Atmospheric and Oceanic Conditions Associated with Preseason Tropical Storms in the North Atlantic (1980–2023)

The period between 1 June and 30 November has been established by the National Oceanic and Atmospheric Administration (NOAA) as the operational hurricane season in the North Atlantic (NA), reflecting the environmental conditions typically conducive to tropical cyclogenesis and tropical cyclone (TC) intensification. However, historical records indicate that cyclonic activity may occasionally begin prior to the official season start. This study investigates 12 TCs at the tropical storm (TS) stage that occurred in April or May (defined here as the preseason) in the NA from 1980 to 2023 (NOAA’s HURDAT2). The analysis of this particular stage is motivated by the fact that it represents the highest intensity reached by preseason TCs during the study period. The research focuses on associated atmospheric and oceanic conditions derived from ERA5 reanalysis data available in the Copernicus Climate Data Store, including sea surface temperature (SST), 600-hPa relative humidity (RH) and vertical wind shear (VWS) between 200 and 850 hPa. Anomalies associated with preseason TSs were identified, characterized by unusually high SST (up to 1°C above the climatological mean) and mid-tropospheric RH (up to 40% above average), accompanied by significantly reduced VWS (up to 10 m/s below average). Additionally, positive seasonal trends in both mean and percentile values of SST (up to 0.4°C per decade) and RH (up to 3% per decade), along with decreasing VWS (up to −2 m/s per decade), were observed in regions where preseason TSs typically occurred. The waters northeast of the Florida Peninsula emerged as a particularly sensitive area, as half of the analysed preseason TSs occurred there. Furthermore, the Florida Peninsula and its surrounding region exhibited statistically significant trends in the examined variables, all of which are associated with the occurrence of preseason TSs. Results indicate that the environmental window for the occurrence of TCs in the NA may continue to expand, potentially increasing the likelihood of such events. These results may contribute to improving long-range preseason TC outlooks and to identifying regions potentially vulnerable to an extension of the hurricane season. Observed springtime trends may be connected with climate change (rising SST and RH) or reflect complex circulation changes (decreasing VWS in certain areas), highlighting the need for further research, particularly through climate projections assessing the persistence of these trends and the physical mechanisms underlying the observed anomalies.