Stratospheric warmings: Observations and theory

Winter stratospheric warming observations and associated theories are reviewed. Historically, major warmings occur on the average every other year and may thus be considered an important climatological component of the winter stratosphere. The warming results from eddy heat transport from equatorial latitudes into the polar regions. The eddies chiefly responsible for the transport are the planetary scale waves which may attain amplitudes twice their monthly average values in the lower stratosphere prior to the warming. In the troposphere this amplitude increase is associated with the development of blocking patterns. The warming is shown to have a strong nonzonal component in the upper stratosphere, a feature not fully recognized by modelers. The critical level theory of the sudden stratospheric warming provides a simple dynamic explanation of the event. Yet mechanistic numerical models which have reproduced many features of the warming exhibit results which tend to indicate that the interaction of planetary waves and the mean flow occurs through a variety of mechanisms including transience and damping. The close connection between blocking in the troposphere and the development of the sudden warming is likely to be a result of resonance of free planetary waves in a stratospheric wind cavity.