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

厄尔尼诺/南方涛动(ENSO)事件发生在中太平洋和东太平洋赤道上海面温度(SST)变暖(El Nino)或变冷(La Nina)时(分别为CP和EP)。从热带季风的中断到美国和加拿大冬季天气严酷程度的变化，这些事件都会在世界范围内产生影响。海流、地面风和降水之间的相互作用已被深入研究，并确定了其潜在的动力机制。但重要的问题仍然存在，特别是关于ENSO行为的事件对事件的差异，这损害了预测工作。术语“ENSO复杂性”被创造出来，指的是通常在各个事件之间观察到的差异。以往关于这一主题的工作强调了ENSO事件高峰期SST模式的差异，特别是在CP或EP中最大SST异常的位置。这里的工作通过观察事件转换的系统差异来补充这一分析，试图解释为什么一些暖事件紧随其后的是冷事件，反之亦然，而其他事件，特别是冷事件持续数年。首席调查员(PI)的初步研究确定了ENSO振荡在暖事件和冷事件之间以及相同符号的持续性事件之间的不同爆发机制。在充放电(CD)机制中，暖水在暖事件发生之前聚集在赤道太平洋的上层海洋中，随后被排放到更高的纬度。这一机制导致了暖事件紧随冷事件之后的振荡。在季节足迹(SF)机制中，中纬度冬季天气异常引起海洋温度变化，进而引起地面风的变化。地面风的变化传播到热带太平洋并导致ENSO事件，PI的工作将这一机制与多年的冷事件联系起来。进一步的工作表明，自1980年以来ENSO行为的变化与SF机制的增强有关，气候模式过度描述了CD机制，从而产生了过于规律性的ENSO事件。本项目将解决五个相关目标：1)了解CD和SF机制对ENSO复杂性的综合影响以及随后的ENSO演变；2)了解SF机制正负阶段不对称的原因；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的ENSO预测，并发展新的模型相互比较的度量标准；(5)利用ENSO复杂性动力学来理解古气候数据集中的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。解决目标5的工作使用来自上一个千年再分析的数据来检查950-1250年期间的中世纪气候异常与小冰期(1300-1850年)之间ENSO复杂性的差异。这项研究具有社会意义，因为ENSO事件在世界范围内的影响以及对它们进行熟练预测的必要性，以便进行适当的准备。这项工作与预报直接相关，因为重点是冷暖过渡，而不是多年持续性，这是ENSO预测的核心挑战。近几十年来，SF机制变得更加突出，这一发现也对预测ENSO行为的未来变化有影响。该项目将支持和培训一名博士后研究员以及一名研究生和一名研究生，从而为这一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优势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.