OCE-PRF: Steps Toward Quantitative ENSO Reconstruction: Modeling Ocean Circulation Near Coral Proxy Sites

概述：厄尔尼诺/南方涛动(ENSO)是太平洋气候变化的主要模式，通常根据热带珊瑚的氧同位素比率(O-18增量)重建。这些记录为气候模型的独立验证提供了关键信息，因为珊瑚三角洲O-18是仅有的高分辨率热带指标之一。但由于涉及的工作量和费用，许多重建只使用了少数几个珊瑚记录；然后，要根据ENSO行为解释O-18增量变化，必须根据当地环境条件(如模型所代表的)和珊瑚礁上的同位素信号进行转换。在过去，这是通过对来自网格仪器数据的温度和盐度进行线性校准来完成的，但与ENSO幅度相关的误差是巨大的；事实上，如此之大，以至于很难确定对模型物理的具体改进。转换误差是由于以下几个因素的组合造成的：观测产品中的不确定性、珊瑚礁局部的小尺度效应(涡流/混合)或未考虑到的大气影响。在这个项目中，研究员将在一个现实的“完美模式”框架下，通过在各种空间尺度上量化ENSO特性和增量O-18异常之间的关系，来评估每个过程的贡献。这一结果将改善未来的模型验证工作和现有珊瑚三角洲O-18记录的可解释性。智力价值：为这项研究开发的实验将在一个同位素启用的耦合大气环流模型(GCM)中嵌套一个区域海洋模型，该模型被约束为与20世纪的观测相匹配。这将提供一个空间上完整的三角洲O-18数据集，其中包含几个不同网格尺度的信息，这将作为一种20世纪的再分析？O-18三角洲；成果将作为公共资源提供给感兴趣的研究人员。区域范围将包括巴尔米拉环礁和基里蒂马蒂岛，这两个赤道中部太平洋珊瑚环礁被广泛用于古气候研究，对ENSO变异性高度敏感，并受到热带和亚热带水团之间有趣的动态交换的影响。区域模拟除了提供有关20世纪厄尔尼诺/拉尼娜事件对珊瑚礁环境的海洋学影响的详细信息外，还将提供关于热带太平洋环礁周围环流的首批基于模型的综合特征之一。Palmyra和Kiritimati也都有大量的当地环境监测数据，这些数据将用于模型验证。通过构建三角洲O-18的预算，将在全球和区域模式中诊断影响海水三角洲O-18的主要过程。这将决定，作为空间尺度的函数，哪些过程对用于代理收集的站点附近的增量O-18异常最重要。然后，这些结果可以用于未来针对代理数据验证GCM的工作：是否有必要运行同位素启用的模型，如果是，分辨率是什么？德尔塔O-18的预算还将用于评估化石珊瑚可能如何记录厄尔尼诺/拉尼娜事件的不同性质(即幅度、冷/暖不对称)，这最终可以为改进GCM的物理参数提供指导。更广泛的影响：该项目代表了一种全新的、高度跨学科的定量古气候重建方法，其结果将适用于各种不同的应用；所有模型输出将向公众公布。与NOAA太平洋岛屿渔业科学中心的合作将确保研究结果与生态应用相关，与太平洋海洋科学教育卓越中心(COSEE-Pacific)合作，在瓦胡岛周围未被充分代表的学生群体中开展的外展工作将有助于提高公众对珊瑚礁保护的认识。

Overview: Reconstructions of the El Nino/Southern Oscillation (ENSO), the dominant mode of Pacific climate variability, are often created from the oxygen isotopic ratio (delta O-18) in tropical corals. These records provide crucial information for independent validation of climate models, as coral delta O-18 is one of the only high-resolution tropical proxies available. But because of the effort and expense involved, many reconstructions use only a handful of coral records; then to interpret delta O-18 variability in terms of ENSO behavior, a conversion must be made from local environmental conditions (as represented by the model) and the isotopic signal on the reef. In the past, this has been done using a linear calibration against temperature and salinity derived from gridded instrumental data, but the associated errors in ENSO amplitude are enormous; so large, in fact, that it becomes extremely difficult to identify specific improvements to model physics. The conversion errors are due to some combination of: uncertainties in observational products, small-scale effects (eddies/mixing) local to the reef, or unaccounted-for atmospheric influences. In this project, the fellow will assess the contribution of each of these processes, by quantifying the relationship between ENSO properties and delta O-18 anomalies on a variety of spatial scales in a realistic, "perfect-model" framework. The results will improve both future model validation efforts and the interpretability of existing coral delta O-18 records. Intellectual Merit: The experiments developed for this study will nest a regional ocean model within an isotope-enabled, coupled general circulation model (GCM) constrained to match 20th century observations. This will provide a spatially complete delta O-18 dataset with information on several different grid scales, which will function as a sort of 20th century ?reanalysis? of delta O-18; output will be provided as a public resource for interested researchers. The regional domain will cover both Palmyra Atoll and Kiritimati (Christmas) Island, two central equatorial Pacific coral atolls which have been used extensively for paleoclimate research, are highly sensitive to ENSO variability, and are influenced by dynamically interesting exchanges between tropical and subtropical water masses. The regional simulations will provide one of the first comprehensive model-based characterizations of circulation around tropical Pacific atolls, in addition to providing detailed information on the oceanographic impacts of 20th century El Nino/La Nina events on coral reef environments. Both Palmyra and Kiritimati also have extensive local environmental monitoring data available, which will be used for model validation. The dominant processes affecting