Underestimated relationship between westerly wind bursts and ENSO in CMIP6 models

Abstract

Observations show that westerly wind bursts (WWBs) in the tropical Pacific play a key role in the triggering, development, and diversity of El Niño/Southern Oscillation (ENSO). Hence, realistic reproduction of WWBs in coupled models is desired for improved ENSO simulation, prediction, and even projection. Here, the authors demonstrate that the ensemble mean of CMIP6 coupled models reproduces well the zonal distributions of the climatology and interannual variability of WWB frequency in the tropical Pacific. However, the WWB–ENSO relationship is largely underestimated in most CMIP6 models; the correlation coefficient between the annual WWB frequency at around 150°E and the October–December Niño3.4 index is 0.82 in the observations but only 0.31 in the multi-model ensemble mean for a 31-year period. This is probably because the air–sea coupling is weaker in most CMIP6 models compared to the observed, and the inter-model correlation coefficient between the strength of air–sea coupling and the WWB–ENSO linkage reaches up to 0.91. Further analyses are still needed in future to understand the model biases responsible for the weakened air–sea coupling in the CMIP6 models.摘要观测表明, 热带太平洋的西风爆发(WWBs)在厄尔尼诺南方涛动(ENSO)的发生, 发展和多样性中起着关键作用. 因此, 在耦合模式中真实地再现WWBs对于改进ENSO的模拟和预测有重要意义. 在本研究中, 作者发现CMIP6的耦合模式的集合平均能很好地再现了热带太平洋WWB发生频率的纬向分布及其年际变动. 然而, 大多数CMIP6模型极大低估了WWB和ENSO的线性关系. 这可能是因为大多数CMIP6模式里海气耦合强度低于观测: 海气耦合强度与WWB-ENSO关系的模式间相关系数高达0.91.