Relationship between Warm Air Mass Transport into the Upper Polar Atmosphere and Cold Air Outbreaks in Winter

This study investigates dominant patterns of daily surface air temperature anomalies in winter (November to February) and their relationship with the meridional mass circulation variability using the daily ECMWF Interim reanalysis in 1979-2011. Mass circulation indices are constructed to measure the day-to-day variability of mass transport into the polar region by the warm air branch aloft and out of the polar region by the cold air branch in the lower troposphere. It is shown that weaker warm air mass transport into the upper polar atmosphere is accompanied by weaker equatorward advancement of cold air in the lower troposphere. As a result, the cold air is largely imprisoned within the polar region, responsible for anomalous warmth in mid-latitudes and anomalous cold in high latitudes. Conversely, stronger warm air mass transport into the upper polar atmosphere is synchronized with stronger equatorward discharge of cold polar air in the lower troposphere, resulting in massive cold air outbreaks in mid-latitudes and anomalous warmth in high latitudes. There are two dominant geographical patterns of cold air outbreaks during the cold air discharge period (or 1-10 days after a stronger mass circulation across 60°N). One represents cold air outbreaks in mid-latitudes of both North America and Eurasia and the other is the dominance of cold air outbreaks only over one of the two continents with abnormal warmth over the other continent. The first pattern mainly corresponds to the first and fourth leading Empirical Orthogonal Functions (EOFs) of daily surface air temperature anomalies in winter, whereas the second pattern is related to the second EOF mode.