Few reconstructions of sea-surface temperature (SST) have focused on seasonal and interannual variability, two major components of the global climate system, due to the limited temporal resolution of proxies. This study presents a combined 228-year-long, monthly resolved strontium to calcium ratios (Sr/Ca) record covering the period of 2070–1740 a BP (years before 1950 CE (Common Era)) extracted from three U-series-based sub-fossil Porites corals located on the Xisha Islands, northern South China Sea (SCS). The composite time series allowed for accurate assessment of the natural range of SST variations during snapshots of the Roman Warm Period (RWP). Reconstructed SST revealed that the RWP was characterized by cooler conditions compared with the 20th century, consistent with climatic variations in the western Pacific and East Asia. The amplitude of SST seasonality was within the modern range, except for a lower value at 1980–1928 a BP. Interannual variability associated with El Niño–Southern Oscillation (ENSO) activity was enhanced by 39% relative to 1980–2014 CE. The results of the sliding window demonstrate that ENSO variability persistently strengthened during 2070–2010 a BP, followed by an overall fluctuating attenuation during 1980–1928 a BP. Then, the trends of rising first before descending twice during 1852–1800 a BP appeared. Furthermore, ENSO activity played a leading role in steering short-term changes in SST seasonality in the northern SCS, manifested as stronger ENSO activity with more frequent El Niño events and decreased SST seasonality. Considering that the frequency of extreme ENSO events may strengthen in the future under global warming, the climate in the northern SCS might become more variable and complex.

由于代理的时间分辨率有限，海面温度 (SST) 的重建很少关注季节性和年际变率，这是全球气候系统的两个主要组成部分。本研究提供了一个长达 228 年、每月解析的锶钙比 (Sr/Ca) 记录，涵盖 2070-1740 年 BP（公元 1950 年（共同时代）之前的几年）从三个基于 U 系列的亚化石滨位于南海北部（SCS）西沙群岛的珊瑚。复合时间序列允许准确评估罗马暖期 (RWP) 快照期间海温变化的自然范围。重建的 SST 显示，与 20 世纪相比，RWP 的特征是凉爽的条件，与西太平洋和东亚的气候变化一致。SST 季节性的幅度在现代范围内，除了 1980-1928 a BP 的较低值。与 1980-2014 年 CE 相比，与厄尔尼诺-南方涛动 (ENSO) 活动相关的年际变化增加了 39%。滑动窗口的结果表明，ENSO 变异性在 2070-2010 a BP 期间持续增强，随后在 1980-1928 a BP 期间整体波动衰减。然后，1852-1800年出现了先升后降的趋势。此外，ENSO 活动在引导南海北部 SST 季节性的短期变化方面发挥了主导作用，表现为 ENSO 活动更强，厄尔尼诺事件更频繁，SST 季节性减弱。考虑到未来全球变暖条件下极端 ENSO 事件的发生频率可能​​会增强，南海北部的气候可能会变得更加多变和复杂。