A deeper insight into the dynamics of the fall transition in the MLT

Abstract

In autumn the prevailing wind in the middle atmosphere at mid and high latitudes changes from summer easterly to winter westerly. This process is not smooth but interrupted by the Hiccup of the fall transition with characteristics similar to a mini sudden stratospheric warming (SSW) which occurs in fall even though the zonal mean zonal wind does not reverse to easterly again. Combining global reanalysis data and satellite observations we improve our knowledge and understanding of the dynamics of the Hiccup of the fall transition in the middle atmosphere. The introduction of a new definition for the onset of the Hiccup focusing now on its core region in the lower mesosphere allows us the automatic detection of a Hiccup in almost every year and thus a deeper insight into its dynamics. For example, we found a latitudinal and altitudinal shift in the zonal wind regime during the Hiccup. We also investigate its 3D-structure and compare the characteristics of the Hiccup in the Northern hemisphere with those in the Southern hemisphere. We found that the latitudinal and altitudinal shift of the zonal wind regime occurs in both hemispheres but is more pronounced in the Northern hemisphere and smoother in the Southern hemisphere. Additionally, we discuss the possible impact of the Hiccup on the D-region.