Isentropic Analysis of Cold Air Outbreaks from the Arctic to North American Mid-Latitudes

Intermittent outflows of polar cold air sometimes cause cold waves and heavy snowfall in mid-latitudes. Iwasaki et al. (2014) proposed a method to quantitatively estimate cold air mass (CAM) amount and its horizontal flux using isentropic coordinate. The results showed two climatological mean cold air streams in boreal winter, termed the East Asian stream and the North American stream. Cold air outbreaks (CAOs) in East Asia are measured by a meridional component of CAM flux across 45°N between 90°E and 180°E (Shoji et al., 2014). The characteristics of interannual and intraseasonal variability of CAOs in East Asia have been already studied, but not yet in North America. This study examines the variability of CAOs in North America.Climatological mean North American stream originates from the Arctic Ocean, grows over the northern part of North America, and flows toward the east coast of North America (Iwasaki et al., 2014). North American CAOs are measured by a meridional component of the CAM flux across 60°N and between 50°W and 130°W because the North American stream is located between Rocky mountain and Greenland. In addition, the equatorward CAM flux over North America takes its maximal value at a latitude of 60°N. We also focus on the variability of extremely colder air masses which cause serious damages in mid-latitudes. The extremely colder air masses are characterized by much lower designated potential temperature. We highlight the interannual and intraseasonal variability of North American CAOs and relationship with the extremely colder air masses.