Eddy transfer processes in the stratosphere during major and “minor” breakdowns of the polar night vortex

Abstract

An analysis of the large-scale eddy processes which may be responsible for the northward and downward transport of trace substances in the stratosphere is performed for cases of a major and a “minor” breakdown of the polar night vortex. By using vertical velocities obtained from the thermodynamic equation, eddy covariances between the vertical velocity and the northward component of the wind were computed on a daily basis for middle and higher latitudes. The results indicate that this quantity reverses very rapidly from positive to large negative values just after the major breakdown of the polar vortex, thus providing an efficient mechanism for northward and downward trace substance transport at this time of the year. This result is also corroborated through large-scale trajectory calculations. A similar analysis is performed for a fall period in which the intensifying polar vortex circulation deforms into a regime dominated by large-scale cyclones and anticyclones (“minor” breakdown), but then reforms into an intense midwinter polar vortex. This situation also appears to be favorable for northward and downward transport, but the processes are considerably weaker than for the major breakdown case. This result may indicate why the largest buildup of trace substance concentrations in the high-latitude lower stratosphere occurs during late winter and early spring.