Collaborative Research: Extratropical Triggering of El Nino/Southern Oscillation (ENSO) Events Through the Trade-Wind Charging Mechanism

厄尔尼诺/南方涛动(ENSO)事件影响世界各地的天气和气候，厄尔尼诺事件影响美国，包括加利福尼亚州的洪水，阿拉斯加和北部的温和冬季，以及俄亥俄山谷更干燥的条件，在拉尼娜或冷相ENSO事件期间，情况基本上相反。ENSO的可预测性是一个具有实际意义和科学意义的问题，人们已经做出了许多努力来确定海面温度(SST)、海平面气压(SLP)和其他变量的前兆模式，以便可以提前几个月到几个季度预测ENSO事件的发生。PIS提出了一种新的机制，通过这种机制，北太平洋表面风的变化与北太平洋涛动(NPO，北半球气候变率的一个显著模式)有关，可以在9至12个月后导致ENSO事件。这种机制被称为信风充电(TWC)，涉及由与NPO(一种Sverdrup输送)有关的地面风应力异常的卷曲驱动，将相对温暖的次表层水从中央副热带北太平洋向南输送。向南的热量输送使赤道太平洋中部的海洋温跃层变暖，从而为厄尔尼娜事件的发展创造了有利条件(类似的事件链可以促进从NPO相反阶段开始的拉尼娜事件)。对TWC机制的检验包括检验三个相关假设：1)赤道太平洋热含量的正和负信风收费(TWC)孤立地足以分别产生暖的和冷的北半球冬季ENSO事件；2)正的和负的TWC增强了初始次表层海洋条件在产生暖和冷的冬季ENSO事件中的有效性；以及3)正的TWC增强了子午线模式(MM)动力学在产生北暖冬季ENSO事件中的有效性。这项研究是通过使用气候模型进行实验进行的，该模型是由NOAA地球物理流体动力学实验室开发的耦合模型2.5版。在一组实验中，根据从再分析产品获得的地表大气条件对海洋成分模式进行积分，从而使海洋状态带有观测到的NPO事件的印记，之后将海洋与大气成分模式耦合，并对完全耦合的气候模式进行额外几个月的积分，以跟踪由TWC引起的ENSO事件的发展(或缺乏)。如上所述，由于对ENSO事件进行更好的长期预测具有更好的社会价值，这项工作具有更广泛的影响。具体地说，这项研究为用于ENSO预测的业务预报模式的评估提供了具体的目标，PIS计划开发一个可用于监测与ENSO预测相关的TWC行为的指数。此外，该项目还支持一名研究生和一名博士后研究助理，从而为这一研究领域的未来劳动力发展提供支持。

El Nino/Southern Oscillation (ENSO) events affect weather and climate worldwide, with El Nino impacts over the United States including flooding in California, mild winters in Alaska and the northern tier, and drier conditions in the Ohio valley, with largely opposite conditions during La Nina, or cold phase ENSO, events. The predictability of ENSO is a matter of both practical and scientific interest, and there have been many efforts to identify precursor patters in sea surface temperature (SST), sea level pressure (SLP), and other variables so that the onset of ENSO events can be anticipated several months to seasons in advance. The PIs have proposed a novel mechanism through which changes in surface wind over the North Pacific associated with the North Pacific Oscillation (NPO, a prominent mode of Northern Hemisphere climate variability) can lead to ENSO events 9 to 12 months later. The mechanism, dubbed trade-wind charging (TWC), involves the southward transport of relatively warm subsurface waters from the central subtropical North Pacific, driven by the curl of the surface wind stress anomalies associated with the NPO (a Sverdrup transport). The southward heat transport warms the ocean thermocline in the central equatorial Pacific, thus creating favorable conditions for the development of an El Nina event (a similar chain of events can promote a La Nina event starting from the opposite phase of the NPO). The examination of the TWC mechanism involves testing three related hypotheses: 1) Positive and negative trade wind charging (TWC) of the equatorial Pacific heat content in isolation is sufficient to generate warm and cold boreal winter ENSO events, respectively; 2) Positive and negative TWC enhances the efficacy of initial subsurface ocean conditions in generating warm and cold boreal winter ENSO events; and 3) Positive TWC enhances the efficacy of Meridional Mode (MM) dynamics in generating warm boreal winter ENSO events. The research is performed through experiments with a climate model, the Coupled Model version 2.5 developed at the NOAA Geophysical Fluid Dynamics Laboratory. In one set of experiments, the ocean component model is integrated subject to surface atmospheric conditions taken from reanalysis products, so that the ocean state bears the imprint of an observed NPO event, after which the ocean is coupled to the atmospheric component model and the fully coupled climate model is integrated for several additional months to track the development (or lack thereof) of a TWC-induced ENSO event. The work has broader impacts due to the societal value of better long-lead predictions of ENSO events, as noted above. More specifically, the research offers specific targets for the evaluation of operational forecast model used for ENSO prediction, and the PIs plan to develop an index that can be used to monitor TWC behavior relevant to ENSO prediction. In addition, the project supports a graduate student and a postdoctoral research associate, thereby providing future workforce development in this research area.