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The 4.2 ka BP Event is an abrupt climate change that might have contributed to the collapse of ancient civilizations and marks the transition between the mid‐ and late‐Holocene. Despite considerable research on this event, our understanding remains primarily based on terrestrial paleoclimate reconstructions, leaving a significant gap in understanding the role of the ocean in this event. Here, we present paired sea surface temperature (SST) and seawater δ18O reconstructions based on four fossil corals from the South China Sea. Our results demonstrate that the climate during the event was cooler, and there were meridional dry‐wet patterns in East Asia, indicating a weakened summer monsoon. Furthermore, our examination of additional coral records from the Pacific and Indian Oceans suggests that low‐latitude forcing (i.e., SST anomalies in the tropical Pacific) plays a crucial role in driving hydrology shifts in East Asia over the 4.2 ka BP interval. Plain Language Summary: Around 4,200 years ago, a major climate event occurred, resulting in a global megadrought that might have caused the collapse of ancient civilizations. However, most evidence for this event comes from land‐based paleoclimate studies, leaving a gap in our understanding of how it impacted the ocean. In this study, we analyzed fossil coral samples from the South China Sea and found that the area was cooler and wetter during the event. While the cooling appears to have been consistent across East Asia, the hydrological changes exhibited a meridional pattern, with Central China also experiencing wetter conditions while North and South China were dry. By examining coral records from the Pacific and Indian Oceans, we concluded that the Central Pacific El Niño was likely occurring during this event, weakening the East Asian Summer Monsoon and causing the associated climate and hydrology changes in East Asia. Key Points: Paired fossil coral Sr/Ca and δ18O records indicate a cold and wet climate in the northern South China Sea during the 4.2 ka BP eventCoral and terrestrial reconstructions reveal consistent cooling but meridionally‐varied hydrological changes in East Asia around 4.2 ka BPLow‐latitude sea surface temperature anomalies in tropical Pacific significantly contribute to the shift in hydroclimate in East Asia [ABSTRACT FROM AUTHOR]

The South China Sea (SCS) is the major source of water vapor to China and East Asia. Modern observations, numerical simulations, and paleoclimate records suggest that on seasonal and annual timescales, variability in SCS precipitation is controlled mainly by the East Asian Monsoon and El Niño–Southern Oscillation (ENSO), respectively. Nevertheless, due to the lack of long-term observational records, our understanding of its decadal variability is limited. Because the understanding of decadal variability in SCS precipitation is fundamental in accurately predicting precipitation patterns in China and East Asia using climate models, it is necessary to obtain preinstrumental records of precipitation using climatic proxies in high resolution archives. We demonstrate the potential of coral Δδ18O, derived from Sr/Ca and δ18O measurements in the coral skeleton, as a proxy for precipitation in the SCS on different timescales. We then use this proxy to reconstruct decadal variability throughout the 20th century. The results suggest that the monsoon, Pacific Decadal Oscillation, ENSO, and Atlantic Multidecadal Oscillation are not significant forcings of decadal variability in precipitation over the SCS. In constrast, the North Atlantic Oscillation (NAO) might have modulated the number of tropical cyclones passing through the SCS and induced more (fewer) tropical cyclones and more (less) precipitation during its negative (positive) phases on decadal timescales during the past century. However, future numerical modeling studies are still required to improve our understanding of the underlying mechanism behind the relationship between the NAO and decadal variability in SCS precipitation. • Coral Δδ18O records can be used to reconstruct decadal variability in precipitation in the SCS during the 20th century. • The monsoon, PDO, ENSO, and AMO are not significant forcings of decadal variability in precipitation over the SCS. • The dominant control on decadal variability in SCS precipitation is the NAO via modulation of tropical cyclone activity. [ABSTRACT FROM AUTHOR]