The El Niño–Southern Oscillation (ENSO) dominates interannual climate variability worldwide and has important environmental and socio‐economic consequences. However, determining the evolution of ENSO variability and its long‐term response to climate forcing remains an ongoing challenge owing to the limited instrumental records. In this study, we quantified ENSO variability via an empirically calibrated threshold and sliding variance windows using monthly sea‐surface temperature (SST) anomalies based on Porites coral Sr/Ca records from the Xisha Islands in the northern South China Sea. Instrumental SST anomalies from the Xisha Islands correctly captured increasing ENSO variability in the twentieth century, with ENSO detection skills similar to those for Niño3.4 regions. Coral Sr/Ca‐SST anomalies can also serve as sensitive and robust proxies for ENSO variability. Sub‐fossil coral Sr/Ca‐SST anomalies indicated intensified ENSO variability at the end of the Medieval Climate Anomaly (MCA) from 1149 to 1205 ± 4.9 (2σ) Common Era (CE). Combining our records with other ENSO‐sensitive proxy reconstructions from the tropical Pacific, we observed fluctuating ENSO variability during the MCA and intensified ENSO variability for the late MCA. Considering the fewer and low intensity fluctuations associated with external climate forcing and the absence of a coherent temporal correspondence of ENSO activity with solar irradiance and volcanic eruption during the MCA, we hypothesized that the internal dynamics of the climate system play a prominent role in modulating ENSO variability and its evolution, which is supported by unforced climate model simulations and coral reconstructions across the tropical Pacific.

中文翻译：

---

南海北部的多孔珊瑚记录的中世纪气候异常期间的ENSO变异性

厄尔尼诺-南方涛动（ENSO）主导着全球范围内的年际气候变化，并具有重要的环境和社会经济后果。然而，由于仪器记录有限，确定ENSO变异性的演变及其对气候强迫的长期响应仍然是一项持续的挑战。在这项研究中，我们使用基于Porites的每月海表温度（SST）异常，通过经验校准的阈值和滑动方差窗口对ENSO变异进行了量化。南海北部西沙群岛的珊瑚Sr / Ca记录。来自西沙群岛的仪器SST异常正确地捕捉到了20世纪ENSO不断增加的变化，其ENSO检测技术与Niño3.4地区相似。珊瑚Sr / Ca-SST异常也可以作为ENSO变异的敏感且可靠的代理。亚化石珊瑚的Sr / Ca-SST异常表明，中世纪气候异常（MCA）结束时ENSO变异性增强，从1149年到1205±4.9（2σ）共同时代（CE）。将我们的记录与来自热带太平洋的其他ENSO敏感的替代物重建相结合，我们观察到MCA期间ENSO的波动性有所变化，而MCA晚期则使ENSO的变异性增强。