La Niña returns for a second winter – El Niño’s cooler sister

December 1, 2021

This coupling of the atmosphere and the ocean alters atmospheric circulation and jet streams to increase precipitation in some areas and cause drought in others.

For the second year in a row, the coolest sister of El Niño showed up at the winter festival in the eastern Pacific. La Niña is expected to remain at least until spring 2022 in the northern hemisphere.

Part of the El Niño-Southern Oscillation cycle, La Niña occurs as energized easterly trade winds intensify the upwelling of cooler water from the depths of the eastern tropical Pacific, causing large-scale cooling of the eastern and central Pacific Ocean surface near the equator. These stronger than usual trade winds are also pushing warm equatorial surface waters west towards Asia and Australia. This dramatic cooling of the surface layers of the ocean then affects the atmosphere by altering the moisture content across the Pacific. This La Niña coupling of the atmosphere and the ocean modifies global atmospheric circulation and may cause changes in the mid-latitude jet stream trajectory in a way that increase precipitation in some areas and bring drought to others.

In the Western Pacific, precipitation can increase significantly over Indonesia and Australia during La Niña. Clouds and precipitation become more sporadic over the central and eastern Pacific Ocean, which can lead to dry conditions in Brazil, Argentina and other parts of South America and wetter conditions in Central America . In North America, cooler and stormier conditions often set in in the Pacific Northwest, while the weather generally becomes warmer and drier in the southern United States and northern Mexico. (These and other trends are reflected on the map later in this story.)

The image above shows the conditions in 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 from ">NasaJet Propulsion Laboratory (JPL). The globe represents height of sea surface anomalies. Shades of blue indicate below average sea levels; normal sea level conditions appear in white; and reds indicate areas where the ocean was higher than normal. The expansion and contraction of the ocean’s surface is a good indicator of temperatures as warmer water expands to fill more volume, while cooler water contracts.

“This moderate La Niña force can be seen in the Sentinel-6 data as an area of ​​below normal sea level along and below the equator in the central and eastern Pacific,” said Josh Willis, climatologist and oceanographer at JPL. . He noted that the deep trough (blue) above the equator is not the La Niña water body; it is a change in the North Equatorial Counter-Current, which tends to strengthen during La Niña events.

December 1, 2021

“This La Niña probably means bad news for the American Southwest, which is expected to experience below normal precipitation this winter,” said Willis. “This La Niña may not be a big deal, but it’s still an unwanted sign for a region that is already deep in a drought. “

The La Niña event that began in late 2020 is part of a larger climate pattern that has lasted for almost two decades – a cold (negative) phase of the Pacific Decadal Oscillation (PDO). During much of the 1980s and 1990s, the Pacific was locked in a hot PDO phase, which coincided with several powerful El Niño events. But since 1999, a cold phase has dominated. Long-term drought in the American Southwest coincides with this trend, Willis noted.

In one published report On December 9, 2021, the NOAA Climate Prediction Center noted that November sea surface temperatures in the eastern tropical Pacific ranged from 0.7 to 1.2 degrees. Celsius below the long-term mean and 0.9 ° C below the mean in the Niño region 3.4 tropical Pacific (170 ° to 120 ° west longitude). Forecasters have predicted that La Niña conditions will persist through the northern hemisphere winter, with a 60% chance that the ocean will return to neutral conditions from April to June.

This La Niña is the first to be observed by Sentinel-6 Michael Freilich, which was launched in November 2020. “The new satellite gives us a great picture of this La Niña,” said Willis. “With the public dissemination of the mission climate quality data, we are now in a position where Sentinel-6 Michael Freilich can soon resume the climatic record for sea level rise, which dates back to the early 1990s. “

Engineers and scientists have spent the past year calibrating and analyzing data from the new satellite against the existing Jason-3 mission. The team ensures that new, more advanced data is correctly correlated with long-term recordings. New high-resolution Sentinel-6 Michael Freilich datasets were released at the end of November 2020.

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


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