Downward transport in the upper stratosphere during the minor warming in February 1979

Abstract

On February 6, 1979, a small region of dry air, referred to below as a dry tongue, appeared in Limb Infrared Monitor of the Stratosphere data directly below the stratopause at approximately 75°N when a zonal average between 40°E and 160°E was applied. Isentropic analysis of Ertel's potential vorticity and H2O mixing ratio does not support the hypothesis of the dry tongue being caused solely by adiabatic sinking from the dry mesosphere into the upper stratosphere. Therefore a three‐dimensional mechanistic model of the middle atmosphere, which is mainly driven by the net heating rate and the effects of breaking gravity waves, is used to simulate the minor warming in January and February 1979. The model data clearly support the existence of the dry tongue mentioned above as a real phenomenon. Moreover, they allow the dry tongue to be interpreted as the result of the combined adiabatic and diabatic downward movements east of the center of the polar vortex.