Beyond the Forecast – El Niño and La Niña cycles

El Niño brings warmer water further east favoring more wet weather in the southern part of the country

Happy Monday and welcome to another edition of Beyond The Forecast!

There are plenty of forecasting tools that we use each day. Direct observations from radar, satellite and weather balloons are immensely helpful in figuring out what the atmosphere is doing right now. That real-time data can give forecasters a picture of where our air is coming from and what changes it will bring.

The measurements are also useful in giving computer models information to start with. The models we use each have a different set of equations that give an idea of how conditions change over a specified period. Some models give data for more than a week at a time.

These tools are most useful for forecasting in a relatively short time frame. When it comes to seasonal forecasting, another set of helpful tools is teleconnections. Broad air patterns over oceans or continents give indications as to how air will move over the next few months. Instead of giving a specific temperature, these patterns can tell us if we generally have more or less rain and relatively cool or warm temperatures in the near future.

One of the most helpful teleconnections for forecasting in the United States is the El Niño-Southern Oscillation (ENSO) cycle. ENSO most directly describes a temperature difference in the waters of the Pacific Ocean.

La Niña concentrates warm water towards Indonesia letting much cooler water from deep in the ocean rise near South America

The standard temperature pattern in this part of the world has warmer water near the Indonesian coast with cooler water close to Peru. An El Niño phase means warm water concentrates closer to South America, and a La Niña phase pushes the warm water even closer to Indonesia than it usually is leaving waters off Peru colder than average.

In fact, El Niño gets its name from a South American fisherman that recognized warmer coastal waters in December. The name started as El Niño de Navidad (Spanish for little boy in connection with the Christmas holiday), and scientists joined it with the Southern Oscillation after more research in the 20th century.

Virginia sits on the edge of El Niño effects, but typically an El Niño pattern means more snowfall for us
La Niña generally lets cooler air sink into the Midwest during winter while we have warmer, drier weather

The ENSO cycle most directly affects rainfall in the tropics of the globe. El Niño concentrates rain toward South America and slows the usual east-to-west winds in that part of the world. La Niña means more Indonesian rain and stronger than average winds.

The differences in heat concentration and rainfall affect air across the Pacific Ocean. In the northern hemisphere, El Niño means the polar Jet Stream drops southward in North America and causes more precipitation and cooler air across the northern part of the US. La Niña keeps the Jet Stream further north; in Virginia, it keeps conditions warmer and drier than average.

The ENSO cycle can be neutral when the Pacific waters follow a more average trend. Winters in a neutral phase are cold in the north, and they tend to bring extra rain and snow to the south.

When neither El Niño or La Niña are in control of the weather the northern part of the country stays cold with snow and rain in the southeast throughout the winter

In this year’s Winter Weather Outlook, our team will describe how the ENSO cycle will affect snowfall through the winter. For details on how this week shapes up in the short term, you can download our weather app and get Meteorologist Chris Michaels’ latest updates online.

You can always get specific forecast details for your zone, whether it’s the Roanoke Valley, Lynchburg area, the New River Valley or elsewhere around Southwest and Central Virginia, anytime at Know your zone!

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-- Marshall Downing

About the Author:

Marshall Downing presents the weather Saturday and Sunday evenings at 6:00 PM and 11:00 PM, and you can see him during the week at 12:00 PM and 5:30 PM.