Get Ready for a La Nina Winter

Andrew Freedman

By Andrew Freedman

Last month we gave you a heads up that a La Nina event in the Pacific Ocean was strengthening and would help shape weather patterns for the next several months. Since then, it has become clearer that this La Nina is likely to remain as a moderate or even strong event, and may not abate until the spring of 2011. In other words — you’re going to be hearing a lot more about La Nina in the coming months.

In fact, you may start blaming it (not entirely unfairly, I might add) when you get soaked on your way home from work (more likely to happen this year in the Pacific Northwest and the Ohio Valley), or are told not to take long showers due to worsening drought conditions (more likely this year in the Southwest).

When present, La Nina, along with her better-known sibling, El Nino, acts as a principle architect of the planet’s weather patterns, especially during the fall and winter months. These events, which are part of a larger ocean and atmosphere cycle known to scientists as the “El Nino-Southern Oscillation” or ENSO, tend to occur about every two to seven years. They’re a key source of natural climate variability.

Sea surface temperature departures from average (in °C) from August 29 to September 25, 2010. Credit: NOAA/ESRL.

The chief hallmark of La Nina is a region of cooler-than-average water temperatures in the central to eastern equatorial Pacific Ocean, extending down along the northwest coast of South America. The cooler waters and associated changes in airflow and precipitation patterns can significantly impact the world’s weather. La Nina can even cause a dip in global average temperatures, and the current event may keep 2010 from becoming the hottest year in the surface temperature record.

Throughout the area of the Pacific that meteorologists hone in on to monitor El Nino and La Nina events, water temperatures are currently running about 1.5°C below average, said Tony Barnston, lead forecaster at the International Research Institute for Climate and Society (IRI) in Palisades, New York. This puts La Nina in the moderate to strong category.

Barnston said this La Nina event came on quickly and strengthened fast, but the ocean waters have not continued to cool during the past few weeks. “It has been kind of plateaued at that reading for about a month,” he said.

Barnston said that if you look at conditions from June through August, this La Nina ranks in about the top three for summertime. Typically, La Nina events reach their peak strength in the late fall or winter. “We’re in a small basket of the strongest events if you measure them by July, August, September,” Barnston said.

The key questions now are: how strong is this La Nina is going to get – in other words, are sea surface temperatures likely to cool even more? Also, how long will such conditions last, and how will this affect the upcoming winter?

According to Barnston as well as forecasters at the National Oceanic and Atmospheric Administration (NOAA), computer models all show that this La Nina event will persist for quite a long time, possibly until next spring, but there is less agreement about whether it will grow any stronger. According to IRI’s forecast, the probability for La Nina conditions is 90 percent or greater all the way through March of 2011.  

As for the weather this winter, here’s how La Nina events typically influence average weather conditions in the U.S.:

  • There tends to be below average precipitation across the southern tier, and above average temperatures in the Southeast (This is not good news for the drought stricken Southwest, where new water restrictions may soon go into effect). 
  • The Pacific Northwest tends to be wetter than average.
  • There tends to be an area of colder than average conditions from Oregon and Washington eastward to the western portion of North Dakota and central Montana.
  • La Nina winters tend not to be major snow producers in the big cities (there are exceptions to this rule, however).
  • The Ohio Valley tends to be wetter-than-average.

Some of the typical global impacts of La Nina include increased odds for drought in eastern equatorial Africa, drought in southeastern South America during the Southern Hemisphere winter, above average rainfall in southern Africa, and drought in Southwest Asia including northern Afghanistan and Pakistan.