Abstract
Abstract. Changes in stratospheric ozone concentrations and increasing concentrations of greenhouse gases (GHGs) alter the temperature structure of the atmosphere and drive changes in the atmospheric and oceanic circulation. We systematically investigate the impacts of ozone recovery and increasing GHGs on the atmospheric and oceanic circulation in the Southern Hemisphere during the twenty-first century using a unique coupled ocean–atmosphere climate model with interactive ozone chemistry and enhanced oceanic resolution. We use the high-emission scenario SSP5-8.5 for GHGs under which the springtime Antarctic total column ozone returns to 1980s levels by 2048 in our model, warming the lower stratosphere and strengthening the stratospheric westerly winds. We perform a spatial analysis and show for the first time that the austral spring stratospheric response to GHGs exhibits a marked planetary wavenumber 1 (PW1) pattern, which reinforces the response to ozone recovery over the Western Hemisphere and weakens it over the Eastern Hemisphere. These changes, which imply an eastward phase shift in the PW1, largely cancel out in the zonal mean. The Southern Hemisphere residual circulation strengthens during most of the year due to the increase in GHGs and weakens in spring due to ozone recovery. However, we find that in November the GHGs also drive a weakening of the residual circulation, reinforcing the effect of ozone recovery, which represents another novel result. At the surface, the westerly winds weaken and shift equatorward due to ozone recovery, driving a weak decrease in the transport of the Antarctic Circumpolar Current and in the Agulhas leakage and a cooling of the upper ocean, which is most pronounced in the latitudinal band 35–45∘ S. The increasing GHGs drive changes in the opposite direction that overwhelm the ozone effect. The total changes at the surface and in the oceanic circulation are nevertheless weaker in the presence of ozone recovery than those induced by GHGs alone, highlighting the importance of the Montreal Protocol in mitigating some of the impacts of climate change. We additionally compare the combined effect of interactively calculated ozone recovery and increasing GHGs with their combined effect in an ensemble in which we prescribe the CMIP6 ozone field. This second ensemble simulates a weaker ozone effect in all the examined fields, consistent with its weaker increase in ozone. The magnitude of the difference between the simulated changes at the surface and in the oceanic circulation in the two ensembles is as large as the ozone effect itself. This shows the large uncertainty that is associated with the choice of the ozone field and how the ozone is treated.
Highlights
Ozone depletion has been the major driver of change in the Southern Hemisphere (SH) atmospheric circulation during the last decades of the twentieth century (e.g., Polvani et al, 2011b)
The combined effects of the two forcings were compared between the configuration of Flexible Ocean Climate Infrastructure (FOCI) with interactive ozone chemistry and the configuration in which the CMIP6 ozone field consistent with the SSP5-8.5 pathway was prescribed
The springtime Antarctic total column ozone in FOCI returns to 1980s levels in 2048 as increasing greenhouse gases (GHGs) following SSP5-8.5 accelerate the recovery of the ozone hole by about 2 decades
Summary
Ozone depletion has been the major driver of change in the Southern Hemisphere (SH) atmospheric circulation during the last decades of the twentieth century (e.g., Polvani et al, 2011b). A number of previous studies employed singleforcing model simulations to explicitly separate the effects of ozone recovery and increasing GHGs on various aspects of the SH climate during the twenty-first century (Shindell and Schmidt, 2004; Perlwitz et al, 2008; Oman et al, 2009; Karpechko et al, 2010; McLandress et al, 2010, 2011; Polvani et al, 2011a; Oberländer et al, 2013; Polvani et al, 2018, 2019), this is the first time that these effects are studied in a state-of-the-art coupled ocean–atmosphere chemistry climate model that includes both interactive ozone chemistry and an ocean that contains a high-resolution, mesoscaleresolving nest in the South Atlantic and the western Indian Ocean This allows us to clearly link stratospheric ozone changes to changes in mesoscale-dominated oceanic features, such as the Agulhas leakage, for the first time in the same model. The study is structured as follows: Sect. 2 presents our model simulations and methodology, Sect. 3 gives an estimate of ozone recovery in our model, Sect. 4 examines the impacts of ozone recovery and increasing GHGs on the atmospheric and oceanic circulation in the SH and compares the combined impact in simulations with prescribed and interactive ozone, and Sect. 5 provides our summary and discussion
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