Weather 101: What is a La Niña?

Weather 101

What Is A La Niña And How Does It Impact Our Weather?

Last week, NOAA (National Oceanic and Atmospheric Administration) announced we officially have a La Niña pattern in place. A La Niña refers to a pattern change in the equatorial Pacific, off the coast of South America.

Part of the El Niño Southern-Oscillation (ENSO), which is also responsible for El Niño, La Niñas are part of a normal cycle in the atmosphere and occur on average every 2-7 years. The last one was observed in 2017-2018.

How Does This Happen?

In the tropics, winds are dominantly out of the east (called the easterly trade winds). This pushes warm water on the ocean’s surface to the west towards Asia and Australia.

Specifically, scientists look at what is referred to as Niño Region 3.4 when determining if a La Niña (or El Niño) is being observed. This is a region along the equator in the Pacific, south of Hawaii.

Niño regions in the Pacific Ocean. Niño 3.4 is the area scientists look at for sea surface temperatures (SST). When the average SST is below-average by at least 0.90°F (0.50°C), we are in a La Niña pattern. Image: National Climatic Data Center

When the winds are stronger, more water is pushed west and cooler water from the seafloor comes to the surface (known as upwelling). This leads to cooler water temperatures across the equatorial Pacific. If this happens long enough, and average sea surface temperatures inside Niño Region 3.4 are at least 0.90°F (0.50°C) below-average, we get an La Niña.

Below-average sea surface temperatures in the equatorial Pacific can impact weather patterns in the U.S.

The opposite occurs when the winds are weaker. The warm water at the equator does not get pushed as much to the west, leading to warmer water temperatures. If this happens long enough, and average sea surface temperatures inside Niño Region 3.4 are at least 0.90°F (0.50°C) above-average, we get an El Niño.

Above-average sea surface temperatures in the equatorial Pacific can impact weather patterns in the U.S.

How does a La Niña Impact the United States?

A persistent high pressure in the Gulf of Alaska could affect the way the Polar Jet Stream (northern jet stream) comes across the United States. On average, a La Niña pattern leads to the following in the U.S. during the winter months

  • Cooler temperatures across western Canada, northern Rockies, and northern Plains.
  • Wetter than average conditions in the Pacific Northwest and Ohio River Valley
  • A dry, warm winter across the southwest, southern Plains, and southeast.
La Niña affects in the United States during the winter months.

Important: while a La Niña has been known to lead to overall changes to seasonal patterns, other factors will still affect our weather during the winter in Northwest Arkansas. So don’t worry snow lovers, we can still get really cold, snowy weather this winter

Do you have a weather question you would like answered? Send an email to us at weather@knwa.com!

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