Mechanisms for El Nino and La Nina Evolution Asymmetry and Formation of Super El Ninos

厄尔尼诺/南方涛动（ENSO）事件是指赤道太平洋中部和东部大部分海域的海表温度变暖或变冷，影响全球的天气和气候，包括对美国的重大影响。ENSO的潜在机制已经确定，预测工作也取得了一些成功，但基本问题仍然存在。其中一个问题是，为什么厄尔尼诺事件（赤道太平洋异常温暖的海温）结束的方式不同于拉尼娜事件（寒冷的海温），因为厄尔尼诺之后往往是拉尼娜，而拉尼娜更有可能持续或重新发展。该项目的初步工作已经确定了在厄尔尼诺和拉尼娜事件成熟期和衰退期，中太平洋地面通量和风应力模式的差异。这里进行的工作结合了海洋混合层热收支分析和模式模拟（主要是不耦合的海洋和大气模式），以确定这些差异的来源及其对事件终止差异的重要性。第二个问题是，为什么一些被称为“超级厄尔尼诺”的厄尔尼诺现象会迅速增长到比典型事件更大的幅度（超过两个标准差）。初步工作已经确定了超级厄尔尼诺和普通厄尔尼诺的发生和发展阶段的差异，包括超级事件中的赤道外温跃层深度异常，并通过耦合和非耦合海洋-大气模拟实验的组合来检验这些差异。研究的另一项任务是在温室气体引起的未来气候变化模拟中检验ENSO振幅的变化。来自耦合模式比较项目第5版（CMIP5）的模拟调查显示了实质性的分歧，大致相同数量的模式预测ENSO事件强度的增加或减少。这里的工作旨在确定反馈机制的差异（例如Bjerknes反馈），这可以解释模型到模型的振幅差异。一旦这些被确定，它们将通过混合层热收支的线性分解与模型背景状态的变化联系起来。考虑的一种假设是，振幅差异与ENSO海温型经向宽度的变化有关，而这又与太平洋副热带气旋的强度有关。由于ENSO事件的全球后果和改进ENSO预测的可取性，这项工作具有更广泛的社会影响。2014年有预测称超级厄尔尼诺现象即将到来，但并没有成为现实，这表明我们特别需要更好地了解超级事件的本质。提高对气候变化如何影响ENSO事件的理解的工作也具有社会价值，PI打算根据该奖项的工作制定ENSO幅度变化的预测。此外，该项目为研究生提供支持和培训，从而促进该研究领域的劳动力发展。

ABSTRACTEl Nino/Southern Oscillation (ENSO) events, in which the sea surface temperatures (SSTs) warm or cool over a large portion of the central and eastern equatorial Pacific, affect weather and climate worldwide including substantial impacts over the US. The underlying mechanisms of ENSO have been identified and prediction efforts have met with some success, but fundamental questions remain. One such question is why El Nino events (with anomalously warm SSTs in the equatorial Pacific) terminate differently from La Nina events (with cold SSTs), as El Ninos are often followed by La Ninas while La Ninas are more likely to persist or redevelop. Preliminary work by the PI has identified differences in the pattern of surface fluxes and wind stress in the central Pacific during the mature and decaying phases of El Nino and La Nina events. Work performed here uses a combination of ocean mixed layer heat budget analysis and model simulations (primarily with uncoupled ocean and atmosphere models) to determine the origins of these differences and their importance for differences in event termination.A second question is why a few El Ninos, termed "super El Ninos", grow rapidly to substantially larger amplitude (over two standard deviations) than typical events. Preliminary work has identified differences in the onset and development phases of super and ordinary El Ninos including off-equatorial thermocline depth anomalies in the super events, and work here examines these differences through a combination of coupled and uncoupled ocean-atmosphere modeling experiments.A further task of the research is an examination of changes in ENSO amplitude in simulations of greenhouse gas-induced future climate change. A survey of simulations from the Coupled Model Intercomparison Project version 5 (CMIP5) shows substantial disagreement, with roughly equal numbers of models projecting an increase or decrease in the strength of ENSO events. Work here seeks to identify differences in feedback mechanisms (e.g. the Bjerknes feedback) which can account for model to model amplitude differences. Once these are identified they will be related to changes in the model background state through a linear decomposition of the mixed layer heat budget. One hypothesis considered is that amplitude differences are related to changes in the meridional width of the ENSO SST pattern, which is in turn related to the strength of the Pacific Subtropical Cell.The work has societal broader impacts due to the worldwide consequences of ENSO events and the desirability of improved ENSO predictions. Predictions in 2014 of an imminent super El Nino, which did not materialize, suggest a particular need for better understanding of the nature of super events. Work to improve understanding of how climate change affects ENSO events also has societal value, and the PI intends to develop a projection of ENSO amplitude change based on work under this award. In addition, the project provides support and training for a graduate student, thereby promoting workforce development in this research area.