Rainfall Stable Water Isotope Variability in Coastal Southwestern Western Australia and Its Relationship to Climate on Multiple Timescales.

The factors driving variability in rainfall stable water isotopes (specifically δ18O and deuterium excess, d = δ2H − 8 δ18O) were studied in a 13‐year data set of daily rainfall samples from coastal southwestern Western Australia (SWWA). Backwards dispersion modeling, automatic synoptic type classification, and a statistical model were used to establish causes of variability on a daily scale; and predictions from the model were aggregated to longer temporal scales to discover the cause of variability on multiple timescales. Factors differ between δ18O and d and differ according to temporal scale. Rainfall intensity, both at the observation site and upwind, was most important for determining δ18O and this relationship was robust across all time scales (daily, seasonal, and interannual) as well as generalizing to a second observation site. The sensitivity of δ18O to rainfall intensity makes annual mean values particularly sensitive to the year's largest events. Projecting the rainfall intensity relationship back through ∼100 years of precipitation observations can explain ∼0.2 $0.2$–0.4‰ shifts in rainfall δ18O. Twentieth century speleothem records from the region exhibit signals of a similar magnitude, indicating that rainfall intensity should be taken into account during the interpretation of regional climate archives. For d, humidity during evaporation from the ocean was the most important driver of variability at the daily scale, as well as explaining the seasonal cycle, but source humidity failed to explain the longer‐term interannual variability. Plain Language Summary: In cave deposits, as with several other natural systems, the relative abundance of the heavy isotopes oxygen‐18 and deuterium can be used to determine past changes in climate. This is because the isotopic composition of these systems is linked to that of rainfall, while the abundance of heavy isotopes in rainfall is driven by climate parameters such as temperature and rainfall characteristics. For this to be possible, the factors which drive rainfall isotopic variability need to be well known. This study uses a 13‐year data set of daily rainfall samples from coastal southwestern Western Australia to better understand isotopic variability for this region. Oxygen‐18 variations here are driven mainly by rainfall intensity (the amount of rain each day) both according to measurements at the site and upwind simulations. Deuterium excess, a second order parameter which is often linked to conditions in the evaporation source region, was well‐predicted by source region humidity at the daily scale but not when aggregated to annual totals. The relationship between rainfall intensity and oxygen‐18 appears to be important over the 20th century, based on a comparison between observed rainfall and a cave record. Key Points: Precipitation δ18O variability is driven primarily by rainfall intensity in southwestern Western AustraliaInterannual δ18O variability is strongly influenced by the largest rainfall events of the yearHumidity during evaporation drives deuterium excess variability at a daily, but not interannual, timescale [ABSTRACT FROM AUTHOR]