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 Nino-Southern Oscillation (ENSO) and the Atlantic Meridional Mode (AMM). La Nina and positive AMM are associated with the bottom percentiles of both TC metrics, and vice versa for El Nino and negative AMM. The ENSO Longitude Index explains variability in annual global named storm days and ACE as well as the Nino 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 Nino-Southern Oscillation (ENSO). La Nina, 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 Nino. 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 Nino 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 Nino 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.

Automated summary

This study investigates the link between global TC activity and ocean variability, finding significant correlations with ENSO and AMM. La Nina and positive AMM are associated with the low percentiles of TC activity, while El Nino and negative AMM show the opposite. This research reveals the importance of reliable ENSO projections for understanding global TC frequency changes.