El Niño-Southern Oscillation Modulation of Tropical Cyclone Climatology over the Southwest Pacific

Abstract

Tropical cyclones (TCs) cause major devastation across the Southwest Pacific, a region that includes small island developing states such as Fiji and Vanuatu, as well as New Zealand. TC activity in this basin is strongly influenced by the El Niño–Southern Oscillation (ENSO), which alters sea surface temperature (SST) gradients, the Walker circulation, and vertical wind shear. Through these mechanisms, ENSO modulates TC genesis, frequency, intensity, tracks, and associated impacts on interannual to decadal timescales. Despite growing research attention, several key challenges exist in the Southwest Pacific. First, coarse resolution climate models often fail to represent TCs realistically and may miss important regional teleconnections. Second, observational records are not always analysed with sufficient temporal resolution to capture intra-seasonal ENSO-related differences. Third, some future projection studies do not explicitly account for thermodynamic thresholds relevant to cyclogenesis and intensification. In response, this thesis addresses the following overarching research questions: (1) How do ENSO characteristics (phases, residence time, and magnitude) modulate Southwest Pacific TC genesis, frequency, intensity, and spatial distribution across seasons, in both observations and historical downscaled simulations? (2) How do projected changes in ENSO characteristics under future climate scenarios modify these seasonal ENSO–TC relationships in downscaled model projections? TC genesis is assessed using a Coupled ENSO Index (CEI), constructed from Niño 3.4 SST anomalies and the Southern Oscillation Index (SOI). This is used in a systematic way to evaluate TC behaviour in the Southwest Pacific from the historical period through to future projections. Specifically, the analysis comprises: (1) decadal assessment for 1971 to 2020 using the observational dataset; (2) a historical evaluation for 1965 to 2014 using simulations dynamically downscaled with the Conformal Cubic Atmospheric Model (CCAM) from six CMIP6 global climate models (GCMs), compared directly with observations; and (3) an assessment of projected changes for 2051 to 2100 under the SSP2-4.5 and SSP3-7.0 scenarios, based on the same downscaled model ensemble. Observations indicate that TC frequency is typically higher during El Niño and in the late season than during La Niña and the early season as SST increases. Moreover, the observational record shows a long-term decrease in TC frequency, alongside increasing SSTs at the time of genesis and increasing maximum wind speeds, suggesting a shift toward more intense TCs over time in the Southwest Pacific basin. The results also show that TCs forming during El Niño tend to be more intense than those forming during La Niña, even though there is no statistically significant difference in SST at genesis between the two ENSO phases. In addition, warming raises SST thresholds at TC genesis and is associated with a shift towards higher TC intensity, even as the overall frequency of TCs decreases. Comparison of the downscaled models with observations shows that most models can reproduce the overall ENSO and seasonal modulation of TC frequency reasonably well. Four of the six models underestimate total TC frequency (‑13% to ‑33%), whereas two models overestimate TC frequency by 18.3% and 103.7% relative to observation. Three models also capture the observed increase in TC frequency from the early to the late season. Spatially, observed TC genesis peaks north of 15°S near the dateline, while most models exhibit a slight westward bias in genesis location that varies by season. Consistent with other models at comparable resolution, CCAM under-represents the most intense TCs (Category 4–5). Despite these biases in absolute frequency, most models represent the relative modulation of TC activity by ENSO well. Under future scenarios, most models project an increased occurrence of El Niño phase conditions, with a tendency for events to shift from La Niña to El Niño phases as warming intensifies. Although total TC frequency does not increase significantly, TCs become increasingly concentrated during El Niño phases and the late TC season under both SSP2-4.5 and SSP3-7.0. Projections of intensity suggest increases in Category 1 and Category 4 TCs, alongside decreases in the mid-range categories (2 and 3), broadly consistent with the observational trends towards intensifying TCs in the Southwest Pacific. Spatial analyses further indicate an eastward and equatorward shift in genesis probability density, with peak genesis regions moving from west of the dateline (180⁰) toward the central Pacific (~160°E–160°W). SSTs at TC genesis show a clear warming signal, with higher median SST values in the future period than in the historical period, accompanied by increased SST variability. Overall, these results suggest that the environmental thresholds governing cyclogenesis in the Southwest Pacific are gradually shifting, with implications for more severe TC impacts in the region. In observations, the most intense TCs tend to occur preferentially during El Niño; however, this relationship for the most intense TCs is generally not reproduced by the models. Despite these biases, the process-oriented evaluation presented here indicates that the downscaled models can broadly capture the key ENSO-Seasonal TC relationships in Southwest Pacific basin. Collectively, these findings imply that future TC hazard in the Southwest Pacific will be increasingly influenced by changes in ENSO characteristics, with important consequences for island nations and a need to update climate adaptation and disaster-risk management strategies.