Welcome Back, La Niña

by | 9.29.2011 at 2:35pm
Follow us on Facebook or Twitter

A map showing the sea surface temperatures in the Pacific for September 18-24. Cooler than normal (blue) waters off the coast of Peru are a telltale sign of la Niña. Click on the image to see the latest conditions.

Well it’s nearly official: La Niña is making her second appearance this year. After a few months’ hiatus this summer, ocean temperatures in the eastern equatorial Pacific have dipped back below normal. Does that mean we’ll also see a return of the extreme global weather of this past winter blamed on La Niña? It’s possible but not very likely. During the International Research Institute for Climate and Society’s climate briefing this month, lead forecaster Tony Barnston shared some of the reasons.

La Niña’s return wasn’t unexpected. Looking at the last 60 years of data on the climate phenomenon, a pattern begins to emerge. “It seems like after a strong El Niño, you’re more likely to see multiple La Niñas,” Barnston said. “This pattern is very similar to other multiyear La Niñas.”

A time series shows El Niño (red) and La Niña (blue) events since 1950. A number of La Niña events show a tendency to do a double dip.

In certain instances, the magnitude of the second dip of La Niña can be even greater than the first. This occurred during the double dip La Niña of 1983-5.  However, that series was preceded by one of the strongest El Niño events on record.

A more apt comparison for this year would be the most recent double dip La Niña, which occurred from 2007-09. The second dip was smaller in magnitude and shorter in time than the first, which is what Barnston expects this year.

Teleconnections

La Niña influences certain climate patterns around the world. The map below shows some of the effects it has on precipitation throughout the year. Last winter’s near-record La Niña led to some climate records being broken. One of the big stories last winter was the record flooding in Queensland, Australia, which was caused in part by the extreme strength of the La Niña.


A map showing the changes in precipitation that usually occur during La Niña. Click the image to download a larger version (pdf).

Other effects lingered even as ocean temperatures briefly returned to normal this summer. The southern U.S., which generally experiences dry weather during La Niña, is still in the grip of the

Wildfires outside of Barstop, Texas have contributed to the $5 billion in damages caused by La Niña-influenced drought gripping the region. brucesflickr/Flickr

worst drought in history. Other factors have since caused the drought to linger. Exceptional drought conditions still stretch from Arizona to Louisiana and have caused at least $5 billion worth of damage.

Drought in the Horn of Africa also contributed to the first famine declaration in three decades. La Niña played some role in the drought, though it’s not the only factor. That’s because the region is only affects from November to the following March during when La Niña causes the region to be drier than usual. Outside of that timeframe, other variables play a larger role in the region’s climate.

Unfortunately, the winter is one of two rainy seasons in East Africa. La Niña’s drying influence contributed to a failed rainy season. In the spring, other factors led to a second failed rainy season. Now more than 12 million people are at risk of dying from starvation and disease.

It Takes Two

The return of La Niña doesn’t bode well for these and other regions that normally feel its impacts. But if there’s a silver lining, it’s that this La Niña won’t likely reach the same strength as the previous one.

“A weak La Niña can, but probably won’t, have the same effect as a strong one,” Barnston said. This is partly because La Niña has two pieces: ocean temperatures and the atmosphere. It takes two to tango, and in this case both components have to boleo together to have widespread impacts on global climate.

By the middle of September, ocean temperatures reached the threshold for La Niña. However, the most common measurement of the atmospheric component of La Niña, known as the Southern Oscillation Index (SOI), hovered near normal for most of month.

A graph showing the Southern Oscillation Index. The Australian Bureau of Meteorology declares a La Niña when values are sustained above 8.

This week it finally started to line up with ocean temperatures, edging closer to La Niña. Hold your olé, though. SOI measures the difference in pressure between Tahiti and Darwin, Australia. It’s a notoriously noisy signal (just look at the graph to the right!). Thus, the Australian Bureau of Meteorology, which uses this index and not ocean temperatures, requires the SOI to remain above a certain threshold for a sustained period of time.

The sluggishness of these two components lining up bodes well for areas adversely affected by La Niña. “If coupling doesn’t occur quickly enough, then we won’t get teleconnections we’d ordinarily expect,” said Barnston.

Still, Barnston anticipates some impacts from La Niña in the coming months. They just likely won’t be as severe as this past winter. Whether that means places such as Texas and East Africa will be granted a full reprieve remains to be seen. Visit IRI’s forecast page for the latest predictions for these and other regions around the world.

Stay connected with IRI on Twitter.

Follow us on Facebook or Twitter

Comment Using Social Media

Comment