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

Analysis 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 (O-18, 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 delta O-18 was significantly more enriched (-3.5 parts per thousand) compared to moisture sourced from the ocean (-7.1 parts per thousand). However, there was no statistically significant difference between the delta O-18 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 delta O-18 variability. A synoptic weather classification scheme was used to analyse the relationship with prevailing synoptic meteorology, which showed that the arithmetic mean of delta O-18 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 delta O-18 arithmetic means of the other systems. The delta O-18 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. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.