Arabian Sea tropical cyclones have become stronger over the past 30 years owing to a reduction in vertical wind shear (VWS) brought about by radiative forcing from pollution aerosols1. Wang et al.2 argue that the decline in VWS results from a systematic shift in storm genesis date, which may be part of a natural cycle or another consequence of regional pollution. However, their conclusions2, although interesting, are not supported by our analysis and are probably sensitive to biases in the observational record.

Throughout the year, average sea surface temperatures in the Arabian Sea are warm enough to support the development of tropical cyclones1, but the atmospheric monsoon circulation and associated strong vertical wind shear limits cyclone development and intensification, only permitting a pre-monsoon and post-monsoon period for cyclogenesis. Thus a recent increase in the intensity of tropical cyclones over the northern Indian Ocean5 is thought to be related to the weakening of the climatological vertical wind shear.