Southern Hemisphere Climate Modes As Drivers Of Synoptic Situations Influencing Auckland Precipitation Patterns

Abstract

Auckland is the largest and one of the fastest growing cities in New Zealand. This rapid growth increases the demand for fresh water, in an environment which has suffered water shortages in the past. The vulnerability of water resources in Auckland is likely to increase under conditions of ongoing climate change. This vulnerability makes it important to understand the driving forces behind Auckland droughts and wet periods, since such events directly influence water supply. Long-term variability in Auckland rainfall is primarily modulated by two Southern Hemisphere climate modes; El Niño – Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). However, on a daily basis Auckland precipitation patterns are largely determined by the synoptic situations passing over New Zealand. This research, therefore, examines the relationship between different synoptic situations and anomalously wet and dry periods in Auckland. Once this link has been established, the relationship of ENSO and the SAM to variability in Auckland rainfall can then be analysed through the impacts of these drivers on the frequency of various synoptic conditions (as ENSO and the SAM influence synoptic situations). A compositing analysis approach is undertaken to analyse the association between synoptic type frequency (represented by the Kidson classification scheme) and long-term precipitation data for Auckland, as well as monthly Southern Oscillation Index (SOI) and SAM index values. Individual events, such as noteworthy storms, droughts, and El Niños, are also analysed. Results indicate a significant relationship between the frequency of certain synoptic situations and anomalously dry and wet periods in Auckland, a relationship which is amplified in association with extreme rainfall events (though not with drought periods). In terms of the climate modes, the SAM is shown to have a much stronger relationship with synoptic type frequency (and Auckland rainfall) than does ENSO, a relationship which is amplified when both oscillations are in complementary phases. Given this, future research should focus on the relationship between the SAM and New Zealand rainfall. Trends in the SAM, likely the product of climate change, have been observed over recent decades; these changes will have associated impacts on New Zealand rainfall and water resource availability. Key Words: Synoptic classification, Auckland precipitation, El Niño-Southern Oscillation, the Southern Annular Mode, Compositing analysis