Arabian Sea tropical cyclones intensified by emissions of black carbon and other aerosols

Throughout the year, average sea surface temperatures in the Arabian Sea are warm enough to support the development of tropical cyclones, but the atmospheric monsoon circulation and associated strong vertical wind shear limits cyclone development and intensification, only permitting a pre- and post-monsoon period for cyclogenesis. Thus a recent increase in the intensity of Northern Indian Ocean tropical cyclones is thought to be related to the weakening of the climatological vertical wind shear. At the same time, anthropogenic emissions of aerosols have increased 6-fold since the 1930s, leading to a weakening of the southwesterly lower-level and easterly upper-level winds that define the monsoonal circulation over the Arabian Sea. In principle, this aerosol-driven circulation modification could affect Arabian Sea tropical cyclone intensity, but to date, no such linkage has been shown. Here we report an increase in the intensity of pre-monsoon Arabian Sea tropical cyclones over the past 30 years, and demonstrate that this change in storm strength is a consequence of a simultaneous upward trend in anthropogenic black carbon and sulfate emissions. We use a combination of observational, reanalysis, and model data to demonstrate that the anomalous circulation, which is radiatively forced by these anthropogenic aerosols, reduces the basin-wide vertical wind shear, creating an environment more favorable for tropical cyclone intensification. As most Arabian Sea tropical cyclones make landfall our results suggest an additional human health impact from regional air pollution.