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

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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.