Paired coral Sr/Ca and δ18O records reveal increasing ENSO influence on Malaysian Borneo’s hydroclimate

The El Niño Southern Oscillation (ENSO) is a worldwide climate phenomenon impacting temperatures and precipitation regimes in Australia, Southeast Asia, and America. Previous studies have shown this climate phenomenon and the Indo–Australian monsoon to have a strong influence on Malaysian Borneo’s hydroclimate. In a context of climate change and increasingly strong extreme ENSO events, understanding the influence of ENSO on this region, as well as its evolution through time, is essential to better constrain the possible future impacts it will have on the Maritime Continent’s hydroclimate. To compensate for the limited availability of dependable instrumental data in the first half of the 20th century, we used coupled δ18O and Sr/Ca records from massive corals’ carbonate calcium skeletons to build a proxy for past hydroclimate: δ18Oseawater (δ18Osw). We first assessed our two 90 and 60-year-long δ18Osw records’ quality as proxies for hydroclimate by correlating them with different instrumental datasets before performing moving windowed correlations with the NINO3.4 index, an indicator of ENSO state. Results show variable agreement with local instrumental data depending on the distance from the river mouth, monsoon season, and instrumental dataset used. When correlated against the NINO3.4 index, our δ18Osw records showed a nonstationary increasing influence of ENSO from the 1980s and onwards on the local hydroclimate with correlation coefficients r > 0.8 using month groups towards the end of the year. Our findings highlight the differences in results depending on the chosen dataset, time scale, or period of the year, as well as the usefulness of these geochemical archives to better understand the impacts of climate phenomena across periods predating reliable instrumental data.