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Abstract A catastrophic heatwave struck North America (NA) in the summer of 2021, the underlying cause of which currently remains unclear. The reanalysis data (1980–2021) is analyzed to elucidate the mechanism modulating the summer heatwaves. We find the heatwaves over western NA tend to occur concurrently with quasi-barotropic ridges (QBTRs). The 2021 record-breaking heatwave, in particular, coincides with an extended eight-day QBTR event. The frequency of QBTRs is modulated by large-scale forcing. During the period of 1980–2000, it is correlated with the Arctic Oscillation. After 2000, however, the QBTR frequency is highly associated with sea ice variations. Specifically, the negative sea ice anomalies in the Chukchi Sea are usually associated with stronger net surface shortwave radiation and low cloud cover, triggering upward motion and a low-pressure center in the low- and mid-troposphere. The low pressure strengthens a stationary wave response, concomitant with two alternately high- and low-pressure centers, inducing more frequent QBTRs over western NA. These findings indicate that further Arctic sea ice loss under a warming climate will likely lead to more devastating heatwaves over western NA. 

China has been experiencing severe ozone pollution problems in recent years. While a number of studies have focused on the ozone-pollution-prone regions such as the North China Plain, Yangtze River Delta, and Pearl River Delta regions, few studies have investigated the mechanisms modulating the interannual variability of ozone concentrations in Shandong Province, where a large population is located and is often subject to ozone pollution. By utilizing both the reanalysis dataset and regional numerical model (WRF-CMAQ), we delve into the potential governing mechanisms of ozone pollution in Shandong Province—especially over the major port city of Qingdao—during summer 2014–2019. During this period, ozone pollution in Qingdao exceeded the tier II standard of the Chinese National Ambient Air Quality (GB 3095-2012) for 75 days. From the perspective of meteorology, the high-pressure ridge over Baikal Lake and to its northeast, which leads to a relatively low humidity and sufficient sunlight, is the most critical weather system inducing high-ozone events in Qingdao. In terms of emissions, biogenic emissions contribute to ozone enhancement close to 10 ppb in the west and north of Shandong Province. Numerical experiments show that the local impact of biogenic emissions on ozone production in Shandong Province is relatively small, whereas biogenic emissions on the southern flank of Shandong Province enhance ozone production and further transport northeastward, resulting in an increase in ozone concentrations over Shandong Province. For the port city of Qingdao, ship emissions increase ozone concentrations when sea breezes (easterlies) prevail over Qingdao, with the 95th percentile reaching 8.7 ppb. The findings in this study have important implications for future ozone pollution in Shandong Province, as well as the northern and coastal areas in China.