Abstract
In recent years, a pair of large-scale circulation patterns consisting of an anomalous ridge over northwestern North America and trough over northeastern North America was found to accompany extreme winter weather events such as the 2013–2015 California drought and eastern U.S. cold outbreaks. Referred to as the North American winter dipole (NAWD), previous studies have found both a marked natural variability and a warming-induced amplification trend in the NAWD. In this study, we utilized multiple global reanalysis datasets and existing climate model simulations to examine the variability of the winter planetary wave patterns over North America and to better understand how it is likely to change in the future. We compared between pre- and post-1980 periods to identify changes to the circulation variations based on empirical analysis. It was found that the leading pattern of the winter planetary waves has changed, from the Pacific–North America (PNA) mode to a spatially shifted mode such as NAWD. Further, the potential influence of global warming on NAWD was examined using multiple climate model simulations.
Highlights
Cold-air intrusion events in eastern North America are common during boreal winter, but severe events, with costly consequences, used to be rare
Given that EOF describes the variance of individual patterns, the observed increase in the amplitude of North American Winter Dipole” (NAWD) suggests that it may have overtaken the Pacific–North America (PNA) as the more common type of winter variability, with increased influence from western North Pacific sea surface temperature (SST)
Variance will gradually amplify alongside its low-frequency natural variability. This result implies that the subseasonal variation of the atmospheric circulations over North America could continue to be dominated by the NAWD mode, with the potential to sharpen the east–west temperature and precipitation division across North America
Summary
Cold-air intrusion events in eastern North America are common during boreal winter, but severe events, with costly consequences, used to be rare. The exceptionally dry conditions in the 2013–2014 winter and the associated high-amplitude atmospheric ridge in the Northeast Pacific was accompanied by an abnormally deepened trough in the eastern USA with strong cold-air outbreaks. This striking division of severe drought in the west and extreme cold in the east has been referred to as the “North American Winter Temperature Dipole” (Singh et al, 2016) [2], while its accompanying circulation pattern is called the “North American Winter Dipole” (NAWD). While the 2013/2014 extreme NAWD event has been extensively studied, the role of large-scale climate variability in terms of the wintertime stationary waves has not been satisfactorily addressed.
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