Co‐Intensification of the East Asian Winter Monsoon and ENSO During the Mid‐Holocene Transition: Coral Evidence From the Northern South China Sea

Abstract

Reconstructing the interactions between El Niño–Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) during the mid‐to late‐Holocene transition is essential for understanding internal climate variability. We present a monthly resolved coral Sr/Ca–derived sea surface temperature (SST) record from the northern South China Sea spanning 3854–3800 years before present (BP). Our results indicate that the region was ∼3.6°C cooler with significantly amplified seasonality. The high‐frequency interannual winter SST variability was ∼54% higher than modern levels, robustly indicating substantially intensified winter cooling, likely driven by enhanced EAWM cold surges. Concurrently, interannual ENSO variance was amplified by ∼57% relative to a modern comparative baseline. Synthesizing paleoclimate records across the tropical Pacific confirms this period marked a transition from a La Niña‐to an El Niño‐like mean state during 4400–3700 years BP. Rather than a one‐way driver, we propose a synergistic ocean‐atmosphere reorganization. Orbitally induced high‐latitude background cooling and the African Humid Period termination collectively favored both an intensified EAWM and shifted tropical Pacific conditions. Within this framework, the EAWM functioned as a dynamic amplifier: frequent cold surges facilitated extreme El Niño events, which progressively shifted the long‐term Pacific mean state through non‐linear rectification. Ultimately, this co‐evolution provides a mechanistic context for the widespread climatic disruptions associated with the 4.2 ka event.