On the role of climate variability on tropospheric ozone

Abstract:

The response of tropospheric ozone to changing atmospheric circulation is poorly understood owing to a lack of reliable long-term observations. There is great current interest in quantifying the extent to which observed ozone trends over recent decades at northern mid-latitude sites are driven by changes in precursor emissions versus shifts in atmospheric circulation patterns. In this talk, I present a detailed analysis of the impact of interannual to decadal climate variability on tropospheric ozone, based on observations and a suite of chemistry-climate model hindcast simulations. Decadal shifts in circulation regimes modulate long-range transport of Asian pollution, leading to very different seasonal ozone trends at Mauna Loa Observatory in the subtropical Pacific Ocean. During autumn, the flow of ozone-rich air from Eurasia towards Hawaii strengthened in the mid-1990s onwards, as a result of the positive phase of the Pacific North American pattern, increasing ozone at Mauna Loa. During spring, weakening airflow from Asia in the 2000s, tied to La-Niña-like decadal cooling in the equatorial Pacific Ocean, offsets ozone increases at Mauna Loa that otherwise would have occurred due to rising Asian emissions. The circulation-driven variability in Asian pollution over the subtropical North Pacific regions manifests mainly as changes in the mean as opposed to in transport events. At high-elevation Western U.S. sites, intrusions of stratospheric ozone deep into the troposphere during spring exert a greater influence than Asian pollution, particularly on the high tail of observed surface ozone distribution. We show that year-to-year variability in springtime high-ozone episodes measured in Western U.S. surface air is tied to known modes of climate variability, which modulate meanders in the polar frontal jet conducive to deep stratospheric ozone intrusions. Specifically, the La Niña-related increase in the frequency of deep stratospheric intrusion events plays a greater role on western U.S. surface ozone variability than the El Niño-related increase in lower stratospheric ozone burden. Finally, I will discuss the implications of these findings for detecting and attributing long-term ozone trends in free tropospheric and surface air.