Synoptic-scale climate patterns associated with rainfall δ18O in southern Australia

A detailed examination of the relationship between synoptic patterns and the oxygen isotopic composition (δ18O) of rainfall is made using a 5-year long dataset of daily-event based rainfall samples from Tasmania, southern Australia. Rainfall δ18O varies inversely with rainfall amount and has only a weak relationship with site surface temperature. Events with extreme rainfall and δ18O values were identified from monthly anomalies. The synoptic patterns for these events were averaged using NCEP/DOE reanalyses-2 mean sea-level pressure data to define the major atmospheric circulation features. Large amounts of rainfall with low δ18O values occur when mid-latitude Southern Ocean low pressure systems pass close to the southeast of Tasmania in a circulation pattern with a strong meridional component. By contrast, small amounts of rainfall with high δ18O values occur when the centre of these mid-latitude cyclonic systems pass much further south of Tasmania. In the latter, only the distal limits of the fronts associated with these low pressure systems cross Tasmania and airflow has a stronger zonal component. In addition, synoptic patterns causing advection of air from lower latitudes towards Tasmania may raise the δ18O value of precipitation. Oxygen isotope records from southern Australia should not be interpreted exclusively as proxies for temperature changes.