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）事件影响着全球的天气和气候，厄尔尼诺对美国的影响包括加州的洪水、阿拉斯加和北部地区的暖冬以及俄亥俄河谷的干旱，而拉尼娜或冷期ENSO事件的情况则基本相反。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，以确定海表温度（SST）、海平面压力（SLP）和其他变量的前兆模式，以便可以提前几个月到几个季节预测ENSO事件的发生。ppi提出了一种新的机制，通过这种机制，与北太平洋涛动（NPO，北半球气候变率的一个突出模式）相关的北太平洋地面风的变化可以导致9至12个月后的ENSO事件。这种机制被称为信风电荷（TWC），涉及到由与NPO（一种Sverdrup输送）相关的地面风应力异常旋度驱动的北太平洋中亚热带相对温暖的地下水向南输送。向南的热输送使赤道太平洋中部的海洋温跃层变暖，从而为El Nina事件的发展创造了有利条件（类似的连锁事件可以促进从NPO相反阶段开始的La Nina事件）。对TWC机制的检验包括三个相关假设的检验：1)赤道太平洋热含量的正、负信风充注（TWC）单独足以分别产生温暖和寒冷的北方冬季ENSO事件；2)正、负TWC增强了初始海底条件对寒带冬季ENSO事件的影响；3)正TWC增强了经向模态（MM）动力学对北方暖冬ENSO事件的影响。这项研究是通过NOAA地球物理流体动力学实验室开发的耦合模型2.5版气候模型进行的实验。在一组实验中，将海洋分量模式与从再分析产品中获取的地表大气条件相结合，使海洋状态带有观测到的NPO事件的印记，之后将海洋与大气分量模式耦合，并将完全耦合的气候模式再结合几个月，以跟踪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的社会价值，这项工作具有更广泛的影响。更具体地说，研究为ENSO预测的业务预测模型的评价提供了具体的目标，PIs计划制定一个可用于监测与ENSO预测相关的TWC行为的指标。此外，该项目支持一名研究生和一名博士后研究助理，从而为该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.