Abstract
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.
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