Synoptic-scale vortices such as tropical cyclones and monsoon depressions produce some of the most destructive tropical weather, due to their extreme precipitation and intense winds. These vortices typically emerge from weaker pre-existing disturbances that are often called seeds, and the availability of these precursor seeds is a key factor controlling the occurrence frequency of the stronger cyclonic vortices. In this study, we demonstrate that the El Niño–Southern Oscillation (ENSO) is the dominant mode of inter-annual variability in precursor seed frequency, and further investigate its impact across various basins. To accomplish this, we developed a new dataset of precursor seeds using ERA5 reanalysis data. A distinguishing feature of our approach is tracking seeds using the streamfunction of low-level horizontal winds, as it has been shown to be highly effective in tracking at least some types of weaker vortices.

Our findings show that ENSO affects synoptic-scale vortices in multiple regions by altering the availability of precursor seeds. Here, we focus on the influence of ENSO on seed genesis in two basins: the eastern North Pacific and the North Indian Ocean, because of several novel results. First, in the eastern North Pacific, we find an increase in the number of tropical cyclones despite a reduction in seed frequency during El Niño. Second, in the North Indian Ocean, changes during ENSO in the frequency of synoptic-scale vortices closely mirror variations in precursor seeds, but the peak intensity of the vortices changes with ENSO phase. Specifically, during El Niño, the frequency of stronger vortices (known as monsoon depressions) increases near the climatological main development region of these storms, while the frequency of weaker vortices (known as monsoon lows) decrease across most of South Asia. We use genesis indices to help understand these ENSO-associated changes in relation to the variations in large-scale environmental factors. Overall, this work highlights the crucial role played by variations in precursor seed frequency and the importance of large-scale environmental factors governing the development of those seeds into cyclonic disturbances.