Tropical cyclone climatology, variability, and trends in the Tonga region, Southwest Pacific

The focus of several past tropical cyclone (TC) studies in the Southwest Pacific (SWP) had been primarily at the regional scale, with little or no attention to the local-scale TC activity (i.e., at the country level). With the growing coastal population in the South Pacific Island countries, as well as increasing threats from and exposure to climate extremes mostly affecting vulnerable communities, examining TC-related risks at the country level is more imperative now than before. This study catalogues for the first time the climatology, variability and trends of TCs affecting Nuku'alofa, the capital of Tonga using the Southwest Pacific Enhanced Archived for Tropical Cyclone (SPEArTC) dataset for the period between 1970 and 2019. The variability is examined in relation to the El Niño–Southern Oscillation (ENSO) phenomenon, which is the major driver of the year-to-year variability of TC activity in the SWP. A total of 128 TC tracks affected the Tonga region over the study period, with a seasonal average of ∼2.6 TCs per year. Of these, about 50% occurred during the peak months of January and February, and ∼38.8% of the total were of hurricane intensity (Categories 3, 4 and 5). Although differences were found between the average number of TCs per year during El Niño, La Niña and ENSO-neutral events (∼2.9, ∼2.6 and ∼2.3, respectively), they were statistically insignificant. No significant long-term trends were found in the number of TCs, severe TCs, and accumulated cyclone energy (ACE) over the period of study. The findings of this study will provide the information needed for disaster preparedness and TC predictions in Tonga

Clustering tropical cyclone genesis on ENSO timescales in the Southwest Pacific

Tropical cyclones (TCs) as a natural hazard pose a major threat and risk to the human population globally. This threat is expected to increase in a warming climate as the frequency of severe TCs is expected to increase. In this study, the influence of different monthly sea surface temperature (SST) patterns on the locations and frequency of tropical cyclone genesis (TCG) in the Southwest Pacific (SWP) region is investigated. Using principal component analysis and k-means clustering of monthly SST between 1970 and 2019, nine statistically different SST patterns are identified. Our findings show that the more prominent ENSO patterns such as the Modoki El Niño (i.e., Modoki I and Modoki II) and Eastern Pacific (EP) El Niño impact the frequency and location of TCG significantly. Our results enhance the overall understanding of the TCG variability and the relationship between TCG and SST configurations in the SWP region. The results of this study may support early warning system in SWP by improving seasonal outlooks and quantification of the level of TC-related risks for the vulnerable Pacific Island communities.The first author is funded under the Pacific Excellence for Research and Innovation (PERSI) scholarship of the University of the South Pacific (USP)

Clustering tropical cyclone genesis on ENSO timescales in the Southwest Pacific

Tropical cyclones (TCs) as a natural hazard pose a major threat and risk to the human population globally. This threat is expected to increase in a warming climate as the frequency of severe TCs is expected to increase. In this study, the influence of different monthly sea surface temperature (SST) patterns on the locations and frequency of tropical cyclone genesis (TCG) in the Southwest Pacific (SWP) region is investigated. Using principal component analysis and k-means clustering of monthly SST between 1970 and 2019, nine statistically different SST patterns are identified. Our findings show that the more prominent ENSO patterns such as the Modoki El Niño (i.e., Modoki I and Modoki II) and Eastern Pacific (EP) El Niño impact the frequency and location of TCG significantly. Our results enhance the overall understanding of the TCG variability and the relationship between TCG and SST configurations in the SWP region. The results of this study may support early warning system in SWP by improving seasonal outlooks and quantification of the level of TC-related risks for the vulnerable Pacific Island communities

Influence of the Madden–Julian Oscillation (MJO) on Tropical Cyclones Affecting Tonga in the Southwest Pacific

The modulating influence of the Madden–Julian oscillation (MJO) on tropical cyclones (TCs) has been examined globally, regionally, and subregionally, but its impact on the island scale remains unclear. This study investigates how TC activity affecting the Tonga region is being modulated by the MJO, using the Southwest Pacific Enhanced Archive of Tropical Cyclones (SPEArTC) and the MJO index. In particular, this study investigates how the MJO modulates the frequency and intensity of TCs affecting the Tonga region relative to the entire study period (1970–2019; hereafter referred to as all years), as well as to different phases of the El Niño southern oscillation (ENSO) phenomenon. Results suggest that the MJO strongly modulates TC activity affecting the Tonga region. The frequency and intensity of TCs is enhanced during the active phases (phases six to eight) in all years, including El Niño and ENSO-neutral years. The MJO also strongly influences the climatological pattern of genesis of TCs affecting the Tonga region, where more (fewer) cyclones form in the active (inactive) phases of the MJO and more genesis points are clustered (scattered) near (away from) the Tonga region. There were three regression curves that best described the movement of TCs in the region matching the dominant steering mechanisms in the Southwest Pacific region. The findings of this study can provide climatological information for the Tonga Meteorological Service (TMS) and disaster managers to better understand the TC risk associated with the impact of the MJO on TCs affecting the Tonga region and support its TC early warning system