Decadal modulation of El Nino Southern Oscillation and its global impacts

该项目将调查气候变率模式之间的耦合及其对季节到年代际预报技巧的影响。遥相关描述了地球上遥远地点的气候异常之间的相互联系。遥相关常常是由于大气环流对不断变化的海洋条件的反应而产生的。如果能够准确地预测这些模式，这些信号将为季节到十年的预测提供一个主要的预测技能来源(Scaife等人，2016)。因此，有必要确定可预测的气候信号，并了解它们对全球气候灾害风险的影响，以改进从季节到十年的预测。以前的大多数研究都单独考虑了变率模式引起的遥相关。然而，越来越多的证据表明，变异性模式及其在不同时间尺度上的遥相关之间存在耦合(蔡等人，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际变化的主要模式，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报很重要。然而，关于ENSO-NAO遥相关的AMV调制是否强大以及它是否能与当地AMV对NAO的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，利用最先进的大型集合气候模拟来评估AMV对ENSO-NAO遥相关的影响。这将提供到目前为止AMV对ENSO-NAO耦合的年代际调制的最全面的评估。另一个主要研究主题是ENSO如何影响包括北大西洋在内的偏远地区的气候。很少有研究考虑会改变ENSO本身的遥相关。利兹大学最近的研究表明，AMV的暖相在十年时间尺度上减弱了ENSO。该项目将在一组更大的气候模型中测试这种效果，并确定它是否得到了强有力的模拟。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型中利用新的“轻推”能力，使特定地区能够被限制在观测范围内，以隔离远程影响。为了结束太平洋-大西洋相互作用的循环，项目中解决的第三个问题将是：北大西洋涛动对ENSO的影响是什么？其动机来自于观察到，在季节性时间尺度上，NAO通过修正的海-气耦合驱动北大西洋海面温度的三极模式。这增加了较长期的数十年NAO趋势对北大西洋以外气候产生影响的可能性，但这一点目前尚未得到探索。这个项目将结合对观测的统计分析和一个类似的模型实验设计来解决这个新问题，在这个模型实验中，NAO异常被‘轻推’，全球响应发生变化。该项目的具体目标将被调整，以适应学生的兴趣，并追求最有希望的探索途径。具体的研究目标可包括：量化大西洋年代际变化如何影响ENSO-NAO关系，包括相关的机制；检验这种关系在气候模式中是否稳健；通过大西洋-太平洋相互作用探索AMV对ENSO的影响及其机制；调查年际至年代际NAO变化的全球影响，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.