Strengthening of favorable environments for North Atlantic tropical cyclogenesis in midlatitudes in a warmer climate

Abstract Anthropogenic climate change is intensifying tropical cyclones, and some studies suggest that they are now impacting regions farther from the equator, though uncertainties remain. This study examines the North Atlantic (NATL) basin’s autumn climatology, focusing on environments conducive to tropical transitions (TTs), as most cyclones affecting Europe that originate from TTs occur during this season. Ten CMIP6 climate models under the historical, SSP2-4.5 and SSP5-8.5 scenarios are used, covering the 1981–2100 period, with the ERA5 reanalysis employed as a reference to support the results. The study introduces the Tropical Transition Favorability Index (TTFI), which is a novel metric that integrates key parameters to quantify environmental favorability for TTs in the NATL. Findings indicate a progressive tropicalization of the NATL basin under both SSP2-4.5 and SSP5-8.5, with a more pronounced effect under the latter, driven by increased sea surface temperatures and humidity, while dynamic constraints weaken. Although in some cases the magnitude of projected future changes is comparable to existing CMIP6 models biases with respect to ERA5, the results suggest a higher likelihood of TTs, increasing the risk from these destructive systems.