Stratospherically induced tropospheric circulation changes under the extreme conditions of the No-Montreal-Protocol scenario
Abstract. The Montreal Protocol and its amendments (MPA) have been a huge success in preserving the stratospheric ozone layer from being destroyed by unabated chlorofluorocarbons (CFCs) emissions. The phase out of CFCs has not only prevented serious impacts on our health and climate, but also avoided strong alterations of atmospheric circulation patterns. With the Earth System Model SOCOLv4, we study the dynamical and climatic impacts of a scenario with unabated CFC emissions by 2100, disentangling radiative and chemical (ozone-mediated) effects of CFCs. In the stratosphere, chemical effects of CFCs (i.e. the resulting ozone loss) are the main drivers of circulation changes, weakening wintertime polar vortices and speeding up the Brewer-Dobson circulation. These dynamical impacts during wintertime are due to low-latitude ozone depletion and resulting reduction of the equator-to-pole temperature gradient. In Southern Hemisphere (SH) summer, the vortex strengthens, similar due to the effects of the Antarctic ozone hole over the second half of the 20th century. Furthermore, the winter and spring vortex variability increases in the SH, whereas it decreases in summer and fall. This seasonal variation of wind speed in the stratosphere has regional implications on the tropospheric circulation modes. We find coherent changes in the troposphere, such as negative Southern Annular mode (SAM) and North Atlantic Oscillation (NAO) during seasons with a weaker vortex (winter and spring); the opposite occurs during seasons with stronger westerlies in the stratosphere (summer). In the troposphere, radiative heating by CFCs prevails throughout the year, shifting the SAM into a positive phase and canceling out the ozone-induced effects on the NAO. Furthermore, global warming is amplified by 1.9 K with regionally up to 12 K increase over Eastern Canada and Western Arctic. Our study sheds light into the adverse effects of a non-adherence to the MPA on the global atmospheric circulation, uncovering the roles of the underlying physical mechanisms. In so doing, our study emphasizes the importance of the MPA for Earth’s climate, to avoid regional amplifications of negative climate impacts.
Franziska Zilker et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2023-326', Anonymous Referee #1, 04 Apr 2023
- RC2: 'Comment on egusphere-2023-326', Anonymous Referee #2, 21 Apr 2023
Franziska Zilker et al.
Franziska Zilker et al.
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Summary: The authors conduct sensitivity simulations using the SOCOLv4 chemistry-climate model of a no-MP scenario involving continuing accelerating increases of atmospheric abundances of ozone-depleting substances. The unique aspect of this paper is that they decompose the modelled response into chemical (caused by ozone responses) and radiative (due to LW absorption by CFCs) effects. They show, for this extreme scenario, some partial cancellation of the effects of these two mechanisms. The results are consistent with and expand what had previously been known thanks to earlier studies of the same subject. I recommend publication of the study in ACP subject to addressing my comments, listed below.
I think there is an interesting difference between some results reported here and what have been understood to be the present dynamical consequences of ozone depletion. Canonical wisdom is that ozone depletion causes a strengthening of the SAM due to cooling of the polar vortex; this influences tropospheric circulation in late spring and summer. Here the authors show that in the extreme ozone-depletion scenario of no Montreal Protocol, this is no longer the case. As ozone depletion becomes global, the cooling difference associated with this between low latitudes and the South Pole reduces, causing a weakening of the SAM. This had been new to me. The net strengthening of the SAM is then not mainly driven by ozone depletion but by the direct radiative heating due to CFCs. More could be made of this, and an explanation could be added that this is actually different from what is seen in the present situation when ozone depletion is not near saturation. This is my understanding; please feel free to agree or disagree with this.
For the Arctic, you discuss the “North Atlantic Oscillation”. Please consider replacing this with the more generic “Northern Annular Mode”. The NAO, in my perception, is just a regional expression of the NAM defined by the pressure difference between Iceland and the Azores. I’d prefer to use the “NAM” terminology here, in analogy with the SAM that you discuss throughout the paper. Also more substantially, under present conditions relatively moderate ozone depletion is not known to drive a trend of the NAM (which generally remains unexplained, see Ch3 of IPCC AR6) but there are associations between extreme ozone states and NAM anomalies, as you correctly state. Under the much more extreme ozone depletion considered here, you find some significant SLP anomalies. Perhaps more could be made of the fact that these effects are significant here but are not significant in the real world?
Title: I’d drop the word “tropospheric” in the title as the paper equally deals with stratospheric circulation changes.
L8: Usually during summer there is no “polar vortex” over Antarctica – it breaks down in spring. You probably want to state first that the polar vortex now persists year-round in the no-MP scenario (if that’s the case).
L12: Insert “anomalies” before “during”.
L49: The “studies” are very interactive, just their models are not. Suggest to rephrase this, such as “Models used in previous studies are not fully interactive…”.
L72-73: Young et al. (2021) is the most recent paper to deal with this; they add another 0.8 degrees of warming under the No-MP protocol, due to biospheric release of carbon under ozone depletion. Also https://www.nature.com/articles/s41598-019-48625-z could be discussed somewhere.
L140: How do you know it’s shortwave absorption? Ozone also absorbs outgoing LW radiation.
L153: Morgenstern et al. (2018) find a similar feature (an increase in ozone in the polar middle stratosphere due to increasing chlorine in spring), across several CCMI models. It seems to be part of the dynamical response to ozone depletion.
Figure 6: I’m impressed at how different the direct and indirect effects of CFCs on SLP are. This is to my understanding quite different again from the historical and present-day situation where I‘m sure the direct effect is smaller than the indirect one. Perhaps you can comment on this. Also here a nod to Velders et al. (PNAS, 2007) might be in order who first stipulated that CFCs would be rivalling CO2 as the leading cause of global warming under a no-MP scenario.
L251: Replace “largely” with “substantially”. Also line 265.
L275: Replace “mostly” with “most”.
Figure A1: The labels for (b), (e), (h) should be “ref” not “noMPA”, and for (c), (f), and (i) probably “(noMPA – ref)/ref”
Figure B1: The usage of CO as a diagnostic probably needs more explanation (or dropping) as CO is not elaborated in the text and is a somewhat separate story.