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，其中海表温度（SST）的温暖或冷却的赤道太平洋中部和东部的大部分地区，影响天气和气候的世界范围内，包括在美国的重大影响。 厄尔尼诺/南方涛动的基本机制已经查明，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（赤道太平洋海温异常温暖）的终止方式与拉尼娜事件（海温异常寒冷）不同，因为厄尔尼诺事件之后通常会出现拉尼娜现象，而拉尼娜现象更有可能持续或重新发展。PI的初步工作确定了在厄尔尼诺和拉尼娜事件的成熟和衰减阶段，中太平洋表面通量和风应力模式的差异。 这里进行的工作使用海洋混合层热收支分析和模式模拟（主要是与非耦合的海洋和大气模式）相结合，以确定这些差异的起源和它们的重要性的事件终止的差异。第二个问题是为什么一些厄尔尼诺现象，被称为“超级厄尔尼诺”，增长迅速，大幅幅度（超过两个标准差）比典型的事件。 初步研究发现，超级厄尔尼诺和普通厄尔尼诺在发生和发展阶段存在差异，其中包括超级事件中的赤道外温跃层深度异常，本文通过耦合和非耦合的海气模式试验研究了这些差异。 对耦合模式相互比较项目第5版（CMIP 5）的模拟进行的调查显示，存在很大的分歧，预测厄尔尼诺/南方涛动事件强度增加或减少的模式数目大致相等。 这里的工作旨在确定反馈机制（例如Bjerknes反馈）的差异，这可以解释模型之间的幅度差异。 一旦这些被识别，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相关。 一个假设认为，振幅差异是有关的变化，在纬向宽度的ENSO SST的模式，这反过来又关系到太平洋副热带环流的强度。这项工作具有更广泛的社会影响，由于全球范围内的ENSO事件的后果和改进ENSO预测的可取性。 2014年预测即将发生超级厄尔尼诺现象，但没有实现，这表明特别需要更好地了解超级事件的性质。提高对气候变化如何影响厄尔尼诺/南方涛动事件的理解的工作也具有社会价值，PI打算根据该奖项下的工作制定厄尔尼诺/南方涛动幅度变化的预测。 此外，该项目还为一名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.