Is the isotopic composition of event based precipitation driven by moisture source or synoptic scale weather in the Sydney Basin, Australia?

• Event based rainfall samples from Mt Werong were analysed for isotopic composition.
• δ18O was more enriched when the moisture was sourced over land compared to the ocean.
• δ18O in rainfall from East Coast Lows was more depleted than other synoptic weather systems.
• Winter and summer depleted events were associated with higher precipitation amount.

SummaryAnalysis of the isotopic composition of five years of event based precipitation samples collected in the Blue Mountains near Sydney Australia is presented. The relationship between the stable isotopic (18O, D-excess) composition of precipitation with moisture source regions and prevailing synoptic scale weather systems was investigated. Back trajectory analysis showed that when the majority of moisture was sourced over land the δ18O was significantly more enriched (−3.5‰) compared to moisture sourced from the ocean (−7.1‰). However, there was no statistically significant difference between the δ18O arithmetic means of different oceanic source regions. The analysis indicated that the large scale rainout during transport of moisture to the measurement site was a more important factor influencing the δ18O variability. A synoptic weather classification scheme was used to analyse the relationship with prevailing synoptic meteorology, which showed that the arithmetic mean of δ18O in precipitation for the East Coast Low (ECL) systems was significantly more depleted than other systems and exhibited much larger variability. There was no statistically significant difference in the δ18O arithmetic means of the other systems. The δ18O of the ECLs was shown to be related to the distance from low pressure system centre to the site and the large scale rainout along the back trajectory. Considering the large amount of precipitation that can result from ECLs, they may contribute significantly to signals observed over longer period collections such as the GNIP records. Winter and summer more depleted samples were from synoptic weather systems from which more precipitation had occurred both at the site and prior to the site. The enriched events could be associated with short duration low intensity precipitation events.