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，即赤道太平洋中部和东部大部分地区的海面温度(SST)变暖或变冷，影响着世界各地的天气和气候，包括对美国的重大影响。ENSO的基本机制已经确定，预测工作也取得了一些成功，但根本问题仍然存在。一个这样的问题是，为什么厄尔尼诺事件(赤道太平洋的海温异常温暖)与拉尼娜事件(海温偏冷)的结局不同，因为厄尔尼诺事件之后往往会出现拉尼娜现象，而拉尼娜现象更有可能持续或重新发展。PI的初步工作发现，在厄尔尼诺和拉尼娜事件的成熟和衰退期，中太平洋表层通量和风应力的模式有所不同。这里的工作结合了海洋混合层热量收支分析和模式模拟(主要是非耦合海洋和大气模式)，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”的厄尔尼诺迅速增长到比典型事件大得多的幅度(超过两个标准差)。初步工作已经确定了超强和普通厄尔尼诺现象的发生和发展阶段的差异，包括超级事件中赤道外温跃层深度异常，这里的工作通过耦合和非耦合海洋-大气模拟实验相结合来检验这些差异。这项研究的进一步任务是在模拟温室气体引起的未来气候变化时检查ENSO幅度的变化。一项来自耦合模式比较项目第5版(CMIP5)的模拟调查显示，有很大的分歧，预测ENSO事件强度增加或减少的模型数量大致相同。这里的工作试图找出反馈机制中的差异(例如，比耶克内斯反馈)，这可以解释模型到模型的幅度差异。一旦确定了它们，它们将通过混合层热量收支的线性分解与模式背景状态的变化相关。一种假设是，幅度差异与ENSO海温型经向宽度的变化有关，而经向宽度的变化又与太平洋副热带气旋的强度有关。由于ENSO事件的全球影响和改进ENSO预测的必要性，这项工作具有更广泛的社会影响。2014年关于超级厄尔尼诺即将到来的预测并未成为现实，这表明特别需要更好地理解超级事件的性质。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打算在该奖项下的工作基础上制定ENSO幅度变化的预测。此外，该项目还为研究生提供支持和培训，从而促进这一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.