Preseason Tropical Cyclones and Springtime Environmental Changes in the North Atlantic (1980–2023)

Abstract

Abstract

ABSTRACT This study examines the atmospheric and oceanic conditions associated with preseason North Atlantic tropical cyclones (TCs)—those forming in April and May prior to the official June 1 season start—over the period 1980–2023. Using HURDAT2 track data and ERA5 reanalysis, we analyse sea surface temperature (SST), mid‐tropospheric relative humidity (RH at 600 hPa), vertical wind shear (VWS), low‐level relative vorticity (RV), and two integrated diagnostics—Potential Intensity (PI) and the Genesis Potential Index (GPI). A total of 12 preseason TCs were identified, all reaching tropical storm intensity but none achieving hurricane strength. A statistically significant trend toward earlier tropical storm onset was detected at −12.3 days per decade ( p < 0.01). Ten of the twelve systems developed from non‐tropical or hybrid precursors—stalled frontal boundaries, upper‐level troughs, or extratropical cyclones undergoing tropical transition—rather than the classical tropical‐wave pathway, distinguishing preseason cyclogenesis fundamentally from peak‐season activity. Composite anomaly analysis reveals that preseason TC formation was associated with modest but significant positive SST and PI anomalies in the western Atlantic, locally elevated RH near Cuba, reduced VWS in parts of the eastern Atlantic, and enhanced low‐level cyclonic vorticity near the Straits of Florida—a region that concentrates the majority of the preseason events. Springtime trend analysis identifies statistically significant SST warming (up to +0.4°C per decade) in the Gulf of Mexico and along the Gulf Stream corridor, RH increases in parts of the western basin, and notably, a significant VWS reduction over the Florida Peninsula (exceeding −2.0 m/s per decade)—a combination that may progressively lower barriers to early‐season cyclogenesis. Positive PI trends exceeding +3 m/s per decade along the Gulf Stream axis reinforce this picture, while GPI positive trends, though spatially more limited, are consistent with an expanding environmental window for preseason activity. Together, these findings suggest that springtime conditions in the western North Atlantic are becoming incrementally more favourable for preseason TC formation, with the Florida Peninsula region emerging as a particularly sensitive hotspot warranting enhanced early‐season monitoring.