The Life Cycle of the Northern Hemisphere Sudden Stratospheric Warmings

Motivated by recent evidence of strong stratospheric–tropospheric coupling during the Northern Hemisphere winter, this study examines the evolution of the atmospheric flow and wave fluxes at levels throughout the stratosphere and troposphere during the composite life cycle of a sudden stratospheric warming. The composite comprises 39 major and minor warming events using 44 years of NCEP–NCAR reanalysis data. The incipient stage of the life cycle is characterized by preconditioning of the stratospheric zonal flow and anomalous, quasistationary wavenumber-1 forcing in both the stratosphere and troposphere. As the life cycle intensifies, planetary wave driving gives rise to weakening of the stratospheric polar vortex and downward propagation of the attendant easterly wind and positive temperature anomalies. When these anomalies reach the tropopause, the life cycle is marked by momentum flux and mean meridional circulation anomalies at tropospheric levels that are consistent with the negative phase of the Northern Hemisphere annular mode. The anomalous momentum fluxes are largest over the Atlantic half of the hemisphere and are associated primarily with waves of wavenumber 3 and higher.