Sullivan Arnold, Zhong Wenxiu, Borzelli Gian Luca Eusebi, Geng Tao, Mackallah Chloe, Ng Benjamin, Hong Chi-Cherng, Cai Wenju, Huang An-Yi, Bodman Roger
CSIRO Oceans and Atmosphere, Aspendale, Australia.
School of Earth, Atmosphere, and Environment, Monash University, Melbourne, Australia.
Sci Rep. 2021 Jan 13;11(1):912. doi: 10.1038/s41598-020-79655-7.
The westerly wind burst (WWB) is an important triggering mechanism of El Niño and typically occurs in the western Pacific Ocean. The Fourier spectrum of the wind field over the western tropical Pacific is characterised by a large variety of peaks distributed from intra-seasonal to decadal time scales, suggesting that WWBs could be a result of nonlinear interactions on these time scales. Using a combination of observations and simulations with 15 coupled models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we demonstrate that the main drivers initiating WWBs are quantifiable physical processes rather than atmospheric stochastic signals. In this study, ensemble empirical mode decomposition (EEMD) from the Holo-Hilbert spectral analysis (HHSA) is used to decompose daily zonal winds over the western equatorial Pacific into seasonal, interannual and decadal components. The seasonal element, with prominent spectral peaks of less than 12 months, is not ENSO related, and we find it to be strongly associated with the East Asian monsoon (EAM) and cross-equatorial flow (CEF) over the Australian monsoon region. The CEF is directly related to the intensity of the Australian subtropical ridge (STR-I). Both the EAM and CEF are essential sources of these high-frequency winds over the western Pacific. In contrast, the interannual wind component is closely related to El Niño occurrences and usually peaks approximately two months prior to a typical El Niño event. Finally, the decadal element merely represents a long-term trend and thus has little to no relation to El Niño. We identified EAM- and CEF-induced westerly wind anomalies in December-January-February (DJF) and September-October-November (SON). However, these anomalies fade in March-April-May (MAM), potentially undermining the usual absence of WWBs in the boreal spring. Similar results are found in CMIP6 historical scenario data.
西风爆发（WWB）是厄尔尼诺现象的一个重要触发机制，通常发生在西太平洋。热带西太平洋风场的傅里叶频谱具有多种峰值，分布在季节内到年代际时间尺度上，这表明西风爆发可能是这些时间尺度上非线性相互作用的结果。通过结合观测数据以及来自耦合模式比较计划第6阶段（CMIP6）的15个耦合模式进行模拟，我们证明引发西风爆发的主要驱动因素是可量化的物理过程，而非大气随机信号。在本研究中，全希尔伯特谱分析（HHSA）中的总体经验模态分解（EEMD）被用于将赤道西太平洋的日纬向风分解为季节、年际和年代际分量。季节分量的显著频谱峰值小于12个月，与厄尔尼诺-南方涛动（ENSO）无关，我们发现它与东亚季风（EAM）以及澳大利亚季风区的越赤道气流（CEF）密切相关。越赤道气流与澳大利亚副热带高压脊强度（STR-I）直接相关。东亚季风和越赤道气流都是西太平洋这些高频风的重要来源。相比之下，年际风分量与厄尔尼诺事件的发生密切相关，通常在典型厄尔尼诺事件前约两个月达到峰值。最后，年代际分量仅代表长期趋势，因此与厄尔尼诺几乎没有关系。我们在12月至次年1月至2月（DJF）以及9月至10月至11月（SON）期间识别出了由东亚季风和越赤道气流引起的西风异常。然而，这些异常在3月至4月至5月（MAM）期间减弱，这可能会破坏北半球春季通常不存在西风爆发的情况。在CMIP6历史情景数据中也发现了类似结果。
