Mechanisms coupling stratosphere and troposphere in observations and modelling-results
DWD-contribution to joint-project
Coupling of dynamics and atmospheric chemistry in the stratosphere: KODYACS
(Förderkennzeichen 07ATF43)
P. Winkler, W. Steinbrecht, B. Haßler
Meteorological Observatory, Deutscher Wetterdienst, 82383 Hohenpeissenberg
Interannual and long-term variations of stratospheric ozone and temperature are separated into natural parts, associated with Quasi-Biennial-Oscillation (QBO), 11-year solar-cycle, El-Nino, Polar Vortex strength, or volcanic eruptions, and anthropogenic changes, e.g. due to increasing chlorine and green-house gases.
The investigation is based on the best available long-term observations (20 years or longer) from about 10 sonde and lidar stations (WOUDC and NDSC databases), and on global data from TOMS/ SBUV and NCEP reanalyses. Model simulations from state-of-the-art fully coupled chemistry climate models (ECHAM4.L39(DLR)/CHEM, KODYACS partner DLR-Oberpfaffenhofen; MAECHAM/CHEM, KODYACS partners MPI Mainz and Hamburg), and from the chemistry-transport model SLIMCAT (U. Leeds), which uses ECMWF analyses, are investigated as well.
Ozone (or temperature) fluctuations are attributed through a multiple linear regression scheme, where different predictor time series describe the different influences. Only statistically significant predictors are retained.
The most important source of long-term variations in the stratosphere, both in observations and models, is a linear trend (representing the increase in chlorine and greenhouse gases). Ozone decreases by -5 to -10% per decade in the lower stratosphere, and by up to -10% per decade in the upper stratosphere. The lower stratosphere has also been cooling by -0.5 to -1 K/decade. Meteorological variations in the troposphere are important in the lower stratosphere (ozone and temperature changes typically 40% or 3K). The models show a similar influence as observed. A clear seasonally dependent, downward propagating QBO influence is found throughout the stratosphere (up to 10 % or a few Kelvin).
Coupling between troposphere and stratosphere is clearly evident through the important influence of tropospheric meteorological conditions on the lower stratosphere. Stratospheric influences from QBO and solar cycle can be found in the troposphere as well. They are, however, small and sketchy in the observations, and probably over-represented in ECHAM. Much clearer coupling appears between the strength of the stratospheric polar vortex and tropospheric conditions at the mid-latitude stations.
Publications:
Steinbrecht, W., Hassler, B., et al., Global distribution of total ozone and lower stratospheric temperature variations. Atmos. Chem. Phys., 3, 1421-1438, 2003.
Steinbrecht, W., Claude, H., Winkler, P., Enhanced Upper Stratospheric Ozone: Sign of Recovery or Solar Cycle Effect? J. Geophys. Res., 109, doi:10.1029/2003JD004284, 2004.