the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Aug 2023
Air quality and radiative impacts of downward propagating sudden stratospheric warmings (SSWs)
Abstract. Sudden stratospheric warmings (SSWs) are abrupt disturbances to the Northern Hemisphere wintertime stratospheric polar vortex that can lead to pronounced regional changes in surface temperature and precipitation. SSWs also strongly impact the distribution of chemical constituents within the stratosphere, but the implications of these changes for stratosphere-troposphere exchange (STE) and radiative effects in the upper troposphere-lower stratosphere (UTLS) have not been extensively studied. Here we show, based on a specified-dynamics simulations from the EMAC chemistry-climate model, that SSWs lead to a pronounced increase in high-latitude ozone just above the tropopause (>25 % relative to climatology), persisting for up to 50 days for the ~50 % events classified as downward propagating following Hitchcock et al. (2013). This anomalous feature in lowermost stratospheric ozone is verified from ozone-sonde soundings and using the Copernicus Atmospheric Monitoring Service (CAMS) atmospheric composition reanalysis product. A significant dipole anomaly (>±25 %) in water vapour also persists in this region for up to 75 days, with a drying signal above a region of moistening, also evident within the CAMS reanalysis. Resultant enhanced STE leads to a significant 5–10 % increase in ozone of stratospheric origin over the Arctic, with a typical time-lag of 50 days. The signal also propagates to mid-latitudes leading to significant enhancements in UTLS ozone, and, of weakening strength, also in free tropospheric and near-surface ozone up to 90 days after the event. In quantifying the potential significance for surface air quality breaches above ozone regulatory standards, a risk enhancement of up to a factor of 2 to 3 is calculated following such events. The chemical composition perturbations in the Arctic UTLS result in radiatively-driven Arctic stratospheric temperature changes of around 2 K. An idealised sensitivity evaluation highlights the changing radiative importance of both ozone and water vapour perturbations with seasonality. Our results imply that SSW-related transport of ozone needs to be accounted for when studying the drivers of surface air quality. Accurate representation of UTLS composition (namely ozone and water vapour), through its effects on local temperatures, may also help improve numerical weather prediction forecasts on sub-seasonal to seasonal timescales.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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(683 KB) - BibTeX
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1175', Anonymous Referee #1, 16 Aug 2023
general comments
This study investigates the impacts of SSWs on air quality at the surface and radiative effects in the UTLS. Regarding air quality at the surface, the focus is placed on the field of near-surface ozone. As to radiative effects in the UTLS, both ozone and water vapor are considered. In particular, they found that SSWs lead to changes in ozone and water vapor in the UTLS, which in turn drive changes in stratospheric temperature. The results presented in this study reveal a 5-10% increase in the stratospheric-origin ozone near the surface and a roughly 2 degrees stratospheric temperature anomaly driven by the radiative effects of ozone and water vapor. These results are new, clear, and robust.
My major concern is that these changes do not seem, or have not proven, to be of critical importance. (a) For the air quality impact part, the full ozone field at the surface does not seem to respond to SSWs, indicating that the SSW-induced changes in the stratospheric-origin ozone are not sufficient to drive changes in the full ozone field. As shown in Fig.1d, even for the most intense and prolonged event on record, the SSW profile is not well separated from the climatology profile below 500 hPa. The signal may be even weaker from a composite-based perspective, which includes multiple less intense and less prolonged events. (b) For the radiative impact part, the radiatively-driven stratospheric temperature change is roughly 2 degrees, the magnitude of which is quite small in comparison to the tens of degrees change associated with SSWs.
Therefore, in my opinion, instead of arguing that SSWs exhibit an impact on surface ozone (which the results do not support), the authors may focus mainly on the signal in the stratospheric-origin ozone and perhaps add some discussion about why surface ozone, or the full ozone field, does not show a clear and robust response to SSWs. Maybe because the stratospheric-origin ozone only accounts for a very small fraction of the full ozone field at the surface, and a 5-10% increase in the former is not able to drive any notable changes in the latter? Regarding the radiative impact part, it would be helpful to show that a 2 degrees change in stratospheric temperature can have a substantial impact on the numerical weather forecasts on sub-seasonal to seasonal timescales. This could be done by adding some relevant references.
specific comments
Line 19: "Resultant enhanced STE"
“Resultant” means there is a reason, which is not clear here.
Lines 19-20: "a significant 5–10% increase in ozone of stratospheric origin over the Arctic, with a typical time-lag of 50 days."
it is not clear in which layer this 5–10% increase in ozone occurs, within UTLS or at the surface.
Lines 25-26: "Our results imply that SSW-related transport of ozone needs to be accounted for when studying the drivers of surface air quality."
