Low Arctic sea ice extent favor the occurrence of extreme cold wave (ECW) events over the southeastern United States (US) via the Northern Hemisphere annular mode (NAM) negative phase. The roles of planetary and synoptic-scale waves in extreme cold wave (ECW) events over the southeastern US are studied using a spherical harmonic decomposition in conjunction with piecewise tendency diagnosis (PTD). Planetary waves and synoptic waves jointly work together to initiate ECW events. Notably, the planetary waves not only provide a direct contribution to circulation field enacting ECW events but also alter the background circulation field in such a manner that promotes synoptic waves growth via increases in regional barotropic deformation. The ECW events over the southeastern US, concurrent with the Northern Hemisphere annular mode (NAM) negative phase, feature high latitude intensification and subsequent southeastward movement of cold surface air temperature anomalies. The planetary-scale pattern provides a sizable contribution to the total wave pattern on both sea level pressure (SLP) and upper level. Moreover, the negative NAM planetary anomaly acts to displace the jet equatorward and thereby increases the barotropic deformation of the synoptic-scale anomaly over southeastern US. PTD confirms that the planetary-scale barotropic deformation plays a key role in deepening the negative height anomaly with a secondary contribution from baroclinic growth.
 

Sea ice,Extreme cold wave,Planetary wave,Barotropic deformation,Piecewise tendency diagnosis