2020—2022年与2010—2012年两个“二次变冷”La Nina事件的特征与机制对比

Characteristic and Mechanism Comparisons of Two Multi-Year-CoolingLa Nina Events in 2020—2022 and 2010—2012

厄尔尼诺-南方涛动(EI Nino-Southern Oscillation, ENSO)是由热带太平洋海-气相互作用产生的显著年际变率,其负位相的拉尼娜(La Nina)事件对中国极端天气有巨大影响。本文利用海洋、大气观测、再分析资料集,对比研究了2010—2012年和2020—2022年两个“二次变冷”La Nina事件演变过程中各阶段的物理过程和机制。结果表明,2010—2012年La Nina事件第一个峰值强于2020—2022年La Nina事件,但第二个峰值较后者弱,且其海表温度异常(Sea surface temperature anomaly, SSTA)偏西,该SSTA的强度和位置特征可从次表层海温异常得到进一步验证。另外,赤道太平洋的东风异常也在2010年夏秋季明显比2020年偏强、偏西;而偏东且增强的东风异常使2020—2022年La Nina事件的第二个冬季峰值强于2010—2012事件。通过海洋混合层热收支诊断分析,纬向平流反馈(海表面热通量异常)是引起La Nina事件发展(衰减)的主要因素。在发展阶段,东风异常引起的纬向海流异常将冷水向西输运,成为赤道中-东太平洋冷SSTA发展的主导因子,垂直方向温跃层反馈过程的贡献也不可忽略。本研究以近十年来的两个“二次变冷”La Nina事件为例进行定性和定量的对比分析,明晰了二者生命演变史过程中的物理机制,为探究La Nina事件生消机理和ENSO不对称性在全球变暖下的可能变化提供了一定的参考。

El Nino-Southern Oscillation(ENSO)is the dominant interannual variability in the tropical Pacific Ocean.Its negative phase,the La Nina event,often exerts substantial impacts on the extreme weather and climate in China.The study compares the main evolutionary features and mechanisms of the 2010—2012 and 2020—2022“secondary cooling”La Nina events using ocean and atmospheric observation/reanalysis datasets.The results show that the first peak of the 2010—2012 La Nina event is stronger than that of the 2020—2022 La Nina event,but the second peak is weaker than that of the latter.The sea surface temperature anomaly(SSTA)of 2010—2012 La Nina event is located westward.The intensity and location characteristics can be further verified from the subsurface temperature anomaly.Consistently,the easterly anomaly is stronger and westward in the summer and fall of 2010 while the intensified and eastward easterlies result in the stronger intensity of second winter peak during the 2020—2022 La Nina event.Based on the mixed-layer heat budget,we reveal that the zonal advective feedback(sea surface heat flux anomaly)is the main factor causing the development(decay)of the La Nina events.In the developmental phase,anomalous westward zonal currents caused by easterly anomalies transport cold water westward and become the dominant factor for the development of the cold SSTA in the equatorial central-eastern Pacific.Moreover,thermocline feedback also plays an important role in the development of two events.This study presents qualitative and quantitative comparisons of the two“secondary cooling”La Nina events in recent decade and clarifies the physical mechanisms in the evolution of two events,providing an important reference to investigate the potential changes of La Nina mechanisms and ENSO asymmetry under global warming.