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<p>Whereas pronounced westerly anomalies appear in the equatorial Pacific during a typical El Niño, abnormal easterly anomalies occurred in July 2023. The cause of the SST-wind “decoupling” was investigated through observational analyses and idealized numerical model experiments. By comparing precipitation and SST anomaly patterns in July 2023 with those of the El Niño composite, two possible factors that led to the abnormal wind pattern were identified. The first is a marked positive SST anomaly (SSTA) and enhanced precipitation in the tropical North Atlantic (TNA). The second is enhanced precipitation over the Indian summer monsoon (ISM) region and the Maritime Continent (MC). Idealized atmospheric general circulation model experiments were conducted, to validate their potential roles. It was found that the abnormal easterly in July 2023 was primarily driven by the convective heating anomaly in ISM/MC via a large-scale east–west circulation, whereas the TNA warming also played a role, through a Kelvin wave response. Together, these two factors overwhelmed the effect of the El Niño induced westerly, resulting in the easterly anomalies. A further examination of historical data since 1979 shows that such a wind-SST decoupling appeared in 9 out of 42 El Niño summer months, during which the ISM, TNA or the accumulated effect of the Madden–Julian Oscillation (MJO) contributed.</p>

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Why zonal wind anomalies were “decoupled” to El Niño in July 2023

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年2月27日

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  • Yuhang Zhao,
  • Tim Li,
  • Xiao Pan

摘要

Whereas pronounced westerly anomalies appear in the equatorial Pacific during a typical El Niño, abnormal easterly anomalies occurred in July 2023. The cause of the SST-wind “decoupling” was investigated through observational analyses and idealized numerical model experiments. By comparing precipitation and SST anomaly patterns in July 2023 with those of the El Niño composite, two possible factors that led to the abnormal wind pattern were identified. The first is a marked positive SST anomaly (SSTA) and enhanced precipitation in the tropical North Atlantic (TNA). The second is enhanced precipitation over the Indian summer monsoon (ISM) region and the Maritime Continent (MC). Idealized atmospheric general circulation model experiments were conducted, to validate their potential roles. It was found that the abnormal easterly in July 2023 was primarily driven by the convective heating anomaly in ISM/MC via a large-scale east–west circulation, whereas the TNA warming also played a role, through a Kelvin wave response. Together, these two factors overwhelmed the effect of the El Niño induced westerly, resulting in the easterly anomalies. A further examination of historical data since 1979 shows that such a wind-SST decoupling appeared in 9 out of 42 El Niño summer months, during which the ISM, TNA or the accumulated effect of the Madden–Julian Oscillation (MJO) contributed.