Coral perspective on temperature seasonality and interannual variability in the northern South China Sea during the Roman Warm Period

Highlights

• 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

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.

Introduction

As the primary components and physical descriptors of climate systems, seasonality and interannual variability in sea-surface temperature (SST) have dramatic impacts on the Earth's surface system (Denton et al., 2005; Giry et al., 2012; Patterson et al., 2010; Yan et al., 2015a). However, the predictability of SST variation on these timescales remains challenging due in part to the incomplete understanding of its long-term changes and forcing mechanisms, which stem primarily from the insufficient length and spatial coverage of modern observations and reanalysis products for SST (Deser et al., 2010; Smith et al., 2008). Therefore, reconstructions of SST seasonality and interannual variability in the preindustrial era, especially over the last two millennia, are crucial for comprehensively understanding past climate change, assessing current climatic conditions, researching forcing mechanisms, and improving the accuracy of future climatic predictions (Dee et al., 2020; Deng et al., 2019; Ge et al., 2013).

The Roman Warm Period (RWP), also called the Roman Climate Optimum episode (Wang et al., 2012), covers the first centuries of the Common Era (CE) (2200–1550 a BP (years before 1950 CE); Steinke et al., 2014). It has been proposed as one of the most recent two natural warm periods in the last two millennia due to the general warmth in the North Atlantic and Europe (Büntgen et al., 2011; Lamb, 1977; Ljungqvist, 2010; Martín-Puertas et al., 2009). Reconstructions of high-resolution SST during the RWP provide an important benchmark for understanding the influence of external forcings and internal climate variability, distinguishing the relative contribution of natural processes and human actions to climate change, and predicting the likelihood of the occurrence of similar climate variations in the future (Wu et al., 2012). In recent decades, considerable progress has been made in the reconstruction of SST during the RWP (Oppo et al., 2009; Neukom et al., 2019; Wu et al., 2012). However, the temporal and spatial distribution of warm climatic conditions during this period remains controversial. Several paleoclimatic reconstructions indicate that relatively cold periods occurred 2000 years ago (Deng et al., 2019; Moberg et al., 2005). Additionally, an overwhelming majority of high-resolution paleoclimatic records that can quantify SST seasonality were obtained from high latitudes (Wanamaker Jr. et al., 2011; Wang et al., 2012). On interannual timescales, El Niño–Southern Oscillation (ENSO) variability during the RWP also exhibited large uncertainties (Cobb et al., 2013; Conroy et al., 2008; McGregor and Gagan, 2004; Steinke et al., 2014; Yan et al., 2017). These results indicate large gaps in SST variations of the tropical Pacific approximately 2000 years ago and emphasize the necessity of reconstructing additional monthly resolved SSTs.

Massive hermatypic corals are one of the excellent high-resolution paleoclimatic and paleoceanographic archives due to their rapid growth rate (up to 2 cm per year), long lifespan (centuries), and clear annual-density bands (Corrège, 2006; Yu, 2012). Among a remarkable array of geochemical tracers within the coral skeleton, strontium to calcium ratios (Sr/Ca) are a powerful tool for reconstructing past sea surface thermal variations because coral Sr/Ca varies primarily as a function of SST during their growth phases (Corrège, 2006; DeLong et al., 2014; Yu et al., 2005a). Precisely dated coral Sr/Ca time series have the ability to track changes in SST with monthly to annual resolution, allowing for continuous reconstruction of seasonal to multidecadal SST variability that spans several centuries (Gagan et al., 1998; Jiang et al., 2021; Kong et al., 2017; Linsley et al., 2000; Yu et al., 2005b). Furthermore, coral Sr/Ca-SST records can perfectly compensate for deficiencies in the coarse resolution of marine sediment archives (Wu et al., 2012) and shorter lived bivalves (Wang et al., 2012; Yan et al., 2015a).

The South China Sea (SCS), one of the largest marginal seas in the western Pacific that contains abundant corals (Fig. 1), is an ideal area for studying multi-timescale SST variations (Mitsuguchi et al., 2008; Yu, 2012). Previous studies derived from Tridacna gigas have revealed SST seasonality and interannual variability in the northern SCS approximately 2000 years ago (Yan et al., 2015a; Yan et al., 2017). However, these records are insufficient to quantify SST variations over the whole RWP due to the short lifespan inherent in individual proxies. The relationship between seasonality and interannual variability has also not been revealed. In this study, a total of 228-year-long, monthly SST records were generated for snapshots of the RWP, with individual time windows of up to 113 years length, based on three spliced Porites coral Sr/Ca time series from the northern SCS. The main objective was to reveal the characteristics of SST variation on seasonal and interannual timescales, reconstruct past ENSO activity, and explore the relationship between SST seasonality and ENSO variability.