Net effect of the QBO in a chemistry climate model H. J. Punge1,2 and M. A. Giorgetta1 1Max Planck Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany 2IMPRS on Earth System Modelling, Bundesstr. 53, 20146 Hamburg, Germany
Abstract. The quasi-biennial oscillation (QBO) of zonal wind is a prominent mode of variability
in the tropical stratosphere. It affects not only the meridional circulation
and temperature over a wide latitude range but also the transport and chemistry
of trace gases such as ozone. Compared to a QBO less circulation, the long-term
climatological means of these quantities are also different.
These climatological net effects of the QBO can be studied in general circulation
models that extend into the middle atmosphere and have a chemistry and transport
component, so-called Chemistry Climate Models (CCMs).
In this work we show that the CCM MAECHAM4-CHEM can reproduce the observed QBO
variations in temperature and ozone mole fractions when nudged towards observed
winds. In particular, it is shown that the QBO signal in transport of nitrogen
oxides NOx plays an important role in reproducing the observed ozone QBO,
which features a phase reversal slightly below the level of maximum of the ozone
mole fraction in the tropics.
We then compare two 20-year experiments with the MAECHAM4-CHEM model that differ
by including or not including the QBO. The mean wind fields differ between the
two model runs, especially during summer and fall seasons in both hemispheres.
The differences in the wind field lead to differences in the meridional circulation,
by the same mechanism that causes the QBO's secondary meridional circulation, and
thereby affect mean temperatures and the mean transport of tracers. In the tropics,
the net effect on ozone is mostly due to net differences in upwelling and, higher
up, the associated temperature change.
We show that a net surplus of up to 15% in NOx in the tropics above
10 hPa in the experiment that includes the QBO does not lead to significantly
different volume mixing ratios of ozone.
We also note a slight increase in the southern vortex strength as well as earlier
vortex formation in northern winter. Polar temperatures differ accordingly.
Differences in the strength of the Brewer-Dobson circulation and in further
trace gas concentrations are analysed. Our findings underline the importance
of a representation of the QBO in CCMs.
Citation: Punge, H. J. and Giorgetta, M. A.: Net effect of the QBO in a chemistry climate model, Atmos. Chem. Phys., 8, 6505-6525, doi:10.5194/acp-8-6505-2008, 2008.