Figure: Sea-level pressure self-organized map (SOM) from 45oN to 90oN. Graphs represent Oct. - Mar. climatology of SOM pattern to the right.
THE CHARACTERISTICS OF COLD AIR OUTBREAKS IN THE EASTERN UNITED STATES AND THE INFLUENCE OF ATMOSPHERIC CIRCULATION PATTERNS
Periods of extreme cold impact the mid-latitudes every winter. Depending on the magnitude and duration of the occurrence, extremely cold periods may be deemed cold air outbreaks (CAOs). Atmospheric teleconnections impact the displacement of polar air, but the relationship between the primary teleconnections and the manifestation of CAOs is not fully understood. A systematic CAO index was developed from 20 surface weather stations based on a set of criteria concerning magnitude, duration, and spatial extent. Statistical analyses of the data were used to determine the overall trends in CAOs. Clusters of sea level pressure (SLP), 100mb, and 10mb geopotential height anomalies were mapped utilizing self-organizing maps (SOMs) to understand the surface, tropospheric Polar Vortex (PV), and stratospheric PV patterns preceding CAOs. The Arctic Oscillation (AO), North Atlantic Oscillation (NAO), and Pacific-North American (PNA) teleconnections were used as variables to explain the magnitude and location of mid-latitude Arctic air displacement. Persistently negative SLP anomalies across the Arctic and North Atlantic were evident 1 – 2 weeks prior to the CAOs throughout the winter. The tropospheric and stratospheric PV were found to be persistently weak/weakening prior to mid-winter CAOs and predominantly strong and off-centered prior to early and late season CAOs. Negative phases of the AO and NAO were favored prior to CAOs, while the PNA was found to be less applicable. This method of CAO and synoptic pattern characterization benefits from a continuous pattern representation and provides insight as to how specific teleconnections impact the atmospheric flow in a way that leads to CAOs in the eastern U.S.