Stratospheric contribution to the summertime high surface ozone events over the western united states

The stratospheric influence on summertime high surface ozone ( O3) events is examined using a twenty-year simulation from the Whole Atmosphere Community Climate Model. We find that O3 transported from the stratosphere makes a significant contribution to the surface O3 variability where background surface O3 exceeds the 95th percentile, especially over western U.S. Maximum covariance analysis is applied to O3 anomalies paired with stratospheric O3 tracer anomalies to identify the stratospheric intrusion and the underlying dynamical mechanism. The first leading mode corresponds to deep stratospheric intrusions in the western and northern tier of the U.S., and intensified northeasterlies in the mid-to-lower troposphere along the west coast, which also facilitate the transport to the eastern Pacific Ocean. The second leading mode corresponds to deep intrusions over the Intermountain Regions. Both modes are associated with eastward propagating baroclinic systems, which are amplified near the end of the North Pacific storm tracks, leading to strong descents over the western U.S.


Introduction
Surface ozone (O 3 ) adversely affects human health and the ecosystem because of its high oxidation capability (U.S. Environmental Protection Agency, 2015). The risk of O 3 pollution on mortality is also significantly raised by high temperatures (Levy and Patz 2015). During summer, surface O 3 level maximizes over the western U.S. (Gaudel et al 2018), mainly attributed to the combination of active photochemical production and noncontrollable sources, such as intercontinental pollution transport, lightning, and wildfire events (Fiore et al 2002, Jaffe et al 2018. Downward transport of O 3 during stratospheric intrusions is also considered to be a contributing factor during summertime (Danielsen 1980, Lefohn et al 2011, Lefohn et al 2012, Zanis et al 2014, Akritidis et al 2016, Yang et al 2016, Škerlak et al 2019. The transport is achieved irreversibly by a tongue-like structure containing high stratospheric O 3 extruding downward, folding into the tropospheric air and descending toward the surface (Danielsen 1968, Johnson andViezee 1981). When a stratospheric intrusion contributes high O 3 to the surface, in addition to that produced by anthropogenic pollution, it could easily push the surface O 3 values beyond the National Ambient Air Quality Standard threshold 70 ppbv (Langford et al 2017(Langford et al , Škerlak et al 2019. Observational and modeling studies have shown that surface O 3 extremes that are directly associated with downward transport from the stratosphere preferentially occur in the western U.S. (Stohl et al 2003, Lin et al 2012, Lin et al 2015, Škerlak et al 2014. Consequently, the joint effects of chemistry and episodic stratospheric transport make the western U.S. a hot spot of O 3 pollution in summer. Due to the large dynamic variability of the tropopause, limited temporal and spatial extent of measurements, and mixing with tropospheric air, the observations of transport due to stratospheric intrusions are challenging (Stohl et al 2003). In addition, most of the previous studies focused on springtime stratospheric influence because tropopause O 3 abundances and downward air mass fluxes maximize during that time (Langford 1999, Prather et al 2011, Langford et al 2009, Lin et al 2012, Lin et al 2015, Langford et al 2017, Albers et al 2018. The linkage between summertime stratospheric intrusion and high surface O 3 events over the western U.S. has received less attention (Lefohn et al 2011, Lefohn et al 2012. By analyzing the output of a state-of-the-art chemistry climate model implemented with an artificial stratospheric ozone tracer (O 3 S), we aim to 1. estimate the contribution of O 3 reaching the surface associated with summertime stratospheric intrusions, 2. understand the space-time behavior of stratospheric intrusion events, and 3. clarify the underlying dynamical mechanism.

