Tropical Oceanic Influences on Observed Global Tropical Cyclone Frequency

The global tropical cyclone (TC) number has historically been relatively constant from year‐to‐year, however, the reason remains unknown. Furthermore, climate projections are inconclusive regarding future global TC frequency changes. Here, we investigated years in which observed global TC activity deviated from the mean and potential links to ocean drivers from 1980 to 2021. We found that the annual global number of named storm days and accumulated cyclone energy (ACE) were significantly linked with El Niño–Southern Oscillation (ENSO) and the Atlantic Meridional Mode (AMM). La Niña and positive AMM are associated with the bottom percentiles of both TC metrics, and vice versa for El Niño and negative AMM. The ENSO Longitude Index explains variability in annual global named storm days and ACE as well as the Niño 3.4 index. This research reveals that reliable future projections of ENSO are necessary, but not sufficient, to understand future changes in global TC frequency. Plain Language Summary Although the number of global tropical cyclones (TCs) has been relatively constant from year‐to‐year in recent decades, the reason remains unknown. It is important to understand what can lead to global TC frequency variations because of its link with TC impacts. We investigated years in which observed global TC activity deviated from the 1980–2021 average. We found that global TC activity is significantly linked with ocean variability, most strongly with El Niño–Southern Oscillation (ENSO). La Niña, which is marked by cool eastern equatorial Pacific sea‐surface temperature (SST) anomalies, is associated with less global TC activity, and vice versa for El Niño. A new physically‐based index for ENSO, the ENSO Longitude Index (ELI), explains annual global named storm days and ACE as well as the SST anomaly‐based Niño 3.4 index. This is because the ELI accounts for the nonlinear response of thunderstorm activity to SST, accounts for changes in the background SST state associated with the seasonal cycle and/or climate change, and better captures ENSO's spatial diversity than Niño 3.4. This research reveals that reliable future projections of ENSO are necessary, but not sufficient, to understand whether global TC frequency may change in the future. Key Points El Niño–Southern Oscillation (ENSO) drives deviations in the relatively constant number of annual global named tropical cyclone (TC) days The ENSO Longitude Index explains annual global named TC day and accumulated cycl