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321675

La Niña returns bringing continuing drought

For the second year in a row, the cooler sister of El Niño showed up at the winter party in the Eastern Pacific. La Niña is expected to stick around until at least spring 2022 in the Northern Hemisphere.

“This La Niña probably means bad news for the American southwest, which should see lower-than-normal rainfall this winter,” says Josh Willis, a climate scientist and oceanographer at NASA’s Jet Propulsion Laboratory (JPL). “This La Niña may not be a whopper, but it’s still an unwelcome sign for an area already deep into a drought.”

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. This dramatic cooling of the ocean’s surface layers then affects the atmosphere by modifying the moisture content across the Pacific. This La Niña coupling of the atmosphere and ocean 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 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. (These and other trends are reflected in the map lower in this story.)

Sea surface height anomaly an it impact on weather
NASA

The image above shows conditions across the central and eastern Pacific Ocean as observed from November 26 to December 5, 2021, by the Sentinel-6 Michael Freilich satellite and analyzed by scientists at NASA’s Jet Propulsion Laboratory (JPL).

The globe depicts sea surface height anomalies. Shades of blue indicate sea levels that were lower than average; normal sea-level conditions appear white; and reds indicate areas where the ocean stood higher than normal.

The La Niña event that started in late 2020 fits into a larger climate pattern that has been going on for nearly two decades — a cool (negative) phase of the Pacific Decadal Oscillation (PDO).

During most of the 1980s and 1990s, the Pacific was locked in a PDO warm phase, which coincided with several strong El Niño events. But since 1999, a cool phase has dominated.

La Nina rainfall patterns across the world
NASA

The long-term drought in the American Southwest coincides with this trend, Willis notes.

In a report released on December 9, 2021, forecasters predicted La Niña conditions would persist through Northern Hemisphere winter, with a 60% chance that the ocean would transition back to neutral conditions during the April through June period.

NASA Earth Observatory images by Joshua Stevens, using modified Copernicus Sentinel data (2021) processed by the European Space Agency courtesy of Josh Willis/NASA/JPL-Caltech, and information adapted from the Famine Early Warning Systems Network.

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