Blame it all on La Nina

For meteorologists and climate scientists, the flooding that has affected the Australian state of Queensland has not been unexpected.

It is seen as collateral damage from the current La Nina, a lowering of sea-surface temperatures in the tropical eastern Pacific Ocean.

Changing patterns

While a typical La Nina, the low-temperature side of the cycle known as the El Nino/Southern Oscillation – usually brings wet weather to the western Pacific, it also brings dry conditions to the Southern United States, including Southern California. But since late December, Southern California has been hit by severe rainstorms, with some areas receiving nearly a normal year’s worth of precipitation in a few weeks.

“Clearly, to this point, the pattern has not been panning out for Southern California,” said Michelle L’Heureux, a meteorologist with the Climate Prediction Centre of the National Oceanic and Atmospheric Administration in Camp Springs, Md.

When La Nina is in effect, as strong easterly trade winds in the tropical Pacific push warmer surface water to the west, colder water rises in the eastern Pacific to replace it. The current La Nina, which began last summer, is considered moderate to strong in intensity, with sea-surface temperatures about 3 degrees Fahrenheit lower, on average, than normal. Colder water in the eastern Pacific means less rainfall there.

“During a La Nina, precipitation almost completely shuts down,” L’Heureux said. The area of rainfall shifts to the western Pacific, and it moves around. “You can kind of think of it as a big blob of precipitation,” she said. At this time of year it moves south of the equator, where it affects parts of Australia.

“Based on the historical record, there’s probably some linkage between La Nina and rainfall over northeastern Australia,” L’Heureux said. The shift in precipitation, in turn, leads to the changes in the jet stream that can affect the United States. In addition to drier-than-average conditions in the South, a La Nina usually brings colder and wetter conditions to the North, particularly the Pacific Northwest. (Climate modelers do not see much of a link between La Nina and the Northeast, however – “There are other climate factors that are out there,” L’Heureux said.)

So far, Southern California has had the kind of winter it might see in a severe El Nino year, when higher-than-normal sea-surface temperatures in the eastern Pacific bring greater rainfall to the West Coast. Yet scientists are far from throwing their models out the window. The explanation for Southern California’s soggy La Nina fortnight lies as much in statistics as anything else, they say. “Climate forecasts are probabilistic,” L’Heureux said. “It’s a tilt in the odds. We’re bound to fail at some point.” David Battisti, a climate scientist at the University of Washington, said that La Nina “shifts the probability distribution around.”

Fewer, high-intensity storms

While the average winter precipitation in the Southwest in a La Nina year is usually lower than normal, the likelihood of extreme storms is actually higher. For Southern California, as the jet stream gets pushed farther north, “it makes the average number of storms go down, but it means the odds of having a really juicy storm go up,” Battisti said.

“It’s still a rare event,” he added. “The jet stream just comes straight into California with a lot of precipitation in it.” He described it as a “statistical alignment,” like a bad day on a freeway when numerous factors combine to make for a major tie-up. It’s possible that after the torrential rains at the start of winter, the rest of the season will be dry in Southern California, as it has been across much of the rest of the Southern United States.

The real question, L’Heureux said, is what happens after spring – whether La Nina persists, or sea temperatures cycle back to normal. A continuation of La Nina could result in below-average summer precipitation over a large part of the country’s midsection, which would not be good news for the United States’ farmers .“We’re certainly not making that forecast now,” she said.