Climate response to COVID-19 emission reductions: A testbed to understand atmospheric circulation adjustments to aerosol forcing 

<p>Many nations declared national lockdowns to limit the spread of COVID-19. This resulted in a dramatic, albeit temporary, reduction of many aerosol precursors and greenhouse gas emissions mainly from transport and industry. We analyse the output of various Earth system models participating in CovidMIP to investigate the impact of COVID-related emission reductions on the near-term climate across the Northern Hemisphere. A key finding is that surface anomalies are associated with a hemispheric-wide upper-tropospheric circulation response, featuring a wave-like pattern in the mid-latitudes and Arctic oscillation-type anomalies in the high latitudes during winter and spring. This is accompanied by changes in the vertical atmospheric temperature structure and consequent meridional shift of the NH jet. These anomalous patterns reverse between 2020 and 2021 and are shown to be consistent with the underlying changes in aerosol emissions. Our findings provide additional evidence to the important role of atmospheric circulation adjustments in realising the aerosol impact, which has significant implications for diagnosing the interplay between anthropogenic forcing and internal variability in the context of climate change. &#160;</p>