ENSO variability around 2000 years ago recorded by Tridacna gigas δ18O from the South China Sea

El Niño-Southern Oscillation (ENSO) is the most important mode of interannual changes in global climate. The knowledge of ENSO variability in different mean climate conditions is essential for us to understand the ENSO mechanism and predict its future trend in a warming world. Here we present a 50-year-long, monthly resolved oxygen isotope record, obtained from a 14C dated (AD 50 ± 40) fossil giant clam, Tridacna gigas, located on Dongdao Island, South China Sea, where the interannual climate anomaly is dominated by ENSO variations. We developed a quantitative method to extract the ENSO events from the regional Sea Surface Temperature (SST) and oxygen isotope records. And then, the ENSO variations about 2000 years ago, located in the so called Roman Warm Period, was calculated from the oxygen isotope series of fossil T. gigas. Our quantitative record shows that ENSO variance in Roman times was similar to the instrumental times, with a total of 11 El Niño events and 12 La Niña events within 50 years.