CESM2(WACCM6) and O 3 S Diagnostic
We analyzed daily surface O 3 and stratospheric O 3 tracer (denoted by O 3 S) from 1995 to 2014 summer months (June-August, JJA) using the Whole Atmosphere Community Climate Model version 6 (WACCM6) of the Community Earth System Model version 2 (CESM2). It is the high-top version of the Community Atmosphere Model version 6 (CAM6), integrating the atmospheric physics and chemistry from the surface to nearly 140 km. The WACCM6 uses the same atmospheric physics as CAM6. The chemical mechanism includes comprehensive troposphere, stratosphere, mesosphere and lower thermosphere chemistry, described by Emmons et al (2020). The standard emissions are based on anthropogenic and biomass burning inventories specified for the Coupled Model Intercomparison Project 6 (CMIP6). WACCM6 is coupled to the interactive Community Land Model version 5 (CLM5), which handles dry deposition. The simulations shown here are fully coupled ocean-atmosphere experiment, and feature 0.95 • × 1.25 • (latitude× longitude) horizontal resolution and 70 layers, with~1.2 km vertical resolution above the boundary layer to the lower stratosphere. We consider WACCM6 is well suited for studying the transport during stratospheric intrusion because that 1. the atmospheric chemistry and deposition scheme for O 3 are well represented and tropospheric O 3 simulations are improved in comparison to observations (Emmons et al 2020); 2. WACCM6 is able to effectively reproduce the observed wind and temperature climatologies as well as stratospheric variability (Gettelman et al 2019); 3. the twenty-year simulation with daily output of O 3 and O 3 S provides us with large samples to study. Detailed model formulations, descriptions, and evaluations can be found in Gettelman et al (2019) and Emmons et al (2020).
To quantify the stratospheric contribution to high surface O 3 events, we study an artificial tracer, O 3 S, for O 3 originating in the stratosphere, which is implemented in a manner as described in Tilmes et al (2016). O 3 S experiences the same loss rate as O 3 in the troposphere but is not affected by NO x photolysis as defined by the Chemistry-Climate Model Initiative (CCMI). Earlier studies have shown that the diagnostics of deep stratospheric intrusions are insensitive to the choice of tropopause definition (Yang et al 2016) or O 3 S tagging methods (Lin et al 2012).

Evaluation of CESM2 (WACCM6)
Here we first evaluate the WACCM6 simulations against observations from the Tropospheric Ozone Assessment Report (TOAR) (Schultz et al 2017). Figure 1(a) is the 1995-2014 mean of 'all$ m ean' variables from the TOAR 5 • × 5 • (latitude× longitude) monthly median products. High O 3 values ( ∼ 45 nmol mol −1 ) are seen over the western U.S. in TOAR measurements. For WACCM6, we have gridded the simulations onto the horizontal grid of TOAR data and calculated median O 3 concentrations using the same metrics to guarantee an apple-to-apple comparison (see figure 1(b). Generally a good agreement is found between the model and observations over the Central U.S. and most of the West Region, with percentage differences within 15% (see figure S1 (https://stacks.iop.org/ERL/15/1040a6/mmedia)). However, WACCM6 overestimates the surface median O 3 over the southeast U.S. by 55% (∼ 15 nmol mol −1 ), which is consistent with the evaluation in Emmons et al (2020). The biased O 3 over the eastern U.S. is a long existing problem that can have various reasons starting with still insufficient complexity of the chemistry, but also the model resolution, deposition scheme, etc (Schwantes et al 2020).  Figure 1(c) is similar to figure 1(b) and shows that high O 3 are concentrated over the western U.S. As shown in 1(d), strong stratospheric impact, ranging from 6 to 10 nmol/mol, is found over the Canada-U.S. border and the Western States, including southern British Columbia,  (d)). The linear trend has been removed before the calculation. As suggested in figure 1(e), high peaks are located over Wyoming, southern Texas, and the Four Corners area (Colorado, Utah, Arizona, and New Mexico). We see in Arizona, for example, O 3 S can explain as high as 54% of surface O 3 variance during days in which surface O 3 exceeds p95. We compare our results with those from prior observational studies. Lefohn et al (2012) studied stratospheric intrusion events associated with daily maximum hourly O 3 in exceedance of 50 nmol mol −1 at both rural and urban monitoring sites of the U.S. Statistically significant relations are found over the southern Texas, Arizona, Colorado, Utah, Nevada, California, and Wyoming in summer months during 2006-2008. Overall, our model simulations support observational findings that stratospheric intrusions coincident with O 3 preferentially take place in the West and Intermountain West during summertime. We also quantify the stratospheric impact when surface O 3 exceeds p95 at each grid cell and find that in regional average over the western U.S. (30 • N − 50 • N, 100 • W-125 • W), the stratospheric contribution is 18.4% (not shown).

