• Fall: Enhanced Eurasian airflow augments O3

Major changes in observed baseline tropospheric ozone over northern mid-latitudes:- An overall increase over the past 20-30 years1-4.- A tendency for ozone pollution season to begin earlier in spring5- A pattern dominated by slow or no growth in the 2000s2,4 However, the rate of ozone increases is highly uncertain (due to large interannual variability, short record, and sparse in situ sampling). Unraveling the drivers of historical ozone variability and trends necessitates process-level understanding on daily to multi-decadal time scales: Crucial for improving seasonal forecasts and designing ozone abatement strategies; Provide insights into how tropospheric ozone will respond to a warming climate. 2. The GFDL AM3 Model Hindcasts (in the CCMI framework) 5. Interannual variability of Western U.S. O34. Model evaluations of U.S. O3 trends • Increase in fall for MLO while in spring for NH mid-latitudes • Spring: Weakened Eurasian airflow offsets O3 increases from rising Asian emission