Stable isotope ratios on foraminifera from Site 138-846@en

There is an urgent requirement to understand how large fluctuations in tropical heat distribution associated with the El Niño-Southern Oscillation (ENSO) will respond to anthropogenic emissions of greenhouse gases. Intervals of global warmth in Earth history provide a unique natural laboratory to explore the behaviour of the ENSO in a warmer world. To investigate interannual climatic variability, specifically ENSO, in the mid-Piacenzian Warm Period (mPWP: 3.26 - 3.03 Ma), we integrate observations from the stable isotopes of multiple individual planktonic foraminifera from three different species from the eastern equatorial Pacific (EEP) with ENSO simulations from HadCM3, a fully coupled ocean-atmosphere climate model. Our proxy data and model outputs show persistent inter-annual variability during the mPWP caused by a fluctuating thermocline, despite a deeper thermocline and reduced upwelling. We show that the likely cause of the deeper thermocline is due to warmer equatorial undercurrents rather than reduced physical upwelling. We conclude that the mPWP was characterized by ENSO related variability around a mean state akin to a modern El Niño event. Furthermore, HadCM3 predicts that the warmer Pliocene world is characterized by a more periodic, regular amplitude ENSO fluctuation, suggestive that the larger and deeper west Pacific warm pool is more easily destabilized eastwards. These conclusions are comparable to the observed trend over the last forty years to more regular and intense ENSO events. Future research must resolve whether global warming alone, or in concert with tectonic factors, was sufficient to alter ENSO variability during warm intervals of the Pliocene.当前存在迫切需求，以厘清与厄尔尼诺-南方涛动（El Niño-Southern Oscillation, ENSO）相关的热带热量分布大幅波动，将如何响应人类活动排放的温室气体。地球历史上的全球温暖期，为探究暖化环境下ENSO的活动特征提供了独一无二的天然实验平台。为研究皮亚琴察期中期暖期（mid-Piacenzian Warm Period, mPWP：3.26~3.03 Ma）的年际气候变率，尤其是ENSO活动，我们整合了来自东赤道太平洋（eastern equatorial Pacific, EEP）3个不同种属的多颗浮游有孔虫的稳定同位素观测数据，以及完全耦合海气气候模式HadCM3的ENSO模拟结果。本研究的代用资料与模式结果表明，尽管温跃层加深且上升流减弱，皮亚琴察期中期暖期仍存在由波动温跃层驱动的持续年际变率。研究揭示，温跃层加深的潜在成因并非物理上升流减弱，而是赤道下层海流温度升高。综上，皮亚琴察期中期暖期的ENSO相关变率以类似现代厄尔尼诺事件的平均状态为核心特征。此外，HadCM3模式模拟显示，暖化的上新世环境中，ENSO波动具有更强的周期性与更规则的振幅，这表明面积更大、深度更深的西太平洋暖池更易向东发生失稳。上述结论与过去40年观测到的ENSO事件愈发规则且强度更高的变化趋势高度吻合。未来研究需厘清：仅全球变暖，还是其与构造因素共同作用，足以改变上新世温暖期的ENSO变率。