Studies of Dynamics for El Nino-Southern Oscillation (ENSO) Diversity and Complexity

厄尔尼诺指的是赤道太平洋海洋表面的异常增暖，而南方涛动是一种海平面气压的拉锯模式，太平洋东侧的异常低气压与盆地西侧的高压相匹配，反之亦然。由于厄尔尼诺现象的发生与南方涛动的波动有关，这种组合现象被缩写为ENSO。虽然ENSO是赤道太平洋的一种现象，但它的影响在世界各地都能感受到，包括热带季风的中断、加利福尼亚州的暴雨和泥石流，以及美国东南部冬季风暴频率的增加。考虑到ENSO对全球的影响，在过去的三十年里，ENSO一直是密集的科学研究的主题，现在可以提前几个月进行熟练的ENSO预测。然而，ENSO的所有行为仍未得到令人满意的解释，对ENSO的预测仍有很大的改进空间。本项目旨在了解ENSO事件中发现的多样性，其中“多样性”一词是指事件的幅度和频率的丰富变异性，以及不同事件中出现的海洋表面温度(SST)异常模式的差异。多样性的一个关键形式是存在两种不同类型的ENSO事件。一种是传统的ENSO事件，海温异常集中在赤道东太平洋。另一种称为“厄尔尼诺现象”、“日期线厄尔尼诺现象”、“中太平洋厄尔尼诺现象”和“暖池厄尔尼诺现象”，其最大的海温印记是在赤道太平洋中部，而不是盆地的东侧，它对全球的影响与传统的厄尔尼诺现象有很大不同。这个项目的研究将试图确定这两种类型是否可以归因于两种不同的动力模式，由不同的物理机制驱动，或者这两种类型是否由单一的动力模式和一套单一的机制产生，这取决于不同事件的随机强迫。另一个导致ENSO行为多样性的因素是ENSO循环和年度循环之间的非线性相互作用。这里开展的工作研究了这种相互作用及其对ENSO多样性的影响。初步工作表明，这种相互作用在与ENSO频率和年周期的总和和差相对应的频率上产生与ENSO有关的变化。需要研究的第三个因素是乘性噪声强迫在多大程度上产生了ENSO行为的多样性，其中随机噪声对ENSO的强迫(例如，西风暴风的影响)取决于ENSO SST异常的状态。这种相关性预计会导致极端幅度的事件，并可能解释为什么厄尔尼诺事件通常比拉尼娜事件(符号相反的ENSO事件)更强。如上所述，由于ENSO事件的全球影响及其对人类活动的影响，这项工作具有更广泛的影响。ENSO多样性对于理解和预测ENSO事件及其对世界各地天气和气候的影响是一个重要的问题，这项工作的结果可能有助于改进ENSO预测。此外，这项工作将支持和培训两名研究生，从而为这一研究领域的未来劳动力提供支持。

El Nino refers to an abnormal warming of the ocean surface in the equatorial Pacific, while the Southern Oscillation is a seesaw pattern in sea level pressure in which abnormally low pressure on the eastern side of the Pacific is matched by high pressure on the western side of the basin and vice versa. As El Nino events occur in association with fluctuations of the Southern Oscillation, the combined phenomenon is referred to by the acronym ENSO. While ENSO is an equatorial Pacific phenomenon, its impacts are felt worldwide and include disruptions of the tropical monsoons, heavy rain and mudslides in California, and increases in the frequency of winter storms over the southeastern US. Given its worldwide impacts, ENSO has been a topic of intense scientific research over the past three decades, and skillful ENSO forecasts can now be made up to a few months in advance. However, the full range of ENSO behaviors has not yet been satisfactorily explained, and there is still much room for improvement in ENSO prediction.This project seeks to understand the diversity found in ENSO events, where the term "diversity" denotes the rich variability seen in the amplitude and frequency of events, as well as differences in the pattern of sea surface temperature (SST) anomalies that appear in different events. A key form of diversity is the existence of two distinct types of ENSO events. One is the conventional ENSO event in which sea surface temperature SST anomalies are concentrated in eastern equatorial Pacific. The other, which is referred to as "El Nino Modoki", "Dateline El Nino", "Central Pacific El Nino", and Warm Pool El Nino", has its largest SST imprint in the central equatorial Pacific, rather than the eastern side of the basin, and its impacts over the globe are quite different from those of the conventional El Nino. Research in this project would attempt to determine if the two types can be ascribed to two distinct dynamical modes, driven by different physical mechanisms, or whether the two are produced by a single dynamical mode with a single set of mechanisms which can produce either type depending on stochastic forcing which varies from event to event.Another factor contributing to diversity in ENSO behavior is the nonlinear interaction between the ENSO cycle and the annual cycle. Work performed here examines this interaction and its impact on ENSO diversity. Preliminary work suggests that the interaction creates ENSO-related variability at frequencies which correspond to the sum and difference of the ENSO frequency and the annual cycle. A third factor to be investigated is the extent to which diversity in ENSO behavior is generated by multiplicative noise forcing, in which the forcing of ENSO by stochastic noise (for instance, the effect of winds from westerly wind bursts) is dependent on the state of ENSO SST anomalies. Such dependence is expected to result in events of extreme amplitudes, and may explain why El Nino events are generally stronger than La Nina events (ENSO events of opposite sign).As noted above, the work has broader impacts due to the global effects of ENSO events and their consequences for human activity. ENSO diversity is an important issue for understanding and predicting ENSO events and their effects on weather and climate around the world, and the results of this work could contribute to improved ENSO prediction. In addition, the work will support and train two graduate students, thereby providing for the future workforce in this research area.