The years 2023 and 2024 are projected to be the warmest on record, coinciding with a significant Pacific climate event known as El Niño. This phenomenon raises surface temperatures in the eastern Pacific Ocean, resulting in excessive heatwaves in the Amazon and heavy rainfall across the southern United States. Conversely, the La Niña event introduces cooler temperatures, bringing wetter conditions to the northern United States.
Typically, during an El Niño, the warm water in the eastern Pacific weakens the trade winds, creating a self-reinforcing cycle that amplifies the warming. However, the El Niño of 2023 is distinct; despite rapid ocean warming, the trade winds have remained strong. Researchers from the Scripps Institution of Oceanography, led by Qihua Peng and Shang-Ping Xie, have explored this unique occurrence.
To understand the changes, the team monitored pressure variations across the Pacific using the Southern Oscillation Index (SOI) established by NOAA. Typically, as the eastern Pacific warms during an El Niño, the pressure differences across the Pacific decrease. However, in 2023, while eastern Pacific temperatures soared more than 3°F (2°C) above average, the pressure drop was only about 31% stronger than anticipated. Additionally, alterations in wind speed and direction accounted for only about 30% of the warming. What then accounts for the robust El Niño in 2023?
To answer this question, researchers expanded their analysis beyond the Pacific, examining satellite data for sea surface temperatures from NOAA. They discovered that the North Atlantic and Indian Oceans also recorded unprecedented heat in 2023, with North Atlantic temperatures exceeding 2°F (1°C) above normal, marking an unusual occurrence. This indicates that El Niño events can be influenced by oceanic conditions globally, not simply confined to the Pacific Ocean.
The team employed a computer program to simulate atmospheric responses to oceanic temperatures using a community atmosphere model. This simulation helped assess how heat from the North Atlantic and Indian Oceans affects the Pacific. Results indicated that heat generated large columns of hot air in these regions, which then cooled at high altitudes before descending over the central Pacific. This enhanced updraft and downdraft loop directed the trade winds westward, fortifying the easterly trade winds by about 30% compared to what Pacific warming would alone suggest. If trade winds remained strong, why was the eastern Pacific so warm in 2023?
To uncover this, researchers scrutinized NOAA’s ocean temperature and sea level data over three consecutive years of La Niña from 2020 to 2023 using the Global Ocean Data System. During this period, the strengthening trade winds transported heat into the western Pacific, leading to thermal expansion of the warming waters, creating a “mountain” of warm water in the western Pacific — the highest level of heat storage recorded since 1982. When the weakening La Niña diminished the trade winds, this accumulated warm water surged eastward, paving the way for the El Niño event.
To ascertain if the stored heat alone could trigger El Niño, researchers utilized a computer simulation to model ocean-atmosphere interactions with a coupled general circulation model. They input observed sea temperatures from April 1, 2023, when La Niña ended, omitting all subsequent wind alterations. Their model adeptly replicated 87% of the observed warming from June to December 2023, indicating that only 13% of the warming resulted from trade wind influences. The stored heat migrated eastward via massive underwater waves along the equator, forcing deeper cold ocean water upwards, which warmed the surface layers. This oceanic dynamics thus enabled the 2023 El Niño to emerge without the typical wind feedback.
The research team posits that in an increasingly warmer world, substantial heat reservoirs in the western Pacific may become more prevalent, potentially leading to a rise in the frequency of strong El Niño events. However, since their analysis focused on this singular phenomenon, the frequency of El Niño occurrences driven purely by oceanic processes remains uncertain. Ultimately, their findings reveal that the ocean is not merely a passive player in El Niño events but can actively influence their development.
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Source: sciworthy.com