ENSO Variability During The Last 2000 Years As Recorded By Porites Corals From The Xisha Islands In The Northern South China Sea

Massive hermatypic corals distributed among tropical oceans are extremely sensitive to climatic and environmental changes.The skeletons of Porites corals can accurately record the climate characteristics during their growth period and are widely used in reconstructing historical climatic and environmental changes.El Ni（?）o–Southern Oscillation（ENSO）,the oscillatory instability of ocean and atmosphere system across the tropical Pacific,is the leading pattern of global interannual variability,with important physical,ecological and human impacts.The knowledge of ENSO variations and its mechanisms that occurred over the last 2000 years are essential for understanding of the process and regular pattern of historical climate variability closely related to modern times,providing necessary background knowledge for improving predictions of future climatic changes in a warming world.To reveal the evolution process,driving mechanism,and the impact on climate change of ENSO activities during the last 2000 years,this study reconstructed a combined 397-year-long,monthly resolved sea surfacetemperature（SST）and a combined 145-year-long,monthly resolved sea surface salinity（SSS）in the northern South China Sea based on a total of 6253 pieces of coral Sr/Ca and a total of 2424 pieces of coral δ¹⁸O from one modern Porites（1980–2007 CE）and five fossil Porites collected from the Xisha Islands.According to uranium series dating,fossil Porites corals were identified to cover the periods of 120–60 BCE,30 BCE–23 CE,98–210 CE,665–749 CE,and 1149–1205 CE,respectively.The major concerned scientific issues are as follows:（1）Change characteristics of SST and its seasonality in the northern South China Sea during the special periods of the last 2000 years.（2）The evolutional features and driving mechanism of ENSO variability during the special periods of the last 2000 years.（3）The impact of ENSO on seasonal SST in the northern South China Sea during the special periods of the last 2000 years.（4）The impact of ENSO on SSS in the northern South China Sea during the typical warm periods of the last 2000 years.（5）The impact of ENSO on humidity patterns in the East Asian monsoon region during the typical warm periods of the last 2000 years.The main conclusions are as follows:（1）Mean SST in the northern South China Sea during the special periods of the last 2000 years did not exceed that of modern times.Coral Sr/Ca records from the Xisha Islands show that mean SST during 120–60 BCE,30 BCE–23CE,98–210 CE,665–749 CE,and 1149–1205 CE was 23.7 ± 1.3℃,27 ±0.9℃,24.8 ± 1.1℃,23 ± 1.1℃,and 25.1 ± 1.4℃,respectively.These reconstructed results are cooler than that of 1990–2020 CE（27.6 ± 0.3℃）.（2）Seasonal SST in the northern South China Sea during the special periods of the last 2000 years was similar to that of modern times,but with fluctuations.Coral Sr/Ca records from the Xisha Islands show that seasonal SST during 120–60 BCE,98–210 CE,665–749 CE,and 1149–1205 CE was4.4 ± 1.1 ℃,4.4 ± 1.3 ℃,4.5 ± 1.7 ℃,and 4.7 ± 1.2 ℃,respectively.These reconstructed results are similar to the present-day conditions（4.4 ± 0.5 ℃ for1990–2020 CE）,while a lower value was present in 30 BCE–23 CE（3.4 ±1.1 ℃）.（3）The frequency and amplitude of ENSO activity during the special periods of the last 2000 years were stronger than those in present day.According to the response of modern instrumental SST in the northern South China Sea to ENSO activity and coral Sr/Ca-SST records from the Xisha Islands,5 El Ni（?）o events and 4 La Ni（?）a events were estimated to have occurred during 1980–2007 CE.A total of 14 El Ni（?）o events and 15 La Ni（?）a events were identified during 120–60 BCE.The total variance of 3–7-year band-pass filtered coral Sr/Ca-SST anomalies shows that ENSO activity for120–60 BCE was enhanced by 39% relative to 1980–2014 CE.A total of 12 El Ni（?）o events and 10 La Ni（?）a events were identified during 30 BCE–23 CE,and ENSO activity was enhanced by 72% relative to modern times.A total of 24 El Ni（?）o events and 24 La Ni（?）a events were identified during 98–210 CE,with ENSO intensity consistent with modern times.A total of 19 El Ni（?）o events and 18 La Ni（?）a events were identified during 665–749 CE,and ENSO activity was enhanced by 39% relative to 1980–2014 CE.A total of 13 El Ni（?）o events and12 La Ni（?）a events were identified during 1149–1205 CE,and ENSO activity was enhanced by 61% relative to modern times.