The Influence of Stratospheric Wave Reflection on North American Cold Spells

Abstract Understanding and predicting midlatitude cold spells is of scientific and public interest, given often associated severe impacts. However, large-scale atmospheric dynamics related to these events are not fully understood. The winter of 2017/18 was characterized by several cold spells affecting large parts of North America and Eurasia. Here, the role of stratosphere–troposphere coupling for the occurrence of cold spells in this winter is investigated using different wave propagation diagnostics. While the European cold spell in late February 2018 was influenced by a major sudden stratospheric warming (SSW) associated with wave absorption, the cold spells over North America at the end of December 2017 and early February 2018 were related to downward reflected waves over the North Pacific. Previously proposed wave reflection indices, however, either miss these reflection events or are not able to distinguish them from the major SSW related to wave absorption. To overcome this, a novel simple index based on eddy heat flux is proposed here, capturing regional wave reflection over the North Pacific. Reflection events detected with this index are shown to be followed by North Pacific blocking and negative temperature anomalies over North America. An improved understanding of the contribution of wave reflection for cold spells is crucial to better predict such events in the future.