19 May 2015

Anomalous Temperature Regime Onset in the Southeast

Summary

During winter anomalous temperature regimes (ATRs), including cold air outbreaks (CAOs) and warm waves (WWs), provide important societal impacts in the Southeast United States. Using composite time-evolution analyses, the study characterizes the structural and dynamical development of ATRs occurring during 1949-2011 in the Southeast. Events are first categorized by the sign and amplitude of relevant low-frequency modes. During the development of CAOs (WWs), a negative (positive) geopotential height anomaly pattern is observed in the upper troposphere over the Southeast with oppositely-signed height anomalies found in the lower troposphere over the central US. In most cases, there is a near-surface east-west height anomaly dipole that provides anomalous northerly (CAO) or southerly (WW) flow into the Southeast leading to the local formation of cold or warm surface air temperature anomalies, respectively. Companion potential vorticity analyses reveal prominent anomaly features in the mid- to upper-troposphere that are related to the coincident geopotential height anomaly patterns. The composite analyses reveal that (a) synoptic-scale disturbances serve as dynamic triggers for ATR events while (b) low-frequency modes provide a favorable environment conducive to ATR onset.

 

The results presented provide a qualitative indication of the physical role of low-frequency modes during ATR onset.  During CAO events influenced by North Atlantic Oscillation (NAO), the North American height anomaly pattern arises, in part, as the regional western manifestation of the negative phase of the NAO pattern. On the other hand, during WW events affected by the Pacific-North American (PNA) pattern, the North American height anomaly pattern is, in part, the downstream manifestation of the negative phase of the PNA pattern. Interestingly, we infer that the NAO pattern likely contributes to both CAO onset and demise. Thus, our results indicate that low-frequency modes also impact event duration. One general asymmetry found for Southeast ATRs is that CAOs involve substantial air mass transport (from Canada) while WW formation is more local in nature.

Contact
Rebecca M Westby
Georgia Institute of Technology School of Earth and Atmospheric Sciences