La Niña is officially declared; tropical effects already being seen

La Niña

ORLANDO, Fla. — As anticipated earlier this summer, La Niña had a good chance of making its appearance just in time for the peak of hurricane season. On Thursday morning, the National Oceanic and Atmospheric Administration (NOAA) officially declared that we are in a La Niña phase.

La Niña is a weather pattern that occurs over the eastern Pacific, but it has worldwide effects. Specifically, for the Atlantic Basin, if there is a La Niña present during hurricane season, more tropical systems tend to develop (we´ve already seen record formations so far this season).

Enhanced tropical formations happen because, under a La Niña, there is weaker vertical wind shear, trade winds, and more instability over the Atlantic.

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On the other ocean, sea surface temperatures tend to be cooler over the eastern Pacific and stronger wind shear which causes less tropical systems to form.

La Niña is not something that is forecast like your daily weather forecast. There are conditions that can help scientists determine if there is a possibility for La Niña to develop. But in order to be declared, certain conditions must be met.

Flowchart showing decision process for determining La Niña conditions. Figure by Fiona Martin, adapted by Climate.gov.
Flowchart showing decision process for determining La Niña conditions. Figure by Fiona Martin, adapted by Climate.gov. (NOAA)

Let´s review those conditions

First, the monthly sea surface temperature anomaly must be equal to or less than -0.5C. Then scientists look for other atmospheric conditions, such as the Walker circulation. This is an atmospheric pattern that brings less rain over the central Pacific and rainier weather over Indonesia.

In the last weeks, scientists have observed a stronger Walker circulation, and the temperature anomaly was -0.6C in August.

Scientists have been observing the gradual downslope trend of the sea surface temperature anomaly since April 2020.

Monthly sea surface temperature in the Niño 3.4 region of the tropical Pacific for 2019-2020 (purple line) and all other years starting from neutral winters since 1950. Climate.gov graph based on ERSSTv5 temperature data.
Monthly sea surface temperature in the Niño 3.4 region of the tropical Pacific for 2019-2020 (purple line) and all other years starting from neutral winters since 1950. Climate.gov graph based on ERSSTv5 temperature data.

Then scientists need to forecast how likely it is for the temperature anomaly to continue for the next several months. They use models to predict that the sea surface temperature anomaly will continue to be below the -0.5C mark through the winter. In fact, the forecast calls for the temperature anomaly to stay below the threshold until March 2021.

The contrast in temperatures between the warm western Pacific and the much cooler Pacific is what drives to a strengthening Walker circulation.

Just as in any other region, warmer water temperatures lead to rising air. The air rises over the western Pacific and travels eastward high in the atmosphere, sinking over the eastern Pacific. Once the air sinks, it travels back to the west closer to the surface. When this huge air circulation is stronger than normal, there are higher trade winds and sinking air over the eastern Pacific, just inhibits tropical storm formation greatly.

La Niña in the U.S. during the winter months

Since la Nina is forecast to stay through the winter months, it is important to highlight that it can also bring big shifts to weather patterns for the U.S.

When La Niña conditions are present in the winter, the polar jet stream tends to shift to the north, which brings wetter and cooler conditions for the northern half of the continental U.S. For the southern half, including California all the way east to Florida, the weather tends to be warmer and drier. This is not great news for California. Although it doesn´t mean California will not get any rain in the winter, it does give them a higher chance for the extreme drought conditions to continue. This drought is playing a huge role in the recent fires.

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