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

厄尔尼诺是指赤道太平洋海面的异常变暖，而南方涛动是海平面压力的跷跷板模式，即太平洋东侧的异常低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，因此这种联合现象被简称为ENSO。虽然ENSO是一种赤道太平洋现象，但它的影响是全球性的，包括热带季风的中断，加州的暴雨和泥石流，以及美国东南部冬季风暴频率的增加。鉴于其全球影响，ENSO在过去三十年中一直是一个激烈的科学研究课题，现在可以提前几个月做出熟练的ENSO预测。然而，ENSO的全部行为尚未得到令人满意的解释，ENSO预测仍有很大的改进空间。该项目旨在了解ENSO事件的多样性，其中“多样性”一词指的是事件的振幅和频率的丰富变异性，以及不同事件中出现的海表温度（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。一是常规ENSO事件，海温海温异常主要集中在赤道东太平洋。另一种被称为“El Nino Modoki”、“Dateline El Nino”、“Central Pacific El Nino”和暖池El Nino”，其海温印记最大的是赤道太平洋中部，而不是盆地东侧，其对全球的影响与常规El Nino有很大不同。该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是由单一的动力模式产生的，该模式具有单一的一套机制，可以根据随机强迫产生任意一种类型，随机强迫随事件而变化。影响ENSO行为多样性的另一个因素是ENSO循环与年循环之间的非线性相互作用。这里开展的工作考察了这种相互作用及其对ENSO多样性的影响。初步研究表明，这种相互作用在频率上产生了与ENSO相关的变率，其频率对应于ENSO频率与年周期的和与差。要研究的第三个因素是ENSO行为的多样性在多大程度上是由乘法噪声强迫产生的，其中随机噪声（例如，西风爆发的风的影响）对ENSO的强迫取决于ENSO海温异常的状态。这种依赖性预计会导致极端振幅的事件，并可能解释为什么厄尔尼诺事件通常比拉尼娜事件（相反符号的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多样性是理解和预测ENSO事件及其对全球天气和气候影响的重要问题，本工作的结果有助于改进ENSO预测。此外，这项工作将支持和培训两名研究生，从而为该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.