“ENSO-Modoki” on the rise under global warming conditions? A view from the past

最近的观测研究表明，除了在东赤道太平洋（EEP）或西赤道太平洋（WEP）以海表温度异常升高和温跃层加深为特征的“经典”厄尔Niño南方涛动（ENSO）外，第三种类型的ENSO已经变得更加突出。在这些事件期间，海温异常位于中赤道太平洋（CEP），导致交替的ENSO状态，称为“El Niño/La Niña Modoki”（或“中太平洋ENSO”）。在El Niño Modoki事件期间，变暖和温跃层加深发生在CEP，而变冷集中在CEP和WEP （La Niña Modoki事件反之）。这导致双沃克环流单元及其共同的上升气流分支以CEP为中心，在热带南美洲和西太平洋地区造成异常干旱条件。相关的全球后果包括飓风袭击墨西哥湾的频率增加，以及两个半球的亚极地地区变暖加剧，可能加速高纬度冰盖的消亡。为了揭示ENSO Modoki的潜在进一步演变，拟议的项目旨在从古角度提供见解。这将通过评估ENSO Modoki样气候状态在不同气候背景状态下的发生情况来完成，这些气候背景状态涉及(i)二氧化碳分压，（ii）全球冰量和（iii）日照，这些因素对经典ENSO和ENSO Modoki动力学有充分的影响。为此，我们将从WEP（站点U1488）、CEP（站点871）和EEP（站点846）上生成ODP/IODP站点的高分辨率海表温度（SST）和次表层温度（subT）变化代用记录，覆盖300 kyr长的时间片，具有明显不同的气候背景条件。(1)中更新世至全新世（0-300 ka）， pCO2浓度相对较低，北半球冰量较高；(2)上新世早期（4.85-5.15 Ma）， pCO2条件与现代相似，北半球基本没有冰盖。这些海温和亚温度记录将基于表层和深层浮游有孔虫的Mg/Ca分析，以及基于烯酮的海温估计（取决于以前研究中每个地点使用的方法，以确保一致性）。海温记录将用于计算过去厄尔尼诺Modoki变率的指数，而subT数据将为相关的温跃层变率提供信息。结合已发表的二氧化碳分压、冰体积和日照的参考记录，我们的代理数据将利用统计方法（线性模型）进行评估。该项目的结果将为厄尔尼诺Modoki变率的驱动因素提供新的见解，并有助于预测其变率对未来全球变暖的响应。

Recent observational studies have shown that aside of the “classical” El Niño Southern Oscillation (ENSO), which is characterized by anomalous sea surface temperature (SST) warming and thermocline deepening either in the Eastern Equatorial Pacific (EEP) or Western Equatorial Pacific (WEP), a third type of ENSO has become more prominent. During these events SST anomalies are located in the Central Equatorial Pacific (CEP) leading to alternate ENSO states coined “El Niño/La Niña Modoki” (or “Central Pacific ENSO”). During El Niño Modoki events, warming and thermocline deepening occurs in the CEP, while the cooling is centered in the CEP and WEP (vice versa for La Niña Modoki events). This leads to twin Walker circulation cells with their common updraft branch centered over the CEP causing anomalously arid conditions in both tropical South America and the western Pacific realm. Related global consequences include a higher frequency of hurricanes hitting the Gulf of Mexico as well increased warming in the subpolar realms of both hemispheres potentially accelerating high latitude ice sheet demise.To shed light onto the potential further evolution of ENSO Modoki, the proposed project aims to provide insights from a paleo-perspective. This will be done via assessing the occurrence of ENSO Modoki-like climatic states during varying climatic background states regarding (i) pCO2, (ii) global ice volume and (iii) insolation, factors with a well-documented impact on classical ENSO as well as ENSO Modoki dynamics. For this purpose, we will generate high-resolution proxy records of sea surface temperature (SST) and subsurface temperature (subT) variability on ODP/IODP Sites from the WEP (Site U1488), CEP (Site 871), and EEP (Site 846) covering 300 kyr-long time slices with distinctly different climatic background conditions: (i) the mid-Pleistocene to Holocene (0–300 ka) with relatively low pCO2 concentrations and high Northern Hemisphere ice volume, and (ii) the Early Pliocene (4.85–5.15 Ma) with pCO2 conditions similar to modern and largely absent Northern Hemisphere ice sheets.These SST and subT records will be based on Mg/Ca analysis of surface and deep dwelling planktic foraminifera as well as alkenone-based SST estimates (depending upon the method used on each site during previous studies to ensure consistency). The SST records will be used to compute an index of the past El Nino Modoki variability while the subT data will inform about associated thermocline variability. In the context with published reference records for pCO2, ice volume, and insolation our proxy data will be evaluated utilizing statistical methods (linear models). The results of this project will provide new insights into the drivers of El Nino Modoki variability and aid the predictions of its variability in response to future global warming.