IMO 2020 and its Impact on Cloud Fraction and Liquid Water Path Adjustments Over the Southeast Atlantic Shipping Corridor

Aerosol cloud interactions (ACI) are a source of high uncertainty when quantifying historical radiative forcing. ACI have a cooling effect that counteracts the warming from the greenhouse effect through both direct and indirect aerosol effects. Aerosols can reflect sunlight due to scattering (direct aerosol effect), and influence cloud formation, impacting the amount of shortwave radiation being reflected and therefore the solar radiation budget (indirect aerosol effect). One way to constrain information on ACI is by using opportunistic experiments, such as ship tracks and shipping corridors as they offer means of detecting clear aerosol perturbations that are not dependent on meteorological or other sources of variability. Cloud sensitivity to aerosol perturbations from shipping fuel aerosols can be measured using remote sensing, which can be used to assess the potential of using salt tracks as a method of marine cloud brightening. In 2020, the International Maritime Organization set a regulation on the sulfur content in shipping fuels, reducing the allowable amount from 3.5% to 0.5%. This change is expected to result in particles that will act as less efficient cloud condensation nuclei. Here we compare observed MODIS Terra and Aqua satellite measurements of total cloud fraction and liquid water path with a counterfactual scenario where there is an expected absence of shipping in the shipping corridor over the southeast Atlantic. Mean climatology values from 2003 to 2019 show a positive cloud fraction anomaly along with a negative liquid water path anomaly that can be explained by secondary indirect effects of aerosols on cloud adjustments such as increased entrainment and drizzle suppression. We expect to find smaller increases in cloud fraction and decreases in liquid water path due to shipping aerosol within the corridor in the post-IMO regulation results as compared with 2003 to 2019 climatology.