Projecting Future Changes in Extratropical Transition of Atlantic Hurricanes in Earth System Models

Tropical cyclones (TCs) often undergo extratropical transition (ET) when they enter the midlatitudes. During this process, storms lose their warm core and develop a frontal structure as they transform into extratropical cyclones (ExTCs). This study explores changes in the climatology of North Atlantic TCs that undergo ET using a free-running, high-resolution model simulation generated by the Community Atmosphere Model version 5 (CAM5). We contrast the historical simulation to two future climate scenarios, Representative Concentration Pathways (RCP) 4.5 and 8.5. The findings indicate that in a warming climate, the ET completion rate is expected to marginally increase. The track density of storms in tropical and transitioning phases will shift poleward, but the track density of extratropical storms will shift equatorward instead. The cyclone phase space distribution analysis reveals shifts toward more asymmetric structures and weaker warm cores, indicating the potential for increasing frequency of hybrid storms, either as separate entities or as part of the TC-ExTC continuum. Among storms that undergo ET, their spatial distribution is projected to shift eastward, likely driven by simulated changes in Eady Growth Rate patterns, which suggest a decrease in baroclinicity in the western Atlantic alongside an increase in the eastern Atlantic. During all phases of ET, storm composites tend to show wetter storms, and composites of near-surface wind fields reveal notably large broadening in the post-ET winds in the southeastern quadrant, suggesting potential hazard increases over the European continent from cyclones of tropical origins.