I don't think the current results provide strong support for this statement. A 5-10% increase in the ozone of stratospheric origin does not necessarily correspond to a substantial or noticeable increase in the full ozone field at the surface, which might be dominated by the ozone of tropospheric origin.
Lines 61-63: "The sensitivity of tropospheric ozone to variations in the Arctic and North Atlantic Oscillations has been widely discussed (Li et al., 2002; Creilson et al., 2003; Duncan et al., 2004), but these variations were explained by purely tropospheric mechanisms."
This seems to imply that SSWs might be able to impact tropospheric ozone by driving variations in the AO/NAO. If this is true, then the ozone anomalies in certain regions (e.g., North America, Europe, Asia) following SSWs shown in this study may not arise from changes in the ozone of stratospheric origin, but from the re-distribution of ozone of tropospheric origin.
Lines 124-125: "For the three selected Arctic stations,..."
Please explain what your criteria were for selecting the three stations.
Line 274: “The 50 hPa level appears throughout much of the time period to be close to an inflexion point”
This is hard to tell because 50 hPa is not marked in the y-axis of Fig.3.
Line 277: “This is even more true for the 250 hPa level,”
Similar to the above, 250 hPa is also not marked in the y-axis of Fig.3.
Lines 279-280: “Following the major PJO-type SSWs over this period, which includes January 2006, January 2009 and January 2013,”
In total, four PJO-type SSWs are shown in Fig. 3. I wonder why only three of them are called “major” events.
Lines 280-281: “an anomalously dry region is found for heights above 250 hPa, which overrides an anomalously moist region immediately below this level.”
I do not see a dry-moist dipole very clearly. Highlighting the layer of 250 hPa would be helpful.
Line 324: tropospheric fraction of ozone of stratospheric origin (O3F) using this tracer (O3F = O3S/O3 x 100).
Since O3F is defined, adding a figure to show the climatology of O3F would be helpful. For example, if stratospheric-origin ozone accounts for a very small fraction of the full ozone, then one may not expect its response to SSWs to drive a notable change in full ozone at the surface, or surface air quality.
Lines 329-330: “with indication of an enhancement in ozone throughout the troposphere and elevated near-surface ozone which may impact air quality (see Sect. 5).”
This is not true. An enhancement in “stratospheric-origin” ozone is not equal to an enhancement in “full” ozone. The authors may want to add “stratospheric-origin” in front of “ozone”.
Lines 482-486 and Fig. 8a,b
The radiative effect of O3 seems to depend on the layer. While O3 increases throughout the vertical layers, it is associated with warming in some layers but cooling in other layers. The authors may provide some explanation for this.
Line 487: “highlighting that such radiative effects appear to be relatively long-lasting and potentially important for NWP.”
This is not clear to me. Please specify what field in NWP may be sensitive to the radiative effects here. SSWs are associated with tens of degrees of changes in stratospheric temperature, whereas the radiative effects here are up to two degrees only.
Citation: https://doi.org/10.5194/egusphere-2023-1175-RC1 - AC2: 'Reply on RC1', Ryan Williams, 08 Nov 2023
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RC2: 'Comment on egusphere-2023-1175', Anonymous Referee #3, 08 Sep 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1175/egusphere-2023-1175-RC2-supplement.pdf
- AC1: 'Reply on RC2', Ryan Williams, 08 Nov 2023
-
AC3: 'Comment on egusphere-2023-1175', Ryan Williams, 08 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1175/egusphere-2023-1175-AC3-supplement.pdf
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1175', Anonymous Referee #1, 16 Aug 2023
general comments
This study investigates the impacts of SSWs on air quality at the surface and radiative effects in the UTLS. Regarding air quality at the surface, the focus is placed on the field of near-surface ozone. As to radiative effects in the UTLS, both ozone and water vapor are considered. In particular, they found that SSWs lead to changes in ozone and water vapor in the UTLS, which in turn drive changes in stratospheric temperature. The results presented in this study reveal a 5-10% increase in the stratospheric-origin ozone near the surface and a roughly 2 degrees stratospheric temperature anomaly driven by the radiative effects of ozone and water vapor. These results are new, clear, and robust.
My major concern is that these changes do not seem, or have not proven, to be of critical importance. (a) For the air quality impact part, the full ozone field at the surface does not seem to respond to SSWs, indicating that the SSW-induced changes in the stratospheric-origin ozone are not sufficient to drive changes in the full ozone field. As shown in Fig.1d, even for the most intense and prolonged event on record, the SSW profile is not well separated from the climatology profile below 500 hPa. The signal may be even weaker from a composite-based perspective, which includes multiple less intense and less prolonged events. (b) For the radiative impact part, the radiatively-driven stratospheric temperature change is roughly 2 degrees, the magnitude of which is quite small in comparison to the tens of degrees change associated with SSWs.
