A case study to investigate the role of aerosols reduction on the East Asian summer monsoon seasonal prediction

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). During the spring 2020, when restrictions to contain the spread of the coronavirus were implemented worldwide, reduced emissions of gases and aerosols were detected and found to be quite extended 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 related restrictions 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 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. The results indicate that in 2020 a more realistic representation of the atmospheric forcing in the spring preceding the core monsoon season improves the skill of the predicted summer precipitation, mostly over East Asia. Beyond the testbed considered in this analysis, the result helps improving the understanding of the processes at work over the Asian monsoons regions, with positive implications on the usefulness of seasonal predictions products.