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.