| The purpose of this study is to investigate horizontal transport
processes in the winter stratosphere using data with a
resolution relevant for chemistry and climate modeling. For this
reason the Freie Universität Berlin Climate Middle Atmosphere Model
(FUB-CMAM) with its model top at 83 km altitude, increased horizontal
resolution T42 and the semi-Lagrangian transport scheme for advecting
passive tracers is used.
A new approach of this paper is the classification of specific
transport phenomena within the stratosphere into tropical-subtropical
streamers (e.g. Offermann et al., 1999) and polar vortex extrusions
hereafter called polar vortex streamers. To investigate the role
played by these large-scale structures on the inter-annual and
seasonal variability of transport processes in northern mid-latitudes,
the global occurrence of such streamers was calculated based on a
10-year model climatology, concentrating on the existence of the
Arctic polar vortex. For the identification and counting of streamers,
the new method of zonal anomaly was chosen. The analysis of the
months October-May yielded a maximum occurrence of
tropical-subtropical streamers during Arctic winter and spring in
the middle and upper stratosphere.
Synoptic maps revealed highest intensities in the subtropics over
East Asia with a secondary maximum over the Atlantic in the
northern hemisphere. Furthermore, tropical-subtropical streamers
exhibited a higher occurrence than polar vortex
streamers, indicating that the subtropical barrier is more
permeable than the polar vortex barrier (edge) in the model, which
is in good correspondence with observations (e.g. Plumb, 2002; Neu
et al., 2003).
Interesting for the total ozone decrease in mid-latitudes is the
consideration of the lower stratosphere for tropical-subtropical
streamers and the stratosphere above ~20 km altitude for polar vortex
streamers, where strongest ozone depletion is observed at polar
latitudes (WMO, 2003). In the lower stratosphere the FUB-CMAM
simulated a climatological maximum of 10% occurrence of
tropical-subtropical streamers over East-Asia/West Pacific
and the Atlantic during early- and mid-winter.
The results of this paper demonstrate that stratospheric streamers
e.g. large-scale, tongue-like structures transporting tropical-subtropical
and polar vortex air masses into mid-latitudes occur frequently
during Arctic winter. They can therefore play a significant role on the strength
and variability of the observed total ozone decrease at mid-latitudes
and should not be neglected in future climate change studies. |