- Title
- Characterising and attributing variability of tropical cyclone tracks in the Southwest Pacific
- Creator
- Sharma, Krishneel Kumar
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Tropical cyclone (TC) hazard is a very relevant topic to the island nations and territories of the southwest Pacific (SWP; 0°–35°S, 135°E–120°W), as it impacts the economy, properties, lives and infrastructure, and accounts for three in four of the reported disasters within the region each year. A particular challenge that island nations face is the inherent degree of variability in TCs from season to season, which hampers adaptation planning due to the difficulty in predicting TC movement post-formation and the subsequent areas of impact. While previous studies have significantly contributed to our understanding of TC track variability, knowledge gaps still remain. This thesis builds on the existing knowledge base by examining long-term trends in TC tracks, local and large-scale influences on track development and the importance of track characteristics on resulting TC impacts. An assessment of pre-satellite era TC track data from the South Pacific Enhanced Archive of Tropical Cyclones (SPEArTC) database confirms the reliability of TC records from 1948-onwards and as such has been adopted for use throughout the thesis. Subsequently, the historical variability of TC genesis and decay points (i.e. the start and end of a TC life cycle) is evaluated on a decadal-scale. It was revealed that from 1948 to 2017, TC genesis has shifted east-northeast by 898 km, whereas the region of TC decay has expanded southward (909 km east-southeast) due to extratropical cyclones (ETCs) travelling further south. These variations strongly coincide with increased sea surface temperature (SST) towards the eastern SWP region and decreased vertical wind shear (VWS) in the higher latitudes. The historical spatio-temporal variability of SWP TC tracks was examined by applying a probabilistic curve-clustering technique to separate TC tracks into distinct clusters and morphometric analysis (i.e. sinuosity analysis), followed by assessing its relationship with El Niño-Southern Oscillation (ENSO). It was discovered that TC tracks have occurred within a number of defined clusters across the Pacific; however, some TCs originated within overlapping clusters but followed different paths post-formation. The frequency of TCs in central and eastern SWP have increased with time, a geometry favoured by El Niño conditions, accompanied by changes in their track morphologies from predominantly straight to more convoluted. Further, the western SWP region has been typically exposed to highly sinuous tracks, that generally corresponded to La Niña conditions. It was also found that the local environmental conditions have more influence on the erratic TCs within the Coral Sea region than the atmospheric conditions. An assessment of steering flow conditions revealed that winds tend to be more zonal in the straight-moving cluster (eastern SWP), and meridional/erratic in clusters with curving/erratic tracks (western SWP). The modulating impact of other large-scale Indo-Pacific climate drivers, present within/outside the region (e.g. ENSO Modoki, Interdecadal Pacific Oscillation (IPO), the Southern Annular Mode (SAM) and Indian Ocean Dipole (IOD)), on SWP TC tracks variability was also assessed. New insights were gained on the temporal components of TC tracks (track length, average speed and duration), being modulated by both individual and the combined influence of large-scale climate modes. For instance, it was demonstrated that when the two climate modes/indices are ‘in phase’ (e.g. El Niño with the positive phase of IPO, eastern pole of IOD (IOD E), Indonesian Index (II) and SAM), TC track length and average speed are enhanced. For cases where either one (e.g. El Niño/negative phase of IPO, IOD E and II) or both (La Niña/negative phase of IPO, IOD E, II and SAM) climate modes/indices were in the negative phase, an increase in TC track duration was observed, implying a dominant impact of negative phases. Lastly, a decision tree model was developed to explore the relative importance of TC track characteristics and associated large-scale ocean-atmospheric interactions on extreme rainfall events over the Fiji Islands. The outcome of the model demonstrated, that while a range of TC characteristics influences extreme rainfall events, only a few could be used for the risk impact analysis and for decision-making. For example, it was found that a TCs minimum distance from land was most important in influencing extreme rainfall, followed by TC tracks cluster grouping, seasonality and duration. Importantly, TC intensity was not identified in the decision tree as a significant factor in producing extreme rainfall, which is often considered (perhaps incorrectly) as a useful guide to TC impacts. The application of this model could result in improved TC risk evaluations and be used by forecasters, and decision-makers on mitigating the TC impacts over the Fiji Islands. Ultimately, the insights from this thesis and the future work proposed may facilitate in optimising TC outlooks for the SWP.
- Subject
- tropical cyclone tracks; southwest Pacific; SAM; IPO; Indian Ocean dipole; multi-modal; decadal variability; extratropical; TC genesis; TC decay; SPEArTC; variability; cluster analysis; sinuosity analysis; decision tree model; classification modelling; extreme rainfall; ENSO
- Identifier
- http://hdl.handle.net/1959.13/1514129
- Identifier
- uon:56815
- Rights
- Copyright 2021 Krishneel Kumar Sharma
- Language
- eng
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