Climatology of Vapour Transport associated with New Zealand Droughts
Drought is a natural hazard which has a wide range of impacts across multiple sectors. While a universal or exact definition of drought is problematic, it is relatively straightforward to classify drought according to the impacted section of the hydrological cycle. Here, hydrological drought is chosen as the focus, as the ecological and socio-economic impacts become more severe as a drought propagates into a hydrological drought. Restraints on hydrological data availability often necessitate the use of standardised indices based on meteorological data, such as the Standardised Precipitation and Evapotranspiration Index (SPEI), as a proxy for hydrological drought. Despite the varied definitions of drought and the complexity in the spatio-temporal development of drought, decreases in precipitation remain a shared characteristic across all drought types, including hydrological drought. Decreases in precipitation are frequently linked to wider atmospheric processes which can act to block the movement of atmospheric moisture, preventing the supply of moisture needed to precipitate out. Thus decreases in precipitation are an outcome of a disruption to the typical pattern of moisture transport over a particular region. Across New Zealand drought research has remained focused on the spatial and temporal expression of drought. With most precipitation across the country coming from the ocean, a focused study of how this transport of moisture is interrupted during drought events could reveal important information on atmospheric causes of drought occurrence. In the present study, the relationship between drought and atmospheric moisture is investigated across New Zealand and regionally. The 3 month accumulation period of the SPEI (SPEI-3) shows utility in tracking hydrological drought onset, while its ability to capture the impact of snowfall and groundwater processes is questionable across the East Cape, East Coast of South Island and Central Otago regions. An environment to climate approach identifies the relationship between drought and atmospheric moisture featuring declining westerly moisture transport, with a weaker connection to moisture flux indicating the significance of PET to the East Cape, East Coast of South Island and Central Otago regions. A climate to environment approach, using the Self Organising Map (SOM) method, supports the initial findings. Collectively, the results provide a first look at atmospheric moisture transport over New Zealand during drought events, revealing important information on possible atmospheric mechanisms associated with drought development such as blocking activity or changes in jet stream movement. Further, this thesis highlights the utility of the SOM method for atmospheric research over New Zealand and provides a basis for further detailed investigation, while the analysis of the SPEI-3 has important implications for the New Zealand Drought Index and its possible usage for monitoring hydrological drought.
Advisor: Kingston, Daniel
Degree Name: Master of Science
Degree Discipline: School of Geography
Publisher: University of Otago
Keywords: Drought; IVT; Hydrological Drought; New Zealand; Vapour Transport
Research Type: Thesis