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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. • 228-year monthly SST derived from coral Sr/Ca for snapshots of the Roman Warm Period. • Cooler conditions and variable SST seasonality characterize the 2070–1740 a BP. • Stronger interannual variability and ENSO evolution for the RWP are investigated. • ENSO activity modulates changes of SST seasonality in the northern South China Sea. [ABSTRACT FROM AUTHOR]

The El Niño-Southern Oscillation (ENSO) is the most influential climatic phenomenon on an interannual timescale. Studying the relationship between sea surface temperature (SST) and ENSO events during typical geological periods is helpful for understanding the trends of ENSO activities in the future, especially under the background of global warming. Here we quantitatively reconstructed a 115-year history of SST and ENSO activity around 282 ka during Marine Isotope Stage 8 (MIS8) by using the growth rate of Porites corals from Xisha Islands in the South China Sea. The results show that the average coral growth rate (6.0 ± 1.0 mm/yr (1σ)) around 282 ka was only 56.1% of the modern average value (10.7 ± 0.9 mm/yr; 1937–2014 CE), and the average SST (26.0 ± 0.3 °C (1σ)) was 0.7 ± 0.4 °C and 1.1 ± 0.4 °C lower than that in the Little Ice Age (LIA; 1520–1676 CE; 26.7 ± 0.2 °C) and the present (1937–2014; 27.1 ± 0.2 °C), respectively. The reconstructed results show that there was significant ENSO activity with moderate intensity or above at 282 ka, while the frequency of ENSO activity at that time was lower than that of the LIA and the present. This seems to indicate that the frequency of moderate intensity ENSO activity increases with rising SST. Given the current rapid warming, the future trend of ENSO activities deserves close attention. • The growth rate of Porites coral is a reliable paleoenvironmental proxy. • The coral growth rate at 282 ka was only equivalent to 56.1% of the present. • Coral–SST at 282 ka was 0.7 °C and 1.1 °C lower than the LIA and present, respectively. • The ENSO frequency at 282 ka was lower than the present. [ABSTRACT FROM AUTHOR]