Letter to the Editor
Seasonal variations and vertical movement of the tropopause in the UTLS region

Abstract. Based on the tracking of the movement of the tropopause over the whole year, the extent/depth of stratosphere-troposphere exchange (STE) events and their seasonal variations is investigated. It is found that a stratospheric signature can be observed at pressures as high as 400 hPa in a hemisphere during its winter to spring period, while a tropospheric signature can be observed at pressures as low as 190 hPa during the hemispheric summer to autumn months. The major implication for such a pronounced vertical movement is that the downward penetration of air from the stratosphere is likely to deposit elevated levels of O₃ into the upper troposphere. Though the analysis at 250 hPa reveals that the values of the stratosphere-troposphere index are similar all year round, a result which is consistent with other studies, it is found that an intrusion from the stratosphere to the troposphere is more likely to occur during the hemispheric winter to spring period than other seasons.

Key words. Atmospheric composition and structure (pressure, density, and temperature; troposphere–composition and chemistry)