（4）ENSO variability during the special periods of the last 2000 years appeared fluctuations,which may be closely related to the internal dynamics of the climate system and natural external climate forcings.The results of 30-year sliding window demonstrate a gradually increased frequency of ENSO events during 120–60 BCE.Meanwhile,the intensity of ENSO doubled from a level similar to modern times.From 30 BCE to 23 CE,the frequency of El Ni（?）o events was consistently higher than that of La Ni（?）a events,but the intensity of ENSO decreased to the level equivalent to modern times.Accompanied with the increased frequency of ENSO events during 98–150 CE,the intensity of ENSO increased to 30–40% higher than it is currently.The intensity of ENSO during 150–170 CE dropped to 30–40% lower than the present level.Then,ENSO activity has gradually risen to the modern level during 170–190 CE,followed by a downward trend during 190–210 CE.From 1149 CE to 1205 CE,the intensity of ENSO increased gradually to 60% higher than that of modern times.Such scenarios were accompanied by an increased frequency of El Ni（?）o events.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 typical warm periods of the last 2000 years,we hypothesized that the internal dynamics of the climate system play a prominent role in modulating ENSO variability and its evolution.However,ENSO variability during 665–700 CE was 5.6% lower than it is currently and remained relatively stable with frequent El Ni（?）o events,while ENSO variability increased rapidly after 700 CE and reached a level double that of the present at749 CE,accompanied with the frequent La Ni（?）a events.This abrupt transformation may be attributed to the increased natural radiation forcing induced by the sudden increase in solar irradiance and decrease in volcanic activity,which may have initiated a dynamic response through the ocean thermostat mechanism,leading to an abrupt increase in ENSO variability and a La Ni（?）a anomaly in the tropical Pacific.（5）ENSO activity over the last 2000 years played a leading role in steering short-term changes in seasonal SST in the northern South China Sea.The relatively stronger ENSO activity and more frequent El Ni（?）o events during 30 BCE–23 CE weakened the amplitude of seasonal SST in the northern South China Sea;while the amplitude of seasonal SST during 120–60BCE,98–210 CE,665–749 CE,and 1149–1205 CE enhanced due to the similar frequency of El Ni（?）o and La Ni（?）a events.（6）The SSS changes in the northern South China Sea during the typical warm periods of the last 2000 years may be contributed to ENSO activity.The stronger El Ni（?）o activity during 1149–1205 CE resulted in relatively high SSS and coral（35）δ¹⁸O in the northern South China Sea by weakening the Walker circulation and strengthening the South China Sea Throughflow.While the relatively weaker ENSO activity during 120–60 BCE resulted in relatively low SSS and coral（35）δ¹⁸O in the northern South China Sea.（7）The meridional spatial pattern of moisture variation over East Asia during the typical warm periods of the last 2000 years revealed the modulation East Asian summer monsoon strength by ENSO.Synthesizing SSS records revealed by coral（35）δ¹⁸O from the Xisha Islands with other published precipitation reconstructions from eastern China revealed a meridional dipole spatial pattern of moisture variation over East Asia during the typical warm periods,The Medieval Climate Anomaly（MCA）appeared a spatial pattern of“dry south and wet north” in East Asia,while the Roman Warm Period（RWP）showed a spatial pattern of “wet south and dry north” in East Asia.This was closely related to the strong El Ni（?）o activity during the MCA,which probably enhanced the East Asian summer monsoon（EASM）and caused a northward shift of the intertropical convergence zone（ITCZ）,possibly resulting in dry south and wet north in East Asian Monsoon region.The relatively weak ENSO activity during the RWP weakened the intensity of the EASM and shifted the ITCZ southward,possibly leading to wet south and dry north in East Asian monsoon region.In conclusion,coral Sr/Ca and δ¹⁸O records from the Xisha Islands revealed that mean SST in the northern South China Sea during the special periods of the last 2000 years did not exceed that of modern times.Seasonal SST was similar to that of the present,and its fluctuations were mainly modulated by ENSO activity.Under the combined influence of internal dynamics of the climate system and natural external forcings,ENSO variability during the special periods of the last 2000 years was stronger and varied.Among them,ENSO activity during the typical warm periods affected the variations of SSS in the northern South China Sea and the meridional distribution of moisture over East Asian monsoon region by modulating the intensity of EASM.We speculate that in the context of intensified global warming caused by human activities,the frequency of extreme El Ni（?）o and La Ni（?）a events would strengthen and ENSO variability would increase in the future.Therefore,the climate in the northern South China Sea and even in East Asia would become more variable and complex.Moreover,coral reef ecosystem would be confronted with more climate stress.