School of Earth Sciences - Theses

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    Tropical cyclone characteristics in the Fiji region
    Chand, Savin Suvnesh ( 2011)
    Tropical cyclones (TCs) are among the most extreme meteorological events, often having devastating impacts on life and property in the southwestern tropical Pacific islands such as Fiji, Samoa and Tonga (the FST region). However, the FST region has garnered little attention in the scientific literature regarding many aspects of TC behaviour when compared with other cyclone basins of the world. In this thesis, we first examine the spatio-temporal variability of TC genesis position, track and intensity due to the two major modes of natural climate variability in this region – the El Niño Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO) phenomena. Statistical forecast models are also formulated using sophisticated methodologies to provide forewarning of TC strike in the FST region. At interannual timescales, TC genesis position, track and intensity are strongly modulated over the FST region by ENSO. During the El Niño phase, for example, TC genesis is enhanced east of the dateline, extending from north of Fiji to over Samoa. The TCs formed during El Niño years take three characteristic paths depending on their mean genesis locations. In the La Niña phase, fewer TCs are observed compared to the El Niño phase and genesis is more common in the west of the region to about 170°E. The TCs formed during La Niña years are often steered over the Fiji islands and Tonga from the Coral Sea region with relatively little or no threat to Samoa. In addition, TC intensity is also considerably influenced by the ENSO signal. It is found that TCs entering the region poleward of 15°S are able to sustain their intensity for a longer period of time in La Niña years as opposed to TCs entering the region in El Niño years, when they decay more rapidly. Equatorward of 15°S, TCs are more intense during El Niño years than in La Niña years. The intraseasonal modulation of TC activity in the FST region by the MJO is also investigated. Results suggest strong MJO-TC relationships. TC genesis patterns are significantly altered over the FST region with approximately five times more cyclones forming in the active phase than in the inactive phase of the MJO. Because the FST region exhibits a marked ENSO-TC relationship, which is identifiable prior to a cyclone season through the use of appropriate ENSO indices, it possible to predict TC strike well in advance. Two models are developed here to make probabilistics forecasts of TC strike in the FST region up to three months in advance: (i) a model to predict TC counts, and (ii) a model to predict the overall TC activity. While both models have a substantial skill to make predictions during El Niño and La Niña years, the latter is more appropriate to make prediction during ENSO-neutral years. Finally, a new methodology is evaluated here to diagnose the probabilistic potential of any tropical depression in the FST region to develop further into a tropical cyclone within up to 72 hours in advance. Evaluation of this methodology suggests its future application in forecasting probabilistic potential for TC formation in the FST region.