This first climatic isotopic analysis of 24-hour daily rainfall samples from southern Australia identifies the proximity of Southern Ocean mid-latitude low pressure systems passing to the south of Tasmania as the dominant control over rainfall d18O. Proximity, or distance between the collection site and low pressure centre, reflects the gradient in rainfall d18O and amount which are greatest nearest to the core of these systems. The correlation between site temperature and rainfall d18O is found to be negligible. However, it is shown that a false correlation between temperature and d18O is produced due to the similarity of their annual cycles when data are aggregated into monthly totals, representing monthly interval collections such as those from the International Atomic Energy Agency Global Isotopes in Precipitation (IAEA/GNIP) program.
The much stronger relationship between d18O and rainfall amount is shown on rain-day sampling intervals indicating a synoptic-scale climatic control. A relationship between d18O, rainfall amount and the proximity of low pressure systems is determined by comparing averaged synoptic patterns created from anomalous rainfall events using numerical NCEP/DOE Reanalyses-2 mean sea-level pressure data. This approach identifies two end-members; firstly, small amounts of isotopically heavy d18O rain are produced during predominantly zonal-flow synoptic patterns, while larger amounts of isotopically light d18O rain are produced during predominantly meridional synoptic patterns. Within these flow patterns, the proximity of passing low-pressure systems is limited in the zonal pattern by the presence of a stable east-west baric ridge across the continent, keeping the lows passing well south of Australia. In the meridional member, low pressure systems pass near to Tasmania in north-south flow between transient highs. In the meridional case, the intensity of the low pressure system bears the greatest impact on the site, producing large amounts of isotopically light rainfall. Exceptions between these end-members occurs when isotopically heavier tropical moisture is injected into the air-flow, producing anomalously large amounts of isotopically heavy d18O rainfall; or when the air mass trajectory over land is lengthened by its approach from the northwest rather than the southwest.