Monday, October 18, 2021

The cold equivalent of El Nio, La Nia affects US winters in the Pacific

La Nia may be the term that casual weather observers as well as aficionados hear meteorologists use from time to time, especially when breaking long-term weather trends or expected during the upcoming winter or hurricane season. Take a sneak peek at the situations. But what exactly is it?

The key to how winter may manifest in the United States often lies thousands of miles away in the open waters of the equatorial Pacific Ocean. This is where the roots of a climatic phenomenon known as La Nia originates and ultimately helps shape weather patterns around the world.

The pattern occurs when sea surface temperatures in the central and eastern Pacific Ocean drop to below average levels for an extended period. This process is just the opposite El Nio, a phase in which sea surface temperatures rise above mean levels.

La Nia, which translates to “little girl” in Spanish, and El Nio (“little boy”) are two phases of three-dimensional natural climate patterns that occur over a large part of the tropical Pacific Ocean known as El Nio. It is – Southern Oscillation. The three phases of ENSO are divided into the cold phase of La Nia, the hot phase of El Nio and a phase that is neither hot nor cold, known as neutral.

Meteorologists often refer to the effects of climate patterns using a concept known as teleconnection, which is a range of factors, such as sea surface temperature, that influence weather conditions in regions far from the origin of those factors. can make an impact.

Want to know what the weather will be like in a few days? You can often look west.

“For most of the season the US first travels to the Pacific Ocean,” explained AccuWeather meteorologist Joe Bauer.

According to the National Oceanic and Atmospheric Administration, the weather effects of La Nia in the United States are often most pronounced during the winter, although the climate pattern typically lasts nine months to a year and can sometimes last for years. .

So, how can La Nia, a phenomenon that occurs thousands of miles away, affect the weather in the United States?

Colder waters in the equatorial Pacific can help increase the northern jet stream, also known as the polar jet stream, which is a major driver of storms across the country during the winter. A more powerful polar jet stream leads to cooler air and more frequent storms in the northern part of the country.

At the same time, the pattern also weakens the southern jet stream, known as the subtropical jet stream, and often leads to dry and mild weather in the southern regions of the United States.

One of the telltale signs of La Nia winter is wet weather in the Pacific Northwest. Generally, western Canada and the northwestern United States face the most stormy conditions during the La Nia winter.

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However, hurricanes usually lose intensity as they travel over the northern Rockies. If a storm is powerful enough, it can reach farther east and bring snow to parts of the Midwest or Great Lakes region. However, on average, storms that occur during the La Nia winter lack sufficient moisture to produce a major storm for areas east of the Great Lakes.

Even though stronger storms can’t make it past the Rockies, a greater number of weaker storms can navigate the Midwest and Great Lakes. It could maintain snowfall totals at near-normal or even above-normal levels with several smaller incidents, rather than one or two blockbuster blizzards.

Winter in the Northeast during the La Nia phase is extremely variable, with conditions ranging from cold and drier to mild and stormy.

The cold equivalent of El Nio, La Nia affects US winters in the Pacific

While an El Nio pattern can boost moisture currents in California, a typical La Nia prevents winter storms from delivering snow and rain to the region.

“During a moderate to strong La Nia, Southern California may be drier than normal throughout the wet season, leading to more drought conditions,” said AccuWeather’s lead long-range forecaster Paul Pastelok.

Similar concerns exist over much of the Southwest during La Nia winters.

“As the active storm track remains well displaced to the north, the southwest remains generally warm and dry,” Bauer said.

Along with a less powerful southerly jet stream, light conditions can spread from the Four Corners region to the southeast and even parts of the north central Atlantic during the La Nia winter.

It is not only during winter that La Nia makes its presence felt. It also has a significant effect on tropical development in the Atlantic basin during hurricane season.

When the La Nia phase occurs, less wind shear is found in areas of the Atlantic basin where tropical weather develops, increasing the likelihood of a higher-than-normal volume of tropical systems.

La Nia was in place for both the prolific 2005 and 2020 Atlantic hurricane seasons. The 2005 season featured the devastating Hurricane Katrina and was also the first time that the predetermined list of named hurricane names was exhausted.

For the first time in history, forecasters had to turn to Greek letters to name storms in 2005. The second and last time forecasters would tap into the Greek alphabet was the record-breaking 2020 season, which saw an unprecedented 11 direct strikes on US soil from tropical cyclones, including the now-infamous Hurricane Laura.

La Nia and El Nio reach peak intensity in the fall, which is a critical time for the effects of the hurricane season through winter.


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