Therefore, in my opinion, instead of arguing that SSWs exhibit an impact on surface ozone (which the results do not support), the authors may focus mainly on the signal in the stratospheric-origin ozone and perhaps add some discussion about why surface ozone, or the full ozone field, does not show a clear and robust response to SSWs. Maybe because the stratospheric-origin ozone only accounts for a very small fraction of the full ozone field at the surface, and a 5-10% increase in the former is not able to drive any notable changes in the latter? Regarding the radiative impact part, it would be helpful to show that a 2 degrees change in stratospheric temperature can have a substantial impact on the numerical weather forecasts on sub-seasonal to seasonal timescales. This could be done by adding some relevant references.
specific comments
Line 19: "Resultant enhanced STE"
“Resultant” means there is a reason, which is not clear here.
Lines 19-20: "a significant 5–10% increase in ozone of stratospheric origin over the Arctic, with a typical time-lag of 50 days."
it is not clear in which layer this 5–10% increase in ozone occurs, within UTLS or at the surface.
Lines 25-26: "Our results imply that SSW-related transport of ozone needs to be accounted for when studying the drivers of surface air quality."
I don't think the current results provide strong support for this statement. A 5-10% increase in the ozone of stratospheric origin does not necessarily correspond to a substantial or noticeable increase in the full ozone field at the surface, which might be dominated by the ozone of tropospheric origin.
Lines 61-63: "The sensitivity of tropospheric ozone to variations in the Arctic and North Atlantic Oscillations has been widely discussed (Li et al., 2002; Creilson et al., 2003; Duncan et al., 2004), but these variations were explained by purely tropospheric mechanisms."
This seems to imply that SSWs might be able to impact tropospheric ozone by driving variations in the AO/NAO. If this is true, then the ozone anomalies in certain regions (e.g., North America, Europe, Asia) following SSWs shown in this study may not arise from changes in the ozone of stratospheric origin, but from the re-distribution of ozone of tropospheric origin.
Lines 124-125: "For the three selected Arctic stations,..."
Please explain what your criteria were for selecting the three stations.
Line 274: “The 50 hPa level appears throughout much of the time period to be close to an inflexion point”
This is hard to tell because 50 hPa is not marked in the y-axis of Fig.3.
Line 277: “This is even more true for the 250 hPa level,”
Similar to the above, 250 hPa is also not marked in the y-axis of Fig.3.
Lines 279-280: “Following the major PJO-type SSWs over this period, which includes January 2006, January 2009 and January 2013,”
In total, four PJO-type SSWs are shown in Fig. 3. I wonder why only three of them are called “major” events.
Lines 280-281: “an anomalously dry region is found for heights above 250 hPa, which overrides an anomalously moist region immediately below this level.”
I do not see a dry-moist dipole very clearly. Highlighting the layer of 250 hPa would be helpful.
Line 324: tropospheric fraction of ozone of stratospheric origin (O3F) using this tracer (O3F = O3S/O3 x 100).
Since O3F is defined, adding a figure to show the climatology of O3F would be helpful. For example, if stratospheric-origin ozone accounts for a very small fraction of the full ozone, then one may not expect its response to SSWs to drive a notable change in full ozone at the surface, or surface air quality.
Lines 329-330: “with indication of an enhancement in ozone throughout the troposphere and elevated near-surface ozone which may impact air quality (see Sect. 5).”
This is not true. An enhancement in “stratospheric-origin” ozone is not equal to an enhancement in “full” ozone. The authors may want to add “stratospheric-origin” in front of “ozone”.
Lines 482-486 and Fig. 8a,b
The radiative effect of O3 seems to depend on the layer. While O3 increases throughout the vertical layers, it is associated with warming in some layers but cooling in other layers. The authors may provide some explanation for this.
Line 487: “highlighting that such radiative effects appear to be relatively long-lasting and potentially important for NWP.”
This is not clear to me. Please specify what field in NWP may be sensitive to the radiative effects here. SSWs are associated with tens of degrees of changes in stratospheric temperature, whereas the radiative effects here are up to two degrees only.
Citation: https://doi.org/10.5194/egusphere-2023-1175-RC1 - AC2: 'Reply on RC1', Ryan Williams, 08 Nov 2023
-
RC2: 'Comment on egusphere-2023-1175', Anonymous Referee #3, 08 Sep 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1175/egusphere-2023-1175-RC2-supplement.pdf
- AC1: 'Reply on RC2', Ryan Williams, 08 Nov 2023
-
AC3: 'Comment on egusphere-2023-1175', Ryan Williams, 08 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1175/egusphere-2023-1175-AC3-supplement.pdf
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Ryan Williams
Michaela Hegglin
Patrick Jöckel
Hella Garny
Keith Shine
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2555 KB) - Metadata XML
-
Supplement
(683 KB) - BibTeX
- EndNote
- Final revised paper