All‐Season Analysis of Extratropical and Arctic Cyclones Over the Northern Hemisphere Oceans During 1940–2024

Abstract Extratropical and Arctic cyclones regulate mid‐to‐high‐latitude climate through the transport of heat, moisture, and momentum; however, their long‐term behavior during spring and autumn seasons remains poorly understood. Using an 85‐year (1940–2024) cyclone climatology derived from ERA5 reanalysis, we identified substantial seasonal and basin‐specific shifts in Northern Hemisphere marine cyclone activity. Most notably, cyclone occurrence and intensity have significantly changed over the central Arctic Ocean, with winter storms becoming fewer but markedly deeper, and summer storms more numerous. Arctic cyclones in spring, previously under‐investigated, have intensified (−0.31 hPa decade −1 ) and exhibit lengthening lifespans (+0.02 days decade −1 ) and trajectories (+29.44 km decade −1 ). In the North Atlantic, cyclone paths have lengthened in winter, genesis has risen (+0.73 decade −1 ) in spring, and storm lifespan has lengthened (+0.03 days decade −1 ) in autumn. In the North Pacific, tracks also have lengthened in winter, while autumn storms have become larger (+10.37 km decade −1 ), longer lived (+0.03 days decade −1 ), and farther traveled (+30.35 km decade −1 ). These observed cyclone changes are closely tied to the coupled responses from surface to upper‐tropospheric levels. Pronounced sea ice loss coincides with enhanced lower‐tropospheric baroclinicity (increased Eady growth rate) and a weaker upper‐tropospheric potential vorticity gradient. Together, these adjustments favor stronger, deeper cyclones and facilitate their meridional propagation. Our results demonstrate that long‐term climate change is reshaping Northern Hemisphere cyclone characteristics far beyond traditionally emphasized seasons. These findings highlight an urgent need for seasonal climate forecasts and long‐term projections to give spring and autumn cyclones equal consideration alongside winter and summer extremes.