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，阿拉斯加和北方层的暖冬，以及俄亥俄州山谷的干燥条件，在拉尼娜或冷相ENSO事件期间，情况基本相反。ENSO的可预测性是一个既有实际意义又有科学意义的问题，人们已经做出了许多努力来确定海表温度（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节预测ENSO事件的发生。PI提出了一种新的机制，通过这种机制，与北太平洋涛动（NPO，北方气候变率的一种突出模式）相关的北太平洋表面风的变化可以导致9至12个月后的ENSO事件。该机制，被称为贸易风充电（TWC），涉及南运输相对温暖的次表层沃茨从中亚热带北太平洋，驱动与NPO（Sverdrup运输）相关的表面风应力异常的旋度。向南的热量输送使赤道中太平洋的海洋温跃层变暖，从而为厄尔尼诺事件的发展创造了有利条件（类似的事件链可以促进从NPO的相反相位开始的拉尼娜事件）。对TWC机制的研究涉及到三个相关假设的检验：1）赤道太平洋热含量的正、负信风充量（TWC）分别足以产生暖、冷冬季ENSO事件：2）正、负TWC增强了初始次表层海洋条件产生暖、冷冬季ENSO事件的效能;正TWC增强了经向模态（MM）动力学在产生暖冬ENSO事件中的作用。这项研究是通过一个气候模型的实验进行的，该模型是在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，海洋成分模式根据再分析产品中的地面大气条件进行整合，使海洋状态带有观测到的NPO事件的印记，之后将海洋耦合到大气成分模式，并将完全耦合的气候模式整合几个月，以跟踪TWC引起的ENSO事件的发展（或缺乏）。如上文所述，由于更好地预测厄尔尼诺/南方涛动事件的社会价值，这项工作具有更广泛的影响。更具体地说，研究提供了具体的目标，用于评估业务预测模式用于ENSO预测，和PI计划开发一个指数，可用于监测TWC的行为有关ENSO预测。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.