Effects of aerosols reduction on the Asian summer monsoon prediction: the case of summer 2020

Northern Hemisphere anthropogenic aerosols influence Southeast and East Asian summer monsoon precipitation. In the late 20^th century, both the East Asian and the South Asian summer monsoons weakened because of increased emissions of anthropogenic aerosols over Asia, counteracting the warming effect of increased greenhouse gases (GHGs). Changes in the anthropogenic aerosols burden in the Northern Hemisphere, and specifically over the Asian continent, may also have affected the sub-seasonal evolution of the summer monsoon. During the spring 2020, when restrictions to contain the spread of the coronavirus were implemented worldwide, reduced emissions of gases and aerosols were detected also over Asia.

Following on from the above and using the EC-Earth3 coupled model, a case-study forecast for summer 2020 (May 1st start date) has been designed and produced with and without the reduced atmospheric forcing due to covid-19 in the SSP2-4.5 baseline scenario, as estimated and adopted within CMIP6 DAMIP covidMIP experiments (hereinafter “covid-19 forcing”). The forecast ensembles (sensitivity and control experiments, meaning with and without covid-19 forcing) consist of 60 members each to better account for the internal variability (noise) and to maximize the capability to identify the effects of the reduced emissions.

The analysis focuses on  the effects of the covid-19 forcing, in particular the reduction of anthropogenic aerosols, on the forecasted evolution of the monsoon, with a specific focus on the performance in predicting the summer precipitation over India and over other parts of  South and East Asia. Changes in the performance of the prediction for specific aspects of the monsoon, like the onset and the length of the season, are evaluated as well.