On the Definition and Tracking of Tropical Cyclone Seeds from a Climate Perspective Bulletin of the American Meteorological Society

Future projections of tropical cyclone (TC) frequency in a changing climate remain uncertain despite substantial advances in climate models (e.g., Knutson et al. 2020). While most climate models project a decrease in global TC frequency through the twenty-first century, several models project an increase in global TC frequency or no significant change (Bhatia et al. 2018; Vecchi et al. 2019). Furthermore, the sign of TC frequency changes in some statistical–dynamical downscaling projections depends on the design of the statistical model, particularly on the choice of humidity variables and the formulation of humidity within the statistical framework (e.g., Lee et al. 2018, 2020), although other models do not show such sensitivity (e.g., Emanuel et al. 2008; Emanuel 2013). As part of a framework to understand the controls of TC frequency in present and future climates, the concept of “TC seeds” has received increasing attention in recent years in the climate community (Vecchi et al. 2019; Hsieh et al. 2020, 2022; Emanuel 2022). TC seeds is a broadly used term to describe the disturbances that precede tropical cyclogenesis. However, the lack of a standardized physics-based definition of TC seeds hinders research into understanding whether TC frequency is mainly controlled by large-scale environmental conditions alone or by both large-scale environmental conditions and the statistics of TC seeds (e.g., Sobel et al. 2021; Emanuel 2022). In this short note, we propose a physics-based definition of TC seeds to inform the development of TC seed tracking methodologies and facilitate research on TC frequency.