,” Grymes said, “we should plan on near-normal rainfall for summer and fall, but there’s always that tropical threat to consider.”
(This article was published in the spring 2008 issue of Louisiana Agriculture.)