Generation of Layering in the Upper Arctic Troposphere away from the Jet Stream

Ozone sounding databases for two stations, So- dankyl¨ a (67 N, 27 E) and Ny- ˚ Alesund (79 N, 12 E) were used in order to investigate the generation of layering in the upper and middle troposphere of the Arctic. We concentrated on dry, ozone-rich and stable layers observed below the ther- mal tropopause under light wind conditions. This condition ensures that the observed layer is not a tropopause fold, a well-known phenomenon that develops within frontal zones near the jet stream. Selection criteria for ozone, humidity and stability anomalies of the tropopause fold detection al- gorithm were used here to pick out for detailed studies the most pronounced examples of laminae. For all these cases the meteorological situations were investigated in order to establish the origin of the observed layers. We found that layers could be classified into two groups. Laminae of the first group were observed equatorward of the jet stream and those of a second group were observed poleward of the jet. The meteorological situation for the first group resembles that for equatorward stratospheric streamer propagation. It was found that this group accounts for only a small fraction of the layers observed at Sodankyland for none of those observed at Ny- ˚ Alesund during the period investigated. A large case-to-case variability in the synoptic situation was observed for the second group of laminae, which were de- tected northward of the jet stream. Nevertheless, in about half of the cases, streamers of tropospheric air were found in the vicinity of the stations on the isentropic surfaces just above the detected stratospheric layers. Back trajectory anal- yses showed that these layers originated in the vicinity of the polar jet stream. We suppose that laminae-like structures in the troposphere were caused, in both groups, by equatorward (poleward) advection of the stratospheric (tropospheric) air, together with differential vertical shear. Forward-trajectory calculations suggest that, subsequently, a part of the strato- spheric layers can mix irreversibly into the troposphere.