Tropical Cyclone Genesis Potential Using a Ventilation-Reduced Potential Intensity

Genesis potential indices (GPIs) are widely used to understand the climatology of tropical cyclones. However, the sign of future changes in commonly-used GPIs is known to depend on the choice of relative humidity versus saturation deficit for the moisture parameter. Recent theory combines potential intensity and mid-tropospheric moisture into a single quantity called the ventilation-reduced potential intensity, which removes this ambiguity and simplifies the thermodynamic component. This work proposes a new GPI that depends on a single parameter, given by the product of the ventilation-reduced potential intensity (thermodynamic component) and the absolute vorticity (dynamic component), raised to approximately the 5th power when fit to data from IBTraCS and ERA5 reanalysis data. When the two components are fit with independent exponents their best-fit values are nearly identical, indicating that their product likely constitutes a single joint parameter. The new GPI performs comparably well to existing indices in reproducing the climatological distribution of tropical cyclone activity and its covariability with ENSO, while only requiring a single fitting exponent. When applied to CMIP6 projections, the GPI predicts that environments globally will become gradually more favorable for TC activity and genesis with warming, though significant changes emerge only under relatively strong warming. Results are nearly identical for a Poisson (exponential) model as for the power law, which reflects the mathematical similarity of the two common formulations. This GPI helps resolve the debate over the treatment of the moisture term and its implication for how TC genesis may change with warming, though it does not resolve the question of future changes in genesis itself. Nonetheless, the joint parameter in the GPI carries a clearer physical interpretation that may offer a step forward towards a more concrete theory for genesis across climate states.