Understanding the Dynamics of El Nino-Southern Oscillation (ENSO) Complexity

厄尔尼诺/南方涛动（ENSO）事件发生时，海表温度（SST）变暖（厄尔尼诺）或冷却（拉尼娜）沿着赤道在太平洋中部和东部（CP和EP，分别）。这些事件在全球范围内产生影响，从热带季风的中断到美国和加拿大冬季天气的严重性变化。 洋流、地面风和降水的相互作用导致ENSO事件已得到深入研究，并已查明了基本的动力机制。但重要的问题仍然存在，特别是关于厄尔尼诺/南方涛动行为的事件与事件之间的差异，这会影响预测工作。“厄尔尼诺/南方涛动复杂性”一词是指在个别事件之间通常观察到的差异。 过去关于这一主题的工作强调了ENSO事件高峰时SST模式的差异，特别是CP或EP中最大SST异常的位置。 这里的工作补充了这一分析的系统性差异，在事件转换，试图解释为什么一些温暖的事件之后立即冷事件，反之亦然，而其他人，特别是冷事件，徘徊多年。 在充放（CD）机制中，暖水在暖事件开始之前积聚在赤道太平洋的上层海洋中，随后被排放到高纬度地区。 这种机制导致了暖事件之后紧接着冷事件的振荡。在季节足迹（SF）机制中，冬季中纬度地区的天气异常导致海洋温度的变化，进而导致表面风的变化。 地面风场的变化传播到热带太平洋并导致ENSO事件，PI的工作将这种机制与多年冷事件联系起来。 进一步的研究表明，自1980年以来ENSO行为的变化与SF机制的增强有关，气候模式对CD机制的过度模拟导致ENSO事件过于规则化。（2）了解SF机制正负位相不对称的原因;（3）了解CD和SF机制的平均状态敏感性和缓慢变化;（4）利用ENSO复杂性动力学来理解未来ENSO预测和发展新的模式相互比较的度量;（5）利用ENSO复杂性动力学研究古气候数据中的ENSO活动。 这项工作是通过分析观测数据集和使用各种配置的气候模型进行模拟来进行的。关于目标5的工作利用上一个千年再分析的数据，审查950年至1250年中世纪气候异常与小冰期（1300年至1850年）之间厄尔尼诺/南方涛动复杂性的差异，这项研究具有社会意义，因为厄尔尼诺/南方涛动事件对全世界产生影响，需要对这些事件进行熟练的预测，以便作出适当的准备。 这项工作与预测直接相关，因为重点是从暖到冷的转变与多年持续性，这是厄尔尼诺/南方涛动预测的一个核心挑战。 SF机制在近几十年来变得更加突出的发现也对预测ENSO行为的未来变化具有影响。 该项目将支持和培训一名博士后研究人员以及一名研究生和一名研究生，从而为该研究领域的未来劳动力提供支持。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

El Nino/Southern Oscillation (ENSO) events occur when sea surface temperature (SST) warms (El Nino) or cools (La Nina) along the equator in the central and eastern Pacific (CP and EP, respectively). These events have worldwide consequences from disruptions in tropical monsoons to changes in the severity of winter weather over the United States and Canada. The interplay of ocean currents, surface winds, and precipitation leading to ENSO events has been intensively studied, and the underlying dynamical mechanisms have been identified. But important questions remain, particularly regarding event-to-event differences in ENSO behavior that compromise prediction efforts. The term "ENSO complexity" has been coined to refer to differences commonly observed between individual events. Past work on the topic has emphasized differences in the SST pattern at the peak of the ENSO event, in particular the location of the maximum SST anomaly in either the CP or EP. Work here complements this analysis by looking at systematic differences in event transition, seeking to explain why some warm events are immediately followed by cold events and vice versa while others, particularly cold events, linger for multiple years.Preliminary research by the Principal Investigator (PI) identifies distinct onset mechanisms for ENSO oscillation between warm and cold events and for persistent events of the same sign. In the charge-discharge (CD) mechanism, warm water accumulates in the upper ocean over the equatorial Pacific prior to the onset of a warm event and is subsequently discharged to higher latitudes. This mechanism leads to oscillations in which warm events are followed directly by cold events. In the seasonal footprinting (SF) mechanism, wintertime weather anomalies in the middle latitudes cause ocean temperature changes which in turn produce changes in the surface winds. The surface wind changes propagate into the tropical Pacific and result in ENSO events, and the PI's work links this mechanism to multi-year cold events. Further work suggests that changes in ENSO behavior since 1980 are associated with enhancement of the SF mechanism, and that climate models overrepresent the CD mechanism and thus produce ENSO events which are too regular.This project will address five related goals: 1) understanding the combined effects on ENSO complexity of the CD and SF onset mechanisms and subsequent ENSO evolution; 2) understanding the cause of the asymmetry between the positive and negative phases of the SF mechanism; 3) understanding the mean state sensitivity and slow variations in the CD and SF mechanisms; 4) using ENSO complexity dynamics to understand future ENSO projections and to develop new metrics for model intercomparisons; and 5) using ENSO complexity dynamics to understand ENSO activity in paleoclimate datasets. The work is performed through analysis of observational datasets and simulations using climate models in a variety of configurations. Work addressing goal 5 uses data from the Last Millennium Reanalysis to examine differences in ENSO complexity between the Medieval Climate Anomaly of the period 950 to 1250 and the Little Ice Age period (1300 to 1850).This research has societal relevance due to the worldwide impacts of ENSO events and the desirability of skillful forecasts of them to allow for appropriate preparations. The work is directly relevant to forecasting given the focus on warm-to-cold transition versus multi-year persistence, a central challenge of ENSO prediction. The finding that the SF mechanism has become more prominent in recent decades also has implications for anticipating future changes in ENSO behavior. The project would support and train a postdoctoral researcher as well as a graduate student and a graduate student, thus providing for the future workforce in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.