R.I.P. La Niña

Attendees observed a brief moment of silence during this month’s climate briefing. Why? Because after nine months, the climate phenomenon La Niña has died. Is there any chance a zombie La Niña could rise from the dead, though? Tony Barnston, IRI’s lead forecaster, answered that question and more.

Sea surface temperatures in the equatorial Pacific for the week of May 15, 2011. Near normal temperatures (white) in the eastern tropical Pacific show the demise of La Niña. Click on the chart to go to an interactive version in the IRI Data Library.

This year’s La Niña was near record setting by one measurement. The Southern Oscillation Index (SOI), which measures the difference in atmospheric pressure between the island of Tahiti and Darwin, Australia, reached nearly three standard deviations above normal. The only time a stronger La Niña-leaning SOI was recorded was in August 1917, when it reached 3.3 standard deviations above normal.

In contrast, sea surface temperature anomalies in the eastern equatorial Pacific, which is the other common metric to measure ENSO events, were only moderate strong compared to past La Niñas.

Even as it was winding down, ocean and atmospheric conditions still indicated a moderately strong La Niña up until two weeks ago. Then suddenly, a pool of warm water surfaced in the eastern equatorial Pacific, breaking up the pool of cooler water that’s indicative of La Niña conditions. Atmospheric conditions decayed quickly afterwards and by the beginning of May, La Niña was officially a goner.

It seems likely to stay that way, too. The IRI’s forecast shows an increased likelihood for neutral conditions in the region through the rest of the year. Neither La Niña nor El Niño has more than a 20-25% chance of forming.

IRI’s ENSO forecast for the rest of the year split up by three month seasons. Green bars show the enhanced likelihood for near normal conditions in the ENSO region. Click on the chart to see IRI’s latest ENSO forecast.

That means a likely reprieve from the torrential rains that have soaked Australia, the Philippines, Colombia and Thailand over the past few months. It also means a possibly break for the Southeastern U.S., which experienced unprecedented tornado and flooding outbreaks in April and May.

Much has been made recently of whether climate change had a hand in the swath of destructive tornadoes. La Niña might be a better place to start looking, though. “Some statistics suggest there’s an enhanced tendency for tornadoes in a La Niña year. However, a general connection does not appear to be overwhelming,” Lyon said.

A paper presented at the 19th Conference of Severe Local Storms supports this view. A recent piece by IRI’s Eric Holthaus in the Wall Street Journal also touches on the connection.

However, Lyon stressed more research needs to be done because not all La Niñas have spawned above normal tornado seasons. One possible area of future research is whether certain factors associated with La Niña, such as SOI, have a greater impact on tornadoes than others.