Arctic Ozone
Ozone depletion occurs in the winter/springtime Arctic lower stratosphere through similar processes to those which occur in the Antarctic – e.g. activation of chlorine in/on the surface of polar stratospheric clouds (PSCs). However, due to the warmer, more variable and more dynamically disturbed polar vortex the extent of ozone depletion varies strongly from season to season.
Much early development of the TOMCAT/SLIMCAT was motivated by the study of polar ozone depletion, and in particular the Arctic, through collaborations in European campaigns.
Quantification of Chemical Loss
In many studies we have diagnosed chemical ozone loss directly from TOMCAT/SLIMCAT either through performing model sensitivity experiments or by using a seasonal ‘passive tracer’. We have also worked closely with observational groups to use our CTM components to help derived observation-based estimates of ozone loss. In particular, the novel development of SLIMCAT, which used a radiation scheme to diagnose diabatic vertical motion, allowed the model heating rates to quantify the impact of vortex descent on tracer fields over the course of Arctic winters. SLIMCAT heating rates have thus been used in a series of MATCH papers which use a dense network of ozonesonde observations to diagnose chemical loss. For example, the MATCH technique was able to reveal the large extent of chemical ozone loss in cold Arctic winters such as 1995/96 (see figure below).
(a) Contour plot showing the MATCH ozone loss rate (p.p.b. per sunlit hour) inside the polar vortex as a function of time and altitude in Arctic winter 1995/96. Potential temperature is used as a measure of altitude. The thin contour lines are isolines of the geographical area covered with temperatures low enough for Type I PSCs to form. The heavy solid contour line is the isoline of the area covered with temperatures below the ice frost point. The dashed lines correspond to surfaces that follow the diabatic descent of the air masses during the winter. The vertical motion of the air masses inside the vortex was calculated from SLIMCAT diabatic cooling/heating rates. (b, c) Data as a section along the double line in (a), reflecting the situation in a subsiding layer of air which started at 535 K potential temperature on 1 January and reached 470 K on 31 March. For this layer (b) shows the geographical area covered with temperatures low enough for the formation of Type I PSCs (light blue) and Type II PSCs (dark blue). (c) Ozone loss rate in p.p.b. per sunlit hour. (d) Ozone loss rate in p.p.b. per day. Figure 1 from Rex et al. (1997).
Role of Dynamics
In addition to the variable extent of chemical loss, Arctic column is also prone to large interannual variations due to dynamics. We used TOMCAT/SLIMCAT to provide the first demonstration and quantification of the relative roles of chemistry and dynamics on Arctic ozone in the 1990s (Chipperfield and Jones, 1999). This worked pointed to the important role of climate-change-induced circulation changes in controlling Arctic ozone levels.
Northern Hemisphere March average column ozone (DU). Results are from the standard TOMCAT/SLIMCAT model run for 1993 (a), 1996 (b), 1997 (c) and 1998 (d). Contour interval is 20 DU. The symbols H and L indicate relative maxima and minima, respectively. Dynamical variations cause a much larger column in 1998, and this variability exceeds that in chemical ozone loss. Figure 2 from Chipperfield and Jones (1999).
Ozone Depletion in Recent Years
Despite the effectiveness of the Montreal Protocol in causing a slow decrease in stratospheric chlorine and bromine abundances, we still experience large chemical ozone loss in cold Arctic winters (e.g. 2010/11, 2015/16, 2019/20). We use TOMCAT to show that this loss is consistent with expectations based on known chemistry for the actual meteorology, and we can also demonstrate the depletion that has been avoided by the Montreal Protocol (e.g. Feng et al., 2021).
Recovery Trend?
The large variability in Arctic column ozone precludes the detection of a robust signal of ozone recovery (e.g. Chipperfield et al., 2017; Chipperfield and Santee et al., 2022). Recently, Pazmino et al. (2023) reported a combination of ground-based ozone observations (SAOZ network) and TOMCAT/SLIMCAT passive ozone simulations to explore different metrics for ozone recovery. This work showed Arctic recovery at the limit of statistical significance when ozone loss is linked to the extent of PSC occurrence.
TOMCAT References
Chipperfield, M.P., and R.L. Jones, Relative influence of atmospheric chemistry and transport on Arctic ozone trends, Nature, 400, 551-554, doi:10.1038/22999, 1999.
Chipperfield, M.P., W. Feng and M. Rex, Arctic ozone loss and climate sensitivity: Updated three-dimensional model study, Geophys. Res. Lett., 32, L11813, doi:10.1029/2005GL022674, 2005.
Chipperfield, M.P., S. Bekki, S. Dhomse, N.R.P. Harris, B. Hassler, R. Hossaini, W. Steinbrecht, R. Thieblemont and M. Weber, Detecting recovery of the stratospheric ozone layer, Nature, 549, 211-218, doi:10.1038/nature23681, 2017.
Chipperfield, M.P., M.L. Santee (Lead Authors), S.P. Alexander, A.T.J. de Laat, D.E. Kinnison, J. Kuttippurath, U. Langematz, and K. Wargan, Polar stratospheric ozone: Past, present and future (Chapter 4), In: Scientific Assessment of Ozone Depletion: 2022, Geneva, World Meteorological Organization, 2022. Link
Feng, W., S.S. Dhomse, C. Arosio, M. Weber, J.P. Burrows, M.L. Santee and M.P. Chipperfield, Arctic ozone depletion in 2019/20: Roles of chemistry, dynamics and the Montreal Protocol, Geophys. Res. Lett., 48, e2020GL091911, doi:10.1029/2020GL091911, 2021.
Pazmiño, A., F. Goutail, S. Godin-Beekmann, A. Hauchecorne, J.-P. Pommereau, M.P. Chipperfield, W. Feng, F. Lefèvre, A. Lecouffe, M. Van Roozendael, N. Jepsen, G. Hansen, R. Kivi, K. Strong, K.A. and Walker, Trends in polar ozone loss since 1989: potential sign of recovery in the Arctic ozone column, Atmos. Chem. Phys., 23, 15655-15670, doi:10.5194/acp-23-15655-2023, 2023.
Rex, M., N.R.P. Harris, P. von der Gathen, R. Lehmann, G.O. Braathen, E. Reimer, A. Beck, M.P. Chipperfield, R. Alfier, M. Allaart, F. O'Connor, H. Dier, V. Dorokhov, H. Fast, M. Gil, E. Kyro, Z. Litynska, I.S. Mikkelsen, M.G. Molyneux, H. Nakane, J. Notholt, M. Rummukainen, P. Viatte and J. Wenger, Prolonged stratospheric ozone loss in the 1995-96 Arctic winter, Nature, 389, 835-838, doi:10.1038/39849, 1997.
Rex, M., R.J.Salawitch, P. von der Gathen, N.R.P. Harris, M.P. Chipperfield and B. Naujokat, Arctic ozone loss and climate change, Geophys. Res. Lett., 31, L04116, doi:10.1029/2003GL018844, 2004.