MCA results and interpretation
We employ MCA to investigate how surface O 3 is related to O 3 S reaching the surface on daily basis. The first two leading modes are summarized in figure 2. Together, these two modes explain 60% of the covariance pattern over the domain of interest. The first mode shows large positive covariances over the eastern Pacific, western and northern tier of the U.S. (figure 2(a). The second mode shows dipole structure with O 3 and O 3 S surplus over the western U.S. and deficit over the east (figure 2(b). Surface level O 3 S anomalies are found to lag the 200-500 hPa O 3 S anomalies by two days (not shown), suggesting the downward influence. No significant lead-lag relationships are found either between the expansion coefficient time series (loadings) of O 3 and O 3 S at the surface or between the first two SVD modes. We identify~20 days per month (1404 days in twenty years for the first two leading modes) when both expansion coefficient time series are greater than 0, indicating that surface O 3 concentration increase is coincident with O 3 S reaching the surface. Lefohn et al Lefohn et al (2011), Lefohn et al (2012) investigated the frequency of surface O 3 enhancements that are associated with stratosphere-to-troposphere transport down to the surface across the U.S. and reported that the average number of days per month ranges from 16 days to 23 days at monitoring sites in the West and Intermountain West in summer months.
Our results of intrusion frequency are in remarkable agreement with their results.
Next, we define high surface O 3 events caused by stratospheric intrusions when both loadings exceed 1.5 standard deviations, as marked by black circles in figure 2. Each event lasts for a few days. Based on this definition, composite analyses according to high surface O 3 events are carried out using O 3 S daily data. The composited patterns are not sensitive to the thresholds we chose for the analyses. Schematics in figure 3 show the large amplitude composited O 3 S structures and near-surface circulations corresponding to both modes. We use neon yellow color to highlight the area with O 3 S larger than 120 nmol mol −1 , which can be treated approximately as the tropopause (Yang et al 2016). During Mode 1, depressed tropopause height followed by enhanced O 3 S can be found around 50 • N, 120 • W (figure 3(a). Since stratospheric air contains higher values of potential vorticity (PV) and O 3 , the intrusion of the tropopause tends to replace the tropospheric air by ozone-rich stratospheric air with large PV (Danielsen 1968, Mote et al 1991, Wimmers et al 2003. Transport of O 3 S to low levels is tied to stratospheric intrusions and strong subsidence in the troposphere. Figure 4(d) shows a map of composited means of 500 hPa vertical velocity (ω) anomalies on the day of the events corresponding to Mode 1. The intrusion is associated with intensified subsidence on the U.S.-Canada border. Coherently, enhanced O 3 S on the northern tier can be seen throughout the troposphere, while the maximum over the Pacific appears below 700 hPa (figure 3(a)). We conduct composited analyses on anomalies of 860 hPa geopotential height as well as surface winds according to events of Mode 1. A quadrupole pattern of low-level geopotential height anomalies is seen over the North American continent. In contrast to the positive correlation between the upper-level cyclonic vorticity and O 3 S (figure S3a), low-level geopotential height and O 3 S reaching the surface are strongly anti-correlated ( figure 3(b)). Dry air (figure S3b) with high PV value (figure S3a) descends on the northern tier of the U.S. as a result of stratosphere-to-troposphere transport by intensified subsidence. Warm and moist tropospheric air is seen (figure S3b-c) downstream (east) of the surface low. In addition to the anomalous descent, we also see a strengthening of northeasterly along the west coast near the surface ( figure 3(b)). This intensified easterly associated with stronger anticyclone facilitates the horizontal transport towards the subtropical Pacific in the mid-to-lower troposphere ( figure 3(a) During both modes, the O 3 S reaching the surface can be 10-18 nmol mol −1 (see surface contours of figure 3(a) and 3(c), respectively). In regional average over the western U.S. (30 • W, the stratospheric contribution is 11 nmol/mol. The amplitude agrees well with the observational record from the California Baseline Ozone Transport Study (CABOTS), in which they found that stratospheric contribution is 10-20 nmol mol −1 to the surface over Northern California during an intrusion case in August 2016 (Clark and Chiao 2019). We also see~10 days per summer season when high surface O 3 events associated with the first two SVD modes occur, with variation across years.

Dynamical mechanism
Next we examine the dynamical mechanism underlying the stratospheric intrusion events associated with together with frequent eastward traveling baroclinic waves (White 1982). The baroclinic waves commonly exhibit life cycles of baroclinic growth and barotropic decay along the storm track regions (Simmons andHoskins 1978, Blackmon et al 1984). Observational and modeling studies have revealed that the baroclinic disturbances leave imprint on atmospheric composition. Aircraft experiments provided evidence that high amounts of stratospheric radioactive debris and ozone were drawn into the troposphere by midlatitude storms (Danielsen 1968). Schoeberl and Krueger (1983) and Mote et al Mote et al (1991) identified the coherent fluctuations in total O 3 and medium scale waves along the wintertime oceanic storm track regions using satellite data. Stone et al (1996) found baroclinic wave features using upper tropospheric water vapor measurement. General circulation model was capable of representing stratosphere-troposphere exchange associated with baroclinic waves in midlatitudes (Mote et al 1994). In our study, baroclinic wave system and its resulting circulation is also found important for the occurrence of deep stratospheric intrusions over the western U.S. in summertime (Sprenger and Wernli 2003).
In both modes, we see that the wave trains originate over the baroclinically unstable central Pacific, grow by baroclinic conversion, radiate energy downstream through ageostrophic geopotential fluxes, and dissipate over the jet exit near the western U.S., as discussed in previous studies (Chang andOrlanski 1993, Chang 1993). The upper-level jet stream acts as a wave guide, constraining the baroclinic system tightly along its core. From day −2 to day 0 in both modes, the intensified descents at 500 hPa, which cor- • N for simplicity. We can see a sequence of downstream developing wave train originating from the North Pacific and propagating along the upperlevel jet. Eastward propagation of Mode 2 is less clear than that of Mode 1. The descending anomalies begin to amplify near the jet exit two days before the deep intrusions over the western U.S. Similar conclusions are also found using upper-level meridional velocity (not shown). The results are consistent with evolving of baroclinic waves, and are in good agreement with the wave train signature diagnosed in Lim and Wallace (1991) and Chang (1993), in terms of their structure, magnitude, and traveling speed. To sum up, the large O 3 events caused by deep intrusions are associated with eastward propagating baroclinic systems tied closely to the North Pacific storm tracks, with enhanced wave amplitudes and descents over the jet exit region near the western U.S.

Conclusions and discussions
Our study has examined high surface O 3 events associated with downward transport from the stratosphere over the U.S. during summertime, when high temperature could further increase the impact of O 3 pollution on mortality. By analyzing a twenty-year (1995-2014) simulation by CESM2 (WACCM6), we have found that the stratospheric O 3 can explain as high as 54% of surface O 3 variability when surface O 3 exceeds 95th percentile, and the regional averaged stratospheric contribution is~18% over the western U.S. We have further analyzed the circumstances when stratospheric intrusions of ozone covary with surface O 3 anomalies over the region of 20 • -50 • N and 70 • -140 • W on daily basis using MCA. The first two leading modes explain 60% of the total covariance pattern. In Mode 1, deep stratospheric intrusions occur preferentially over the northwestern U.S. The associated intensified northeasterly wind anomalies over the west coast further brings continental O 3 S towards the Pacific Ocean. In Mode 2, deep stratospheric intrusions occur over the Intermountain West. The composited O 3 S values reaching the surface associated with these two SVD modes range from 10 to 18 nmol/mol. In regional average over the western U.S., the stratospheric contribution is 11 nmol/mol. Both modes are results of eastward propagating wave trains, originating from the central North Pacific and amplifying near the jet exit, with enhanced subsidence over the western U.S. We have also repeated our MCA analysis to demonstrate the robustness of the dynamical mechanism across seasons. The first leading mode and corresponding anomalous ω evolution prior to intrusion events for winter, spring, and fall seasons are summarized in figures S5-7. The first leading modes during each of the seasons explain~40% of covariance between O 3 and O 3 S. Similarly, we find that the strongest disturbance extends from the central North Pacific to the west coast of the U.S. during day −6 to day −4. During day −2 to day 0, the descents strengthen remarkably over the west coast while new ascending centers develop on the downstream side. The downward O 3 transport is typically regulated by upper-level jet streams across seasons, consistent with previous studies (Langford 1999, Lin et al 2015, Albers et al 2018.
Overall, our study has demonstrated that summertime stratospheric intrusions, though infrequent, can contribute crucially to surface O 3 extremes over the Western U.S. These stratospheric intrusion events are caused by strong subsidence in the region, which is a result of eastward propagating baroclinic waves originating from the central North Pacific Ocean. However, a few caveats have to be noted. WACCM6 overestimates tropospheric O 3 and thus the exact contribution of stratospheric intrusion to surface should be treated with caution. Additionally, our diagnostic is based on one single climate model because of the use of daily O 3 S output. It's worth performing similar analyses in other chemistry climate models to assess the robustness of the conclusions. Future work will also be devoted to 1. studying simulations with high spatiotemporal resolution at certain hot spot areas so that our results can directly benefit air pollution management, and 2. understanding the origination of upstream baroclinic disturbance so that we can improve the predictability of such O 3 extremes associated with summertime stratospheric intrusions.