Variability in the trade wind regime and wet season of Northeastern Queensland
thesisposted on 02.03.2017 by Murphy, Michael James
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The trade wind regime along the coast of eastern Australia is described with an emphasis on seasonal and latitudinal variations, using upper-level wind and radiosonde observations over the periods 1960-2010 and 1976-2010, respectively. The trade winds and their associated temperature inversion dominate the deep tropics in austral winter and the subtropics in late summer with their poleward fringes near Brisbane (27.4°S). The subtropical ridge (STR) over eastern Australia is found to be an important driver of interannual variability of the trade winds themselves as well as the trade wind inversion (TWI). Trends in the annual frequency of low-level winds reveal a significant shift in direction from southeast to east throughout much of the tropics, with a strong increasing trend in the occurrence of the TWI across across eastern Australia with modest increasing trends in the base height of the inversion, strongest in the subtropics. Rainfall and associated convection are examined during the main months of the wet season, November–March, with a focus on regional variations in the northern parts of the states of Western Australia, the Northern Territory, and Queensland (NQL). A similar annual cycle is found across the region, with the highest monthly rainfall during the middle months of the wet season, while a much higher proportion of warm rain precipitation features (PFs) is found over NQL. Strong intraseasonal variability in rainfall associated with the Madden-Julian oscillation (MJO) is found in each month and in all regions except NQL; however, there is a strong relationship between the proportion of warm PFs and the MJO during most of the wet season in NQL. Interannual variability in rainfall is closely associated with El Niño–Southern Oscillation (ENSO) during each month in all regions with the weakest relationship during January. Interannual variability is found to affect the MJO most strongly during January, with an increase in the frequency of the dry and wet phases of the MJO during positive and negative SOI, respectively. The concepts learnt about the trade winds and wet season are applied to precipitation in the ‘Wet Tropics’ of Queensland. Daily rainfall over the region is objectively grouped into six clusters, and the weather associated with each cluster is described and identified as belonging to either the trade wind regime or wet season heavy rainfall. The trades winds are found to be very important, occurring on just over 92% of days and contributing nearly half the total rainfall over the study period. The trade winds remain important even during the wet season, contributing approximately 40% of the total rainfall during the months of November through March. Intraseasonal variability associated with the MJO affects precipitation in the trade winds throughout much of the year, while the MJO’s modulation of wet season heavy rainfall is concentrated in summer. Interannual variability in the rainfall clusters is most closely associated with ENSO throughout the wet season, and the intensity of the STR throughout the winter half of the year. Trends in the frequency of the rainfall clusters were not found.