The response of the North Pacific jet and stratosphere-to-troposphere transport of ozone over western North America to RCP8.5 climate forcing
Abstract. Stratosphere-to-troposphere transport (STT) is an important source of ozone for the troposphere, particularly over western North America. STT in this region is predominantly controlled by a combination of the variability and location of the Pacific jet stream and the amount of ozone in the lower stratosphere, two factors which are likely to change if greenhouse gas concentrations continue to increase. Here we use Whole Atmosphere Community Climate Model experiments with a tracer of stratospheric ozone (O3S) to study how end-of-the-century Representative Concentration Pathway (RCP) 8.5 sea surface temperatures (SSTs) and greenhouses gases (GHGs), in isolation and in combination, influence STT of ozone over western North America relative to a preindustrial control background state.
We find that O3S increases up to 39 % at 700 hPa over western North America in response to RCP8.5 forcing with the largest increases occurring during late winter and tapering off somewhat during spring and summer. When this response is decomposed into the contributions made by future SSTs and GHGs, the latter are found to be primarily responsible for these O3S changes. Both the future SSTs and the future GHGs accelerate the Brewer Dobson circulation, which increases extratropical lower stratospheric ozone mixing ratios. While the GHGs promote a more zonally symmetric lower stratospheric ozone change due to enhanced ozone production and some transport, the SSTs increase lower stratospheric ozone predominantly over the North Pacific via transport associated with a stationary planetary-scale wave. Ozone accumulates in the trough of this anomalous wave and is reduced over the wave’s ridges, illustrating that the composition of the lower stratospheric ozone reservoir in the future is dependent on the phase and position of the stationary planetary-scale wave response to future SSTs, in addition to the poleward mass transport provided by the accelerated Brewer-Dobson Circulation. In addition, the future SSTs account for most changes to the large-scale circulation in the troposphere and stratosphere compared to the effect of future greenhouse gases. These changes include modifying the position and speed of the future North Pacific jet, lifting the tropopause, accelerating both the Brewer-Dobson Circulation’s shallow and deep branches, and enhancing two-way isentropic mixing in the stratosphere.
Dillon Elsbury et al.
Status: final response (author comments only)
RC1: 'Comment on acp-2022-700', Anonymous Referee #1, 03 Jan 2023
- AC1: 'Reply on RC1', Dillon Elsbury, 23 Feb 2023
RC2: 'Comment on acp-2022-700', Anonymous Referee #2, 06 Jan 2023
- AC2: 'Reply on RC2', Dillon Elsbury, 23 Feb 2023
Dillon Elsbury et al.
Dillon Elsbury et al.
Viewed (geographical distribution)
The article examines the response of the stratosphere-to-troposphere transport of ozone to a high-emissions climate change scenario (RCP8.5) using a set of time-slice simulations with WACCM. The focus is over North America and the spring transition of the Pacific jet following previous works, and the response to GHG and SST are separated. The results show increased ozone transport into the troposphere in that region peaking in late winter. The paper is well written and results are clearly presented and adequately discussed. However, there are some important issues that need to be discussed before the paper can be published, which I listed below as major comments.
- Figure 2 shows that the increase in ozone transport into the troposphere is dominated by GHG. This is in contrast with the results in the rest of the paper, which demonstrate a fundamental role of the SST for O3S in the lower stratosphere and upper troposphere (UTLS). How should we interpret this difference? Does it imply that changes in the lower stratospheric reservoir do not translate into tropospheric ozone, contrary to what is typically considered? Or does it reflect a disconnection between UTLS and the deep intrusions reaching the middle-lower troposphere? Or something else? This should be discussed in the manuscript.
- Figure 5 suggests that GHG enhance substantially the ozone concentrations in the extratropical lower stratosphere reservoir, but then in Figures 6 and 7 the GHG have no effect. Does this mean that the ozone local production (rather than the transport by the BDC) is enhanced with GHG increase? If so, it seems quite an important point to make, given that the ozone enhancement is larger than that due to SST.
- The lower stratospheric ozone reservoir changes show a pattern that resemble the stationary wave response to a positive ENSO phase SST anomaly. As stated in the paper, other models could produce different SST patterns leading to different stationary waves and thus ozone changes. Nevertheless, the increase in ozone STT is consistent across models. What does this mean for the role of the ozone wave-like anomalies on the zonal mean trends? The answer could go in the direction of the response to zonal mean SST dominating over the response to SST zonal anomalies, as found in Chrysanthou et al. (2020). A discussion on this point would improve the paper.
- In lines 105-107 the ODS are said to increase from the pre-industrial to the RCP8.5 end-of-century simulations (note that it would be convenient to state here by how much they do increase in order to quantify their effect). However, in Line 437 it is stated “with additional effects from reduced ODS”. I suspect this confusion is due to the different simulations considered in Dietmuller et al. (2021) versus this manuscript. Please, clarify what is the role of ODS if any in your analysis. Also, even though the role of ODS is not explicitly examined in this work given the comparison to pre-industrial conditions, previous studies have shown that ODS decline is the dominant forcing of global ozone STT increase over the century (Banerjee et al. 2016, Meul et al. 2018, Abalos et al. 2020). This is an important point that should be clearly stated in the paper in order to avoid confusion.
- L88: Please specify how stratospheric ozone loss is treated in the troposphere. There are different ways in which this has been made and it is beneficial to explicitly include this information.
- L92-93: “to remove interannual variability driven by the ocean (e.g. variability due to ENSO)” It is true that there is no variability in your experiments. However, a clear ENSO signal shows up in the climatology of the SST change, presumably due to more frequent/intense events. So you are not really removing this effect (and indeed you refer to it later on). So I suggest rephrasing this.
- L121-124 seem in contradiction with L117-120. Please clarify what you mean.
- L241: Is this robust also across climate models?
- L253: Is the amplified surface warming related to changes in sea ice? I assume sea ice is also imposed as a boundary condition?
- Fig. 4: Top row: are there negative values in low latitudes or is it an effect of the colorbar? What are the units of O3S? Caption: “stationary wave (contou*r*s, long-term zonal...) ”. Stationary waves are not a physical magnitude, change to something like “Stationary waves visualized by geopotential height deviation from the long-term mean zonal mean (contours, in meters)”
- L312: Why is the modified flow implied by the wave phase tilt?
- L167: “distribution of median dates” : remove “median”
- Fig. 2 caption: What does the box show?
- L122: “still coincide with roughly 10-35%” → “still imply roughly a 10-35%”
- L259: “purely chemical changes in the atmosphere” Strictly, this applies to all climate change impacts, do you mean chemical changes in the *stratosphere*? - L316: Fig. 4k → Fig. 4g
- L334: 235ºE-260ºE: does this correspond to the box in Fig. 2? If so, please add it.
- L389: “=” should be “)”