Synoptic-scale climate patterns associated with rainfall δ ¹⁸O in southern Australia

A detailed examination of the relationship between synoptic patterns and the oxygen isotopic composition (δ ¹⁸O) of rainfall is made using a 5-year long dataset of daily-event based rainfall samples from Tasmania, southern Australia. Rainfall δ ¹⁸O varies inversely with rainfall amount and has only a weak relationship with site surface temperature. Events with extreme rainfall and δ ¹⁸O 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 δ ¹⁸O 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 δ ¹⁸O 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 δ ¹⁸O value of precipitation. Oxygen isotope records from southern Australia should not be interpreted exclusively as proxies for temperature changes.