Extreme Weather Events in Mid-latitudes: The Role of Arctic Sea Ice, SST due to AMV and Siberian Snow Cover Through Teleconnections Involving the Stratosphere

该项目致力于大陆中纬度地区冬季气候的可预测性，重点关注寒流和极端降雪等极端事件。一次次异常寒冷的气温和大雪对社会经济产生了深远的影响，并可能造成生命损失(例如，2009/10年美国东部/欧洲的寒冬；2012年2月中欧的致命寒流；2013年3月英国的严寒；2014年1月初美国东部的寒流)。以前的研究已经确定了与缓慢演变的大气表面边界条件有关的各种可预测性来源。在许多情况下，可预报性的提高与通过波在非常不同的地理区域之间传播的大气联系有关，即所谓的遥相关。由于下边界条件的缓慢演变，如海冰浓度变化、海面温度变化或积雪变化，地表热通量的变化都可能驱动大气中的遥相关。由此产生的遥相关可以帮助预测平均气候以及极端事件在季节到数十年的时间尺度上的发生，即使在遥远的地区也是如此。平流层也是可预测性的来源，特别是在季节内的时间尺度上，因为平流层异常(有时起源于对流层)在几周后向下传播到对流层。越来越多的观测资料和气候模拟方面的进展有助于更好地了解地表和平流层条件在推动冬季大气环流方面的作用。然而，边界驱动的遥相关的基本方面仍然不确定，因为观测中强迫信号的幅度比大气中的非强迫信号低，而且在使用不同模型和(或)略有不同的边界条件的模拟实验中，结果相互矛盾。有些遥相关在时间上是非平稳的，因为它们在某些时间段被观测到，然后由于仍然未知的原因而消失。不同遥相关之间的破坏性干扰可以解释这种非平稳性，但到目前为止，工作的重点还没有集中在理解地表异常的联合(而不是单独)影响上。此外，人们对平流层在调节这些遥相关中的潜在作用知之甚少。这些问题是本项目的核心。它将审查不同类型的地表异常(北冰洋海冰、北大西洋海面温度、西伯利亚积雪)的影响，这些异常被认为是北美和欧亚冬季环流的可预测性来源。利用观测、全球和简化的气候模型，该项目将探索每一种强迫-以及它们共同作用-可能如何导致大陆中纬度地区冬季寒流的不同频率和/或强度。该项目的一个特别重点将是审查平流层在促进不同地理区域之间的联系方面可能发挥的作用。根据较慢的过程量化极端天气事件的数量和严重程度发生变化的可能性，具有重要的社会经济效益。两名研究生将接受教育和培训。少数民族本科生将继续参与研究小组。

The project addresses the predictability of the wintertime climate over continental mid-latitude regions, with a focus on extreme events such as cold spells and extreme snowfall. Episodes of anomalously cold temperature and heavy snowfall have profound socio-economic impacts and can cause loss of life (for example, the cold winter in 2009/10 over the eastern US/Europe; deadly cold spell of February 2012 over central Europe; frigid temperature in March 2013 over the UK; cold snap of early January 2014 over the eastern US). Various sources of predictability related to slowly evolving surface boundary conditions of the atmosphere have been identified in previous research. In many instances, the increased predictability is related to atmospheric connections through wave propagation between widely different geographic regions, the so-called teleconnections. Changes in surface heat flux due to the slow evolution of lower boundary conditions, such as sea-ice concentration change, sea surface temperature change or snow cover change may all drive teleconnections in the atmosphere. The resulting teleconnections can help foretell the mean climate as well as the occurrence of extreme events at seasonal to multidecadal time scales even in far away regions. The stratosphere is also a source of predictability, especially at the intraseasonal time scale since stratospheric anomalies (sometimes originating in the troposphere) propagate down into the troposphere several weeks later. The increasing availability of observations and progress in climate modeling have helped to better understand the role of surface and stratospheric conditions in driving the wintertime atmospheric circulation. However, fundamental aspects of boundary-driven teleconnections are still uncertain, due to the low amplitude of the forced signal compared to the unforced signal in the atmosphere in the observations and conflicting results in modeling experiments that use different models and/or slightly different boundary conditions. Some of the teleconnections are non-stationary in time as they are observed in certain time periods then disappear for reasons that are still unknown. Destructive interference between different teleconnections can explain the non-stationarity, but to date work has not been focused on understanding the combined (rather than individual) influence of surface anomalies. Moreover, the potential role of the stratosphere in modulating these teleconnections is poorly understood. These questions are at the core of the present project. It will examine the effects of different types of surface anomalies (Arctic sea ice, North Atlantic sea surface temperature, Siberian snow cover) that have been suggested as a source of predictability for the wintertime North American and Eurasian circulation. Using observations, global and simplified climate models, the project will explore how each forcing - as well as together - they may lead to a different frequency and/or intensity of cold spells in winter over continental mid-latitude regions. A particular focus of the project will be to examine the possible role of the stratosphere in facilitating connections between widely different geographical regions. Quantifying the likelihood of a change in the number and severity of extreme weather events based on the slower processes has important socioeconomic benefits. Two graduate students will be educated and trained. Minority undergraduate students will continue to be involved in the research group.