岁差驱动的热带太平洋类ENSO型古气候响应

Previous paleoclimate studies of El Nino-Southern Oscillation (ENSO) mainly focus on the interannual variability and background features of ENSO-like sea surface temperature in the tropical Pacific, but without enough attention to the anomalous annual cycles of the Pacific upper ocean temperature (especially the thermocline water temperature). Our project will deeply trace responses of the tropical Pacific ENSO-like paleoclimate induced by precession, which will be centered as our fundamental scientific question. Using precessional insolation of different spatial-temporal distributions as an external forcing, we will conduct transient accelerated simulations on orbital time scales and equilibrium simulations on special time slices by multiple coupling models of different complexity. We will study the anomalous annual /seasonal mean background states and annual cycles of paleo-ENSO forced by precessional insolation changes, and compare them with paleoceanographic and paleoclimate reconstructions. Particularly, we aim to highlight the anomalous upper water structure of the Pacific upper water temperature, the zonal/meridional transportation of subsurface water temperature anomalies, and their linkage with the surrounding Asian-Indian-Pacific paleoclimate. Our project will provide scientific evidence for the physical mechanism of the Pacific air-ocean coupled system responding to precessional insolation on orbital time scales. Moreover, we will simulate the transient changes of oxygen isotope ratio of sea water through iLOVECLIM model under precessional insolation forcing. By comparing paleoceanographic reconstructions with the seasonal/annual mean results from transient simulations, we will qualitatively evaluate the seasonality of different proxies and rectify deviations in interpreting these proxies， which will provide new knowledge for the future application of these paleoceanographic proxies.

以往的热带太平洋ENSO古气候研究大多关注热带太平洋表层海温的年平均背景态和年际 ENSO变率特征，却忽视了上层水体温度（特别是次表层水温度）的annual循环变化。本研究将围绕关键科学问题“赤道太平洋古ENSO在岁差周期上的变化特征和物理机制到底如何？”深入追溯，用多个海气耦合模式进行瞬变加速模拟或平衡态模拟，系统分析岁差太阳辐射量驱动下古ENSO的annual循环响应特征（特别是次表层海温异常的经/纬向传输过程），探索不同季节的古ENSO响应与亚-印-太区域古气候的大气遥相关联系，试图建立轨道时间尺度上太平洋海气耦合过程对岁差太阳辐射量的物理响应模型，此外还将不同季节／年平均的瞬变模拟结果与古海洋重建记录对比，并利用iLOVECLIM模式直接模拟岁差驱动的海水氧同位素变化，定性评估多项古ENSO替代指标的季节偏向性、减少其解释误差，为古海洋学替代指标的未来应用提供新认识。

厄尔尼诺－南方涛动（ENSO）是现代年际尺度气候变率的主要来源，本项目立足于“气候演变的低纬驱动”这一地球科学领域的前沿热点，围绕关键科学问题“岁差驱动的热带太平洋类ENSO型古气候响应特征和物理机制”，进行了不同复杂程度的海气耦合模拟和古海洋重建资料校验，系统地分析了赤道太平洋古ENSO季节循环与年平均背景态的特征表现，定量评估了不同海气耦合反馈过程的相对贡献。结果表明，岁差辐射量的季节循环可简化为两个正交模态（分点岁差与至点岁差），两者都能驱动赤道太平洋表层海温季节循环的纬向三极型异常格局（类似于现代中太平洋型ENSO），并伴有相似的内部反馈机制，其中赤道温跃层温度的纬向平流反馈和上升流反馈至关重要；但两类岁差驱动的海气异常响应分别集中于春秋季和冬夏季，对赤道太平洋年平均气候有不同影响。我们进一步检验了热带古ENSO与热带以外气候之间的“海洋隧道”联系假设，首次提出岁差至半岁差周期上，印太暖池区温跃层与南、北半球副热带表层水之间存在类似于现代温跃层环流圈（STC）的传输通道，温跃层的温度／厚度决定了暖池上层海洋热含量变化，是经向STC调控纬向ENSO的枢纽。此外，在30万年以来轨道辐射量驱动下，我们利用包含氧同位素的GISS-modelE2模式，首次获得大气降水和海水氧同位素的较高分辨率瞬变模拟结果，提取了岁差驱动的全球季风／Hadley环流的水循环特征响应，据此凝练出气候演变的“Steam Heat Engine”理论模型：太阳辐射量产生经向加热梯度，驱动大气Hadley环流与海洋STC，进而改变暖池温跃层热含量、调控纬向ENSO过程，同时季风／Hadley环流的水蒸汽循环产生潜热反馈，构成气候演变的“蒸汽型热机引擎”。上述研究成果不仅为古海洋替代指标的季节偏向性研究提供了数值模拟依据，也可以“以古论今、论未来”，对全球变暖背景下的长期气候响应有重要理论指导价值。