Abstract
Atmospheric moisture transport is a key driver of hydrometeorological variability, yet understanding of its patterns, drivers, and hydrological impacts remains limited, particularly in complex climatic and topographic setting of New Zealand. While most previous studies have focused on Atmospheric Rivers (ARs), this thesis provides a more complete understanding by examining the entire spectrum of moisture transport processes and its impacts on regional hydrology. Using Integrated Water Vapour Transport (IVT) calculated from 40 years (1981-2020) of ERA5 reanalysis data, this thesis investigated the climatology, spatio-temporal variability, drivers, and hydrological impacts of atmospheric moisture transport over New Zealand. First, a comprehensive climatology of moisture transport showed that while ARs are important drivers of extreme precipitation, they account for less than 10% of total moisture transport at most coastal locations. Notably, extreme water vapour transport (i.e. 90th percentile IVT) showed stronger correspondence with extreme precipitation events than ARs alone. Over the study period, moderate and strong IVT events increased in frequency over the South Island but declined over the northern North Island. Analysis of IVT and its components suggest that changes in thermodynamic (total column water vapour) component being the primary driver of these trends. Next, the thesis identified dominant spatial patterns of IVT variability and its connections to large-scale atmospheric circulation. Using Empirical Orthogonal Function (EOF) analysis, three distinct patterns were identified that explained over 80% of the variance and were strongly linked to the positioning and strength of the subtropical and polar jet streams. At larger spatial scales, the strongest correlations were seen with Southern Annular Mode (SAM) for EOF1 and EOF2, particularly during warm months, whereas correlations with other modes such as El Niño-Southern Oscillation (ENSO) were comparatively weak. Correlation was also established between the IVT EOFs and precipitation patterns, where EOF1 showed negative correlation with precipitation in the western and southern South Island, and EOF2 exhibited positive correlation in central New Zealand. Finally, the thesis investigated the runoff response to atmospheric moisture transport using runoff data from 161 gauging stations across 97 catchments. Strong regional variation emerged in the relationship between catchment averaged IVT (IVTC) and runoff, with the strongest correlations observed along the western Southern Alps where orographic effects enhance moisture-runoff conversion. Multiday moisture transport events generated higher runoff compared to single day events, with catchments in the western South Island showing runoff increases up to 170% during persistent moisture transport conditions. While ARs contributed substantially to annual runoff totals in the North Island regions, high IVTC events played a dominant role in most South Island regions. The study also identified critical IVT thresholds associated with substantial runoff changes and distinct regional patterns of moisture transport pathways during extreme runoff events. Together, these findings advance the understanding of atmospheric moisture transport processes and its hydrometeorological impacts in New Zealand. The interaction between hemispheric-scale atmospheric circulation, regional topography, and moisture transport processes create distinct regional variations in hydrometeorological response. Future research should include how projected changes in atmospheric circulation and moisture content under climate change scenarios will alter the frequency and character of extreme precipitation events in the region and quantification of moisture transport to precipitation conversion efficiency using higher resolution datasets.