Interannual variations in Tropical cyclone activity in the Central South Pacific: is ENSO the whole story?

Abstract Each austral summer, tropical cyclones (TCs) threaten South Pacific Island nations, causing potential socio-economic impacts. Located on the eastern edge of the South Pacific Convergence Zone, French Polynesia (FP) faces highly variable cyclone exposure, experiencing intense activity in some years and none in most, therefore heightening the vulnerability of local populations unaccustomed to such extremes. While El Niño Southern Oscillation (ENSO) strongly affects tropical cyclone activity in the South Pacific, it is hard to quantify its impact in FP due to ENSO diversity, the rarity of TCs, and limited observational data. We show how ENSO flavors influence cyclone emergence in FP, using a region-specific ENSO cluster classification, best track archive data, and cyclogenesis indices. These clusters refine traditional Eastern Pacific (EP)/Central Pacific (CP) categories. We strengthen the statistical robustness of our results by generating a large synthetic TC dataset with a physics-based TC downscaling model and investigating tropical storms from HighResMIP simulations. Cyclogenesis primarily occurs over northern FP during extreme EP El Niño events, due to favorable large-scale vorticity and convective-prone conditions. During CP El Niños, cyclones form farther west but can also reach FP due to favorable steering winds. During mixed El Niños, cyclogenesis is highly sensitive to small shifts in large-scale conditions, causing strong variability in cyclone activity and making seasonal predictions difficult. During La Niña, cyclonic activity decreases, with storms primarily confined to southern FP and disappearing during intense La Niñas. Finally, we discuss the potential influence of volcanic eruptions and background interdecadal variations on cyclone activity in FP.