Abstract: Advancing knowledge on the role of internal climate variability on surface ozone has implications not only for interpreting observed ozone trends over the past few decades but also for understanding uncertainties in projecting the response of surface ozone extremes to global climate change. Climate variability is manifested in changes in the occurrence of atmospheric circulation patterns, which can vary strongly from year to year, but can show trends over multi-decadal periods too. In this talk, I will discuss how interannual to decadal climate variability modulates frequency of deep stratospheric ozone intrusions reaching surface air, hemispheric pollution transport, regional heat waves and their impacts on observed surface ozone trends and extremes at northern mid-latitudes over the past half century (1960-2014). The results are based on an integrated analysis of available long-term ozone observation records and a suite of chemistry-climate model hindcasts designed to isolate the response of ozone to changes in human-induced emissions of non-methane ozone precursors, methane, wildfires, and meteorology.