Dynamics of ENSO-driven stratosphere-to-troposphere transport of ozone over North America

Abstract

Abstract. The El Niño–Southern Oscillation (ENSO) is known to modulate the strength and frequency of stratosphere-to-troposphere transport (STT) of ozone over the Pacific–North American region during late winter to early summer. Dynamical processes that have been proposed to account for this variability include variations in the amount of ozone in the lowermost stratosphere that is available for STT and tropospheric circulation-related variations in the frequency and geographic distribution of individual STT events. Here we use a large ensemble of Whole Atmosphere Community Climate Model (WACCM) simulations (forced by sea-surface temperature (SST) boundary conditions consistent with each phase of ENSO) to show that variability in lower-stratospheric ozone and shifts in the Pacific tropospheric jet constructively contribute to the amount of STT of ozone in the North American region during both ENSO phases. In terms of stratospheric variability, ENSO drives ozone anomalies resembling the Pacific–North American teleconnection pattern that span much of the lower stratosphere below 50 hPa. These ozone anomalies, which dominate over other ENSO-driven changes in the Brewer–Dobson circulation (including changes due to both the stratospheric residual circulation and quasi-isentropic mixing), strongly modulate the amount of ozone available for STT transport. As a result, during late winter (February–March), the stratospheric ozone response to the teleconnections constructively reinforces anomalous ENSO-jet-driven STT of ozone. However, as ENSO forcing weakens as spring progresses into summer (April–June), the direct effects of the ENSO-jet-driven STT transport weaken. Nevertheless, the residual impacts of the teleconnections on the amount of ozone in the lower stratosphere persist, and these anomalies in turn continue to cause anomalous STT of ozone. These results should prove helpful for interpreting the utility of ENSO as a subseasonal predictor of both free-tropospheric ozone and the probability of stratospheric ozone intrusion events that may cause exceedances in surface air quality standards.

Enhancement of Arctic surface ozone during the 2020–2021 winter associated with the sudden stratospheric warming

“…The weakened polar vortex extended from the stratosphere to the troposphere (figures S2(e)-(h)), which is consistent with Baldwin and Dunkerton (2001). Previous studies have shown that geopotential height and ozone anomalies should be anticorrelated for medium-scale waves due to vertical motion and horizontal advection (Schoeberl andKrueger 1983, Salby andCallaghan 1993), which can further influence the stratosphere-to-troposphere transport (Albers et al 2022). Thus, the increase in ground-level ozone in North Eurasia and western North America may be related to the 2021 SSW of the vortex displacement type.…”

“…The model domain extends from the Earth's surface to the lower thermosphere (∼140 km or 5.96 × 10 −6 hPa). WACCM has been widely used in previous studies on the stratosphere-to-troposphere transport of ozone (Hess et al 2015, Yang et al 2016, Abalos et al 2020, Albers et al 2022.…”

The Evolutions and Large‐Scale Mechanisms of Summer Stratospheric Ozone Intrusion Across Global Hotspots

“…For example, a strong intraseasonal variability exists in the MD hotspot, with no events in August in this model simulation. Also, the interannual variability of the summer intrusion at the NA hotspot does not show a connection to ENSO (not shown), whereas the spring deep STT increases during La Ninas (Albers et al., 2022; Lin et al., 2015).…”

“…Our study differs from previous works in that we identified hotspots of stratospheric intrusion with the frequency of extreme ozone events, which is directly relevant to the air quality. Most previous studies focus on a monthly or seasonal average amount of mass or ozone transported from the stratosphere to the troposphere (e.g., Albers et al., 2022; Škerlak et al., 2014; Sprenger & Wernli, 2003). Anomalously high amounts of ozone intrusion on a monthly scale are commonly associated with frequent extreme events.…”

“…However, once O 3 S passes the tropopause and enters the troposphere, O 3 S does not have production routes and is removed at the same rate as ozone. Therefore, tracking the stratospheric contribution throughout time and space is very useful through O 3 S (Akritidis et al., 2016, 2022; Albers et al., 2022; Bartusek et al., 2023; Elsbury et al., 2023; Lin et al., 2012; Zanis et al., 2014). It also allows us to cover intrusion events regardless of their triggering mechanisms.…”