Decadal modulation of El Nino Southern Oscillation and its global impacts

本计画将探讨气候变率模态间的耦合及其对季节至年代际预报技巧的影响。气候关联描述了地球上遥远地点气候异常之间的相互联系。大气环流对海洋条件变化的反应经常引起大气环流的相互联系。如果这些模式可以被准确地预报，则信号提供了季节到十年预报中的预测技能的主要来源（Scaife等人，2016年）。因此，有必要识别可预测的气候信号，并了解其对全球气候灾害风险的影响，以改善季节到年代际的预测。然而，越来越多的证据表明，在不同的时间尺度上，变异模式及其遥相关之间存在耦合（Cai等人，2019年）。最近的一些研究推测，大西洋年代际变化-北大西洋的一种波动模式-增强了冬季北大西洋涛动对厄尔尼诺南方涛动的响应，厄尔尼诺南方涛动是热带地区年度变化的主要模式，ENSO是NAO的关键驱动因素，这种遥相关对欧洲冬季气候的季节性预测很重要。然而，关于ENSO-NAO遥相关的AMV调制是否稳健以及它是否可以与NAO上的局部AMV影响区分开来的问题仍然存在（Simpson等人，2018年）。这个博士项目将开发一个新的框架，使用最先进的大型集合气候模拟评估AMV对ENSO-NAO遥相关的影响。这将提供迄今为止AMV对ENSO-NAO耦合的十年调制的最全面的评估。另一个主要研究课题是ENSO如何影响包括北大西洋在内的偏远地区的气候。很少有研究考虑到遥相关改变ENSO本身。利兹大学最近的研究表明，AMV的暖相在十年时间尺度上削弱了ENSO。该项目将在更大的一组气候模型中测试这种影响，并确定它是否被稳健地模拟。还将进行新的模型模拟，以检验假设和隔离机制。这些实验将在气候模式中利用新的“轻推”能力，使特定地区能够被限制在观测范围内，以隔离遥远的影响。为了结束太平洋-大西洋相互作用的循环，项目中要解决的第三个问题是：北大西洋涛动对ENSO的影响是什么？动机来自于观察到，在季节性的时间尺度上，NAO通过修改海气耦合驱动北大西洋海面温度的三极模式。这增加了NAO长期数十年趋势对北大西洋以外气候影响的可能性，但目前尚未探索。这个项目将通过对观测数据的统计分析和类似的模型实验设计来解决这个新问题，在这个模型实验设计中，NAO异常被“轻推”，全球反应发生变化。项目的具体目标将根据学生的兴趣进行调整，并追求最有希望的调查途径。具体的研究目标可以包括：量化大西洋年代际变率如何影响ENSO-NAO关系，包括相关的机制;测试这在气候模式中是否可靠;通过南极-太平洋相互作用探索AMV对ENSO及其机制的影响;调查NAO年际至年代际变率的全球影响，包括对ENSO的潜在影响。

This project will investigate the coupling between modes of climate variability and their effects on seasonal-to-decadal forecast skill. Teleconnections describe interconnections between climate anomalies at distant locations across Earth. Teleconnections frequently arise through the response of atmospheric circulation to changing oceanic conditions. If these patterns can be accurately forecast, the signals provide a major source of predictive skill in seasonal-to-decadal forecasts (Scaife et al., 2016). Hence, there is a need to identify predictable climate signals and understand their effect on the risk of global climate hazards to improve seasonal-to-decadal predictions.Most previous research has considered teleconnections arising from modes of variability separately. However, there is growing evidence for coupling between modes of variability and their teleconnections on different timescales (Cai et al., 2019). Some recent studies have speculated Atlantic Multidecadal Variability - a fluctuating pattern in the North Atlantic ocean -- augments the response of the winter North Atlantic Oscillation to El Nino Southern Oscillation, the dominant pattern of year-to-year variability in the tropics, ENSO is a key driver of the NAO and this teleconnection is important for seasonal forecasting of European winter climate. However, questions remain about whether the AMV modulation of the ENSO-NAO teleconnection is robust and whether it can be distinguished from a local AMV impact on the NAO (Simpson et al., 2018). This PhD project will develop a new framework for assessing the AMV influence on the ENSO-NAO teleconnection using state-of-the-art large ensemble climate simulations. This will provide the most comprehensive assessment to date of decadal modulation of ENSO-NAO coupling by the AMV.Another major research topic has been how ENSO affects climate in remote regions, including the North Atlantic. Few studies have considered teleconnections that alter ENSO itself. Recent work at the University of Leeds suggests the warm phase of AMV weakens ENSO on decadal timescales. This project will test this effect in a larger set of climate models and identify whether it is robustly simulated. New model simulations will also be performed to test hypotheses and isolate mechanisms. These experiments will exploit new 'nudging' capabilities within a climate model that enable specified regions to be constrained to observations to isolate remote impacts.To close the loop on Pacific-Atlantic interactions, the third question addressed in the project will be: what are the impacts of the North Atlantic Oscillation on ENSO? The motivation comes from the observation that on seasonal timescales the NAO drives a tripolar pattern in North Atlantic sea surface temperatures through modified air-sea coupling. This raises the potential for longer-term multidecadal NAO trends to impact on climate outside the North Atlantic, but this is currently unexplored. This project will tackle this new question using a combination of statistical analysis of observations and a similar model experiment design where NAO anomalies are 'nudged' and the global response evolves.The specific objectives of the project will be adapted to fit the interests of the student and to pursue the most promising avenues of enquiry. Specific research objectives could include:Quantify how Atlantic Multidecadal Variability affects the ENSO-NAO relationship including the associated mechanisms; test whether this is robust in climate models.Explore the impact of the AMV on ENSO and its mechanisms via Atlantic-Pacific interactions.Investigate the global impacts of interannual to decadal NAO variability, including potential impacts on ENSO.