School of Earth Sciences - Theses
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ItemThe relationship between climate and mechanisms of tropical cyclone formation( 2020)A major uncertainty in future projections of tropical cyclone (TC) frequency is due to inadequate understanding of the atmospheric mechanisms leading to a reduction in the TC formation in warmer climates. Although the recently proposed “marsupial pouch theory,” of TC formation indicates that a semi-enclosed recirculating region known as a “pouch” within large-scale disturbances provides necessary conditions for TC formation, it is important to link the frequency of the pouch environments to the evolving climate conditions. Therefore, this thesis examines the changes in the marsupial pouch TC formation environments and their relationship to the large-scale environmental conditions in the current climate and idealized warmer climates. Here we use ERA-interim reanalysis data and high-resolution Australian Community Climate and Earth-System Simulator (ACCESS) climate model simulations of the current climate and idealized warmer climates (aqua-planet simulations) with two fundamentally different TC tracking schemes. The first scheme is the Okubo-Weiss Zeta Parameter (OWZP), a phenomenon-based tracking scheme that detects TC-favorable locations within marsupial pouches using resolution-independent thresholds. The other scheme is the Commonwealth Scientific and Industrial Research Organization (CSIRO), a traditional TC tracking scheme that uses resolution-dependent thresholds. Environmental and structural composite analysis of developing and non-developing tropical depressions (TDs) 48 hours before TD formation, which are detected using the OWZP scheme in the reanalysis, showed that developing circulations have a strong protective layer around the core from the lower to mid troposphere that protects it from external disruptive influences. The relative importance of the environmental variables influencing tropical storm (TS) formation varies across ocean basins due to differences in the large-scale disturbances and surrounding environmental conditions. Statistical TS prediction schemes are also developed using the environmental conditions of developing and non-developing TDs. This analysis notes that random forests and support vector machine algorithms have higher accuracy than decision trees (DTs). Additionally, the hybrid approach of DTs and Markov decision process is proposed which indicates that developing TDs have a higher likelihood of remaining in more favorable environmental conditions than non-developing TDs. In current climate simulation using the climate model, the TCs detected within marsupial pouches are a subset of the traditionally detected TCs. Also, the OWZP scheme performs better in representing the TC frequency statistics and has stronger relationships with the surrounding environmental conditions in the current climate than the CSIRO scheme. In idealized aquaplanet model simulations, we observe both reduced marsupial pouch environments and traditionally detected TCs with increasing sea surface temperatures (SSTs). The increased saturation deficit and increased stability explain the reduction in the frequency of formations with increased SST. The present study also notes a decrease in the frequency of low-intensity storms and an increase in the frequency of intense storms with increasing SSTs. In a drier and more stable atmosphere, the initial vortices need to be stronger for TC formation to occur, with higher low-deformation vorticity and higher upward mass flux. This research indicates that the marsupial pouch theory may be a fundamental paradigm of TC formation due to its better relationships with the large-scale environmental conditions in different climates.