La Niña Times Three

In December 2022, Earth was in the grips of La Niña—an oceanic phenomenon characterized by the presence of cooler than normal sea-surface temperatures in the central and eastern tropical Pacific. The current La Niña, relatively weak but unusually prolonged, began in 2020 and has returned for its third consecutive northern hemisphere winter, making this a rare “triple-dip” event. Other triple-dip La Niña’s recorded since 1950 spanned the years 1998-2001, 1973-1976, and 1954-1956. (...)

Much like El Niño, La Niña events affect weather across the globe. “When the Pacific speaks, the whole world listens,” explained Josh Willis, a climate scientist and oceanographer at NASA’s Jet Propulsion Laboratory (JPL). “Their strongest impacts are on either side of the Pacific Ocean. Floods in northern Australia, Indonesia and southeast Asia are common in La Niña years, as is drought in the American southwest.” Meteorologists have linked the current La Niña to a variety of natural disasters, including drought and food security problems in the Horn of Africa, flooding in Australia, and drought in the U.S. Southwest.

Part of the El Niño-Southern Oscillation cycle, La Niña appears when energized easterly trade winds intensify the upwelling of cooler water from the depths of the eastern tropical Pacific, causing a large-scale cooling of the eastern and central Pacific ocean surface near the Equator. These stronger-than-usual trade winds also push the warm equatorial surface waters westward toward Asia and Australia.

The cooling of the ocean’s surface layers during La Niña affects the atmosphere by modifying the moisture content across the Pacific. It alters global atmospheric circulation and can cause shifts in the path of mid-latitude jet streams in ways that intensify rainfall in some regions and bring drought to others.

In the western Pacific, rainfall can increase dramatically over Indonesia and Australia during La Niña. Over the central and eastern Pacific, clouds and rainfall become more sporadic, which can lead to dry conditions in southern Brazil, Argentina, and other parts of South America and wetter conditions over Central America. In North America, cooler and stormier conditions often set in across the Pacific Northwest, while weather typically becomes warmer and drier across the southern United States and northern Mexico.

La Niña tends to change in sync with the seasons. Both El Niño and La Niña tend to be at their strongest in December. “Then in the spring, the tropical Pacific resets itself and starts building toward whatever is going to happen in the following winter,” explained Willis. “The best bet right now is that this La Niña will last through the winter. Then next spring, we’ll be back in wait and see mode for what happens in winter 2023-2024.”

by NASA Earth Observatory |  Read more:

Image: Joshua Stevens, NASA Earth Observatory