Tropical cyclone genesis potential using a ventilation-reduced potential intensity

Tropical cyclone genesis potential indices combine thermodynamic and dynamic quantities into a single proxy that is used to understand climatological spatiotemporal variations in tropical cyclogenesis on Earth. Such indices typically include the low-level absolute vorticity for the dynamical component, and the maximum potential intensity and a measure of mid-tropospheric moisture for the thermodynamic component. However, climate dependencies, particularly the sign of future changes in tropical cyclone activity with warming, are known to depend on the choice of relative humidity vs saturation deficit for the moisture term. Recent theory demonstrated that potential intensity and mid-tropospheric moisture may be combined into a single quantity called the ventilation-reduced potential intensity, which simplifies the thermodynamic formulation to a single term and eliminates the uncertain formulation of the moisture term. This work proposes a new genesis potential index that depends on a single parameter -- the product of this ventilation-reduced potential intensity and the absolute vorticity -- and demonstrates how this new, simpler index performs comparably well to existing indices in reproducing the climatological distribution of tropical cyclone activity and its covariability with ENSO. Finally, the new index is applied to CMIP6 simulations to yield projections for changes in tropical cyclone favorability across all global basins. As this approach is fully rooted in existing theory, it may help resolve ongoing debate over the treatment of the moisture term and its implications for expected changes in tropical cyclone activity with warming.