Investigating Stratospheric Wave Reflection Events and their Implications for Northern Hemisphere Circulation

Stratospheric wave reflection events involve the upward propagation of planetary waves, which are subsequently reflected downward by the stratospheric polar vortex. This unique phenomenon establishes a crucial connection of large-scale atmospheric dynamics between the troposphere and stratosphere. In this study, wave reflection events are defined via increased poleward eddy heat flux over the Northwest Pacific and increased equatorward eddy heat flux over Canada. While previous research has pointed out a link between these events and an abrupt temperature decrease across North America, the dynamical mechanisms remain less clear. In order to advance the comprehension of the large-scale atmospheric dynamics during stratospheric wave reflection events, meridional eddy heat flux, Rossby wave activity and geopotential height are studied. Around the end of stratospheric wave reflection events an oscillation in meridional eddy heat flux towards opposite values is present over the Northwest Pacific and Canada in the upper troposphere and stratosphere. A westward-propagating ridge, associated with a positive anomaly of geopotential height, and development of a trough downstream can explain this oscillation. East of the ridge, colder air than usual is advected southwards in the lower troposphere over North America. This results in different anomalies of meridional eddy heat flux closer to the surface compared to the upper troposphere. The large-scale circulation anomalies align vertically from the lower troposphere up to the stratosphere and mirror the shift from a Pacific Trough to an Alaskan Ridge. Furthermore, stratospheric wave reflection events exert a far-reaching influence on atmospheric circulation across the mid-latitude and polar Northern Hemisphere. One example is the occurrence of windy extremes over Europe together with changes in mid-latitude jet stream position and strength over the Atlantic at the same time as the temperature decreases to below average values over North America.