On the Second-Year Warming in Late 2019 over the Tropical Pacific and Its Attribution to an Indian Ocean Dipole Event

Abstract

After its maturity, El Niño usually decays rapidly in the following summer and evolves into a La Niña pattern. However, this was not the case for the 2018/19 El Niño event. Based on multiple reanalysis data sets, the space-time evolution and triggering mechanism for the unusual second-year warming in late 2019, after the 2018/19 El Niño event, are investigated in the tropical Pacific. After a short decaying period associated with the 2018/19 El Niño condition, positive sea surface temperature anomalies (SSTAs) re-intensified in the eastern equatorial Pacific in late 2019. Compared with the composite pattern of El Niño in the following year, two key differences are evident in the evolution of SSTAs in 2019. First, is the persistence of the surface warming over the central equatorial Pacific in May, and second, is the re-intensification of the positive SSTAs over the eastern equatorial Pacific in September. Observational results suggest that the re-intensification of anomalous westerly winds over the western and central Pacific, induced remotely by an extreme Indian Ocean Dipole (IOD) event, acted as a triggering mechanism for the second-year warming in late 2019. That is, the IOD-related cold SSTAs in the eastern Indian Ocean established and sustained anomalous surface westerly winds over the western equatorial Pacific, which induced downwelling Kelvin waves propagating eastward along the equator. At the same time, the subsurface ocean provided plenty of warm water in the western and central equatorial Pacific. Mixed-layer heat budget analyses further confirm that positive zonal advection, induced by the anomalous westerly winds, and thermocline feedback played important roles in leading to the second-year warming in late 2019. This study provides new insights into the processes responsible for the diversity of El Niño evolution, which is important for improving the physical understanding and seasonal prediction of El Niño events.

摘要

发生在赤道中东太平洋的厄尔尼诺事件在当年冬季发展成熟之后, 往往于次年春夏季快速衰减并转入拉尼娜状态, 2018/19 年的厄尔尼诺事件却不同于此. 基于多种再分析资料, 本文研究了 2018/19 年厄尔尼诺事件次年热带太平洋不同寻常的二次增暖的时空演变特征及其触发机制.

2018/19 年厄尔尼诺事件在经历 2019 年春季短暂的减弱之后, 赤道中东太平洋正海表温度异常在 2019 年下半年再次增强. 与厄尔尼诺事件合成次年相比, 2019 年的海温演变有两点显著不同. 第一, 5 月份赤道中太平洋海温仍维持暖状态, 第二, 9 月份赤道东太平洋暖海温异常再次加强.

观测资料分析表明, 受超级印度洋偶极子事件影响, 赤道中西太平洋异常西风二次增强, 引发赤道中东太平洋海温二次增暖. 具体来说, 印度洋偶极子在东印度洋的冷海温异常在赤道西太平洋触发并维持异常西风, 异常西风激发下沉开尔文波沿赤道东传, 同时赤道中西太平洋次表层暖异常维持. 混合层热收支分析表明, 异常西风引起的纬向平流和温跃层反馈在 2019 年下半年赤道中东太平洋二次增暖过程中起到重要作用. 其中纬向平流反馈在 9-11 月起主要作用 (~0.7 °C/月), 温跃层反馈在 12 月起到主要作用 (~0.4 °C/月). 该研究为认识厄尔尼诺演变多样性提供了新的视角, 有助于加深对厄尔尼诺事件的理解及预测水平的提高.