Abstract
The occurrence of drought has the potential to cause detrimental impacts on the precious water resources in New Zealand. Climatic conditions associated with drought are influenced by both large and regional scale atmospheric circulation patterns. Through studying how atmospheric circulation patterns influence drought development, atmospheric controls on drought can be identified.
Drought indices are useful tools for analysing the characteristics of drought events. This thesis uses the Standardized Precipitation Evapotranspiration Index (SPEI) to investigate links between atmospheric circulation and drought development in New Zealand. A national average SPEI dataset is created from the Water and Global Change Forcing Dataset (WFD), from which 20 drought events are identified. Drought events are defined as any month which exceeds a national average SPEI value of -0.75 (representing the lowest 10% of values in the dataset). The spatial and temporal patterns of drought identified show consistency with past studies and historical records of drought events, indicating that the SPEI provides an accurate representation of drought conditions.
Composite analysis as a form of environment-to-climate analysis reveals three distinct geopotential height anomaly patterns that occur during the development of drought events. The first pattern (Type 1) consists of positive geopotential height anomalies over or to the east of New Zealand and anomalous north and east airflow. The second pattern (Type 2) consists of negative geopotential height anomalies over or to the east of New Zealand and strong west and southwest airflow. The third pattern (Type 3) is characterised by negative geopotential height anomalies to the south of New Zealand, positive geopotential height anomalies to the west of New Zealand and strong westerly circulation.
Climate-to-environment analysis is carried out involving the Southern Oscillation Index (SOI), Southern Annular Mode (SAM), Interdecadal Pacific Oscillation (IPO), Trenberth indices and Kidson weather regimes in order to identify linkages between specific atmospheric circulation patterns and drought development. Links between large-scale indices and drought development are made through the use of the regional descriptors of atmospheric circulation. Positive modes of the SOI (La Niña) and SAM bring similar conditions to those of Type 1 drought development to the New Zealand region (northeast circulation and positive geopotential height anomalies), along with increased frequencies of the Blocking regime during Type 1 drought development. Negative SOI values (El Niño) are associated with southwest circulation and increased frequency of the Zonal regime occurring during Type 2 drought development. El Niño conditions also influence Type 3 drought development bringing more frequent Zonal regime occurrence and southwest airflow. IPO phases are found to have links to drought development with Type 1 drought mainly occurring during the phase change from positive to negative, Type 2 droughts predominantly occurring during the negative phase and Type 3 droughts occurring most often during the positive phase of the IPO. Correlation analysis is performed between drought index data and atmospheric circulation data. Statistical relationships between national average SPEI values and circulation indices are very weak and only significant for the SAM (coefficient: -0.15, 99% level of significance). The results provide valuable insight into links between atmospheric circulation and large scale drought in New Zealand; showing how regional scale atmospheric circulation indices can be used to understand the influence of large scale atmospheric circulation on the development of drought. The SPEI has proven to be effective for studying New Zealand drought and can be used in the future to provide more detailed insight into atmospheric circulation-drought linkages in New Zealand.