Tropical Cyclone Seed Disturbances in ERA5

Abstract Tropical cyclone (TC) seed disturbances are moist synoptic-scale cyclonic circulation features that can potentially develop into TCs. This study tracks TC seed disturbances in the fifth major global reanalysis produced by the European Centre for Medium-Range Weather Forecasts for the period of 1980–2023 and investigates the characteristics of the tracked disturbances. Using TempestExtremes, the tracking algorithm finds spatiotemporally connected 850-hPa geopotential local minima that satisfy a set of gradient wind, relative vorticity, and relative humidity conditions. Over the 44 years, we found 43 777 seed disturbances with an average survival rate (fraction that becomes a TC) of about 6.5%. The geographical distribution of the genesis and track density of the TC seed disturbances resembles that of TCs, as they predominantly develop in regions characterized by high sea surface temperature (SST), midlevel relative humidity and precipitation, and low vertical wind shear. Among the large-scale environmental variables, relative humidity and SST best explain the interbasin difference in the mean TC seed frequency and survival rate, respectively. Among TC seed disturbances that develop into TCs, those with strong precipitation tend to develop into TCs more rapidly. A comparison to other seed disturbance datasets reveals that the overall number of seed disturbances, hence the survival rate, is highly sensitive to the variables used to define TC seeds, as well as the thresholds considered in the tracking algorithms.