Characteristics of extreme cold air masses over the North American sub-Arctic

Past studies have suggested that possible climate change would be most pronounced and perhaps most easily detected in the arctic regions of the globe. Furthermore, since these changes would be amplified during the winter months, it follows that their signature could appear in extreme cold arctic air masses. To explore this likely possibility, we analyzed National Centers for Environmental Prediction reanalysis surface temperature data spanning a 60 year time period over the source regions for the formation of arctic air masses in North America, and we were able to find variability within the coldest arctic air masses on a decadal timescale. Extreme cold arctic air masses have warmed at least in terms of surface temperature since the beginning of the data record. Extreme cold air arctic outbreaks have decreased in frequency and duration over time. In addition, we have identified criteria that could be used to classify cold air masses in terms of depth and strength, such as lowest kilometer temperature change and inversion height. The 850 and 700 mbar results confirm that inversions are deeper with colder surface temperatures, and the strongest depth indicators are the lapse rates at least in the lowest kilometer; the lapse rates are stronger with colder surface temperatures. Differences between air masses with strong and weak lapse rates in the lowest kilometer are visible on the synoptic scale on sea level pressure maps. No significant relationship between the depth of cold air masses and wind shear was discovered. Finally, a cold bias in the reanalysis data over observations was found in the lowest kilometer of the atmosphere.