Anomalous temperature regimes (ATRs) during winter include cold air outbreaks (CAOs) and warm waves (WWs) that have important impacts in the Southeast United States. This study provides a synoptic-dynamic characterization of Southeast ATRs from 1949-2011 through composite time-evolution analyses. Events are categorized by the sign and amplitude of relevant low-frequency modes. During CAO (WW) onset, negative (positive) geopotential height anomalies are observed in the upper troposphere over the Southeast with oppositely-signed anomalies in the lower troposphere over the central US. In most cases, there is a surface east-west height anomaly dipole, with anomalous northerly (CAO) or southerly (WW) flow into the Southeast leading to cold or warm surface air temperature anomalies, respectively. Companion potential vorticity anomaly analyses reveal prominent features in the mid- to upper-troposphere consistent with the coincident geopotential height anomaly patterns. Ultimately, we found that synoptic-scale disturbances serve as dynamic triggers for ATR events, while low-frequency modes provide a favorable environment for ATR onset.
Our results provide a qualitative indication of the role of low-frequency modes in ATR onset. In WW (CAO) events influenced by low-frequency modes, the North American height anomaly pattern arises in part as a downstream (regional) manifestation of the negative Pacific-North American (North Atlantic Oscillation) pattern. Interestingly, the North Atlantic Oscillation pattern contributes to both CAO onset and demise. Thus, our results indicate that low-frequency modes also affect event duration (CAOs). One general distinction found for ATRs is that CAOs involve substantial air mass transport while WW formation is more regional in nature.