Changes in the Length of the Season with Favorable Environmental Conditions for Tropical Cyclones in the North Atlantic Basin during the Last 40 Years

Analyses of two high-resolution reanalysis products show that high values of hurricane potential intensity (PI) are becoming more frequent and covering a larger area of the Atlantic, which is consistent with the lengthening of the tropical cyclone season previously reported. These changes are especially pronounced during the early months of the storm season (May-July) in subtropical latitudes. The western subtropical Atlantic features increases in mean PI as well as the areal coverage and frequency of high PI throughout the storm season; the length of the season with high PI has grown since 1980. The number of days with low vertical wind shear increases in the tropical North Atlantic during the early and middle months of the storm season, but trends are mixed and generally insignificant elsewhere. A thermodynamic parameter measuring the ratio of midlevel entropy deficits to the strength of surface fluxes that work to eliminate them is sensitive to the choice of the pressure level(s) used to calculate its value in the boundary layer, as well as to subtle differences in temperature and humidity values near the surface in different reanalysis datasets, leading to divergent results in metrics like the ventilation index that depend on its value. Projections from a high-resolution simulation of the remainder of the twenty-first century show that the number of days with high PI is likely to continue increasing in the North Atlantic basin, with trends especially strong in the western subtropical Atlantic during the early and late months of the season.

Automated summary

Analyses of two high-resolution reanalysis products indicate that high values of hurricane potential intensity (PI) are becoming more frequent and covering a larger area of the Atlantic, consistent with the previously reported lengthening of the tropical cyclone season. Specifically, the increase in high PI is most pronounced in the early months of the storm season in subtropical latitudes. The western subtropical Atlantic experiences an increase in mean PI, as well as the areal coverage and frequency of high PI throughout the storm season, with the length of the season with high PI growing since 1980. While the number of days with low vertical wind shear increases in the tropical North Atlantic during the early and middle months of the storm season, trends elsewhere are mixed and generally insignificant. The choice of pressure level(s) used to calculate a thermodynamic parameter, which measures the ratio of midlevel entropy deficits to the strength of surface fluxes, as well as subtle differences in temperature and humidity values near the surface in different reanalysis datasets, can lead to divergent results in metrics like the ventilation index that rely on its value. Projections from a high-resolution simulation for the rest of the twenty-first century suggest that the number of days with high PI is likely to continue increasing in the North Atlantic basin, particularly in the western subtropical Atlantic during the early and late months of the season.