Persistence and photochemical decay of springtime total ozone anomalies in the Canadian Middle Atmosphere Model S. Tegtmeier1,* and T. G. Shepherd2 1Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany 2Department of Physics, University of Toronto, Toronto, Ontario, Canada *now at: Department of Physics, University of Toronto, Canada
Abstract. The persistence and decay of springtime total ozone anomalies over the entire extratropics
(midlatitudes plus polar regions) is analysed using results from the Canadian Middle
Atmosphere Model (CMAM), a comprehensive chemistry-climate model. As in the
observations, interannual anomalies established through winter and spring persist
with very high correlation coefficients (above 0.8) through summer until early autumn,
while decaying in amplitude as a result of photochemical relaxation in the quiescent
summertime stratosphere. The persistence and decay of the ozone anomalies in CMAM
agrees extremely well with observations, even in the southern hemisphere when the model
is run without heterogeneous chemistry (in which case there is no ozone hole and the
seasonal cycle of ozone is quite different from observations).
However in a version of CMAM with strong vertical diffusion, the northern hemisphere
anomalies decay far too rapidly compared to observations.
This shows that ozone anomaly persistence and decay does not depend on how the
springtime anomalies are created or on their magnitude, but reflects the transport
and photochemical decay in the model.
The seasonality of the long-term trends over the entire extratropics is found to be
explained by the persistence of the interannual anomalies, as in the observations,
demonstrating that summertime ozone trends reflect winter/spring trends rather than
any change in summertime ozone chemistry.
However this mechanism fails in the northern hemisphere midlatitudes because of the
relatively large impact, compared to observations, of the CMAM polar anomalies.
As in the southern hemisphere, the influence of polar ozone loss in CMAM increases
the midlatitude summertime loss, leading to a relatively weak seasonal dependence
of ozone loss in the Northern Hemisphere compared to the observations.
Citation: Tegtmeier, S. and Shepherd, T. G.: Persistence and photochemical decay of springtime total ozone anomalies in the Canadian Middle Atmosphere Model, Atmos. Chem. Phys., 7, 485-493, doi:10.5194/acp-7-485-2007, 2007.