What We Can Learn in the Wake of IMO 2020: Aerosols, Clouds, and Convection over Shiptracks

The IMO (International Maritime Organization) instituted new regulations in 2020 limiting fuels to lower maximum sulfur concentrations. This regulation has resulted in dramatically decreased emissions of many atmospheric pollutants from ship traffic, including carbonaceous particles in addition to sulfur. While several studies have linked this change to a global decrease in shiptrack-induced cooling, the local implications of these regulations on aerosol-cloud interactions over major shipping lanes are less certain.

This study investigates impacts of the abrupt and sustained perturbation to aerosol emissions provided by the IMO regulations in this same region on cloud properties and markers of deep convection. For instance, Thornton et al (2017) noted a clear signal of increasing lightning flash rate over two major shipping lanes in the Indian Ocean and the East China Sea, citing convective invigoration by aerosols as a potential contributing mechanism. In principle, the reduction in aerosols should lead to a decrease in observed lightning flash rate over the same shiptracks relative to the background. In practice, it is unclear whether these spatiotemporal signals are directly attributable to convective invigoration by aerosols, or to more mundane markers of aerosol-cloud interactions such as a larger quantity of ice particles. We investigate the possibility of both explanations by correlating lightning flash rate with aerosol loading and other markers of convection to determine whether this natural experiment can provide new insights about the roles of ice microphysics and aerosol invigoration in deep convection.