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 across the globe. Previous studies have shown this climate phenomenon to influence Malaysian Borneo's hydroclimate. In the context of a changing climate and increasingly strong extreme ENSO events, understanding the influence of ENSO on this region, and its evolution through time, is essential to better constrain the future impacts it will have on the Maritime Continent's hydroclimate. Here, we used coupled δ¹⁸O and Sr/Ca records from massive corals' carbonate calcium skeletons to build a proxy for past hydroclimate: δ¹⁸O_seawater (δ¹⁸O_sw) and compensate for the limited dependable instrumental data in most of the 20th century. We assessed our two 90 and 60-year-long δ¹⁸O_sw records' quality as proxies for regional hydroclimate by correlating them with different instrumental salinity datasets before performing moving windowed correlations with the NINO3.4 index, an indicator of ENSO state. Results show that agreement between geochemical proxies and instrumental data highly depends on the chosen dataset, study site location, period, and monsoon season, with stronger agreement with more recent data, pointing towards insufficient data quality when going far back in time. More importantly, when correlated against the NINO3.4 index, our δ¹⁸O_sw records showed a growing correlation for most of their respective lengths. From the 1980s, we found an increasing influence of ENSO on the local hydroclimate with correlation coefficients r > 0.8 during the wet monsoon season. Our findings highlight the differences in results depending on the chosen observational dataset, time scale, or period of the year, and stress the importance of such geochemical archives to better understand the impacts of ENSO across periods predating reliable instrumental data. More importantly, our findings show how the concurrent evolution of the IOD, and the PDV affect ENSO and ultimately, northwestern Borneo's hydroclimate through their teleconnections.