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1. Introduction
Various anomalous synoptic- and planetary-scale patterns have been identified as precursor signals for cold air outbreaks over different regions (e.g., Wexler 1951; Colucci and Davenport 1987; Walsh et al. 2001; Konrad 1996). For instance, cold air outbreaks over North America have been found to be closely related to positive sea level pressure anomalies along the Alaska–Yukon border (Walsh et al. 2001) or the coexistence of a ridge over the Arctic and a trough over the Great Lakes region (Konrad 1996). Elsewhere, in East Asia, the intensification and expansion of the Siberian high are known to be the triggering mechanism for cold air surges (Ding 1990; Zhang et al. 1997; Gong and Ho 2004; Takaya and Nakamura 2005). Palmer (2014) pointed out that intensifying Rossby waves within the jet stream, excited by the latent heat release over the warming tropical west Pacific may have contributed to the extremely cold 2013–14 winter in the United States.
There is also ample evidence indicating a robust relationship between continental-scale cold air outbreaks and the leading oscillation modes in the winter extratropics. It is well recognized that the negative phase of the North Atlantic Oscillation (NAO) coincides with cold anomalies over Europe and warming in the northwest Atlantic (Rogers and van Loon 1979; Hurrell and van Loon 1997). The studies by Walsh et al. (2001) and Cellitti et al. (2006) show that the NAO tends to be in its negative phase 3–6 days prior to cold air outbreaks over different regions of the United States and Europe. Luo et al. (2014) related the cold air outbreak event in January–February 2012 to a positive-to-negative phase transition of NAO. As a broader anomaly pattern than the NAO, the Arctic Oscillation (AO), or the tropospheric northern annular mode (NAM), also tends to be in its negative phase when there are more frequent strong cold air outbreaks in the midlatitudes of Eurasia and North America (Thompson and Wallace 1998, 2001; Wettstein and Mearns 2002; Cohen et al. 2010). Accompanying these leading modes is the oscillation of extratropical zonal mean zonal wind in the troposphere, which was termed the “index cycle” to link variations of the westerly jet to cold air outbreaks in the pioneering work of Namias (1950).
Several promising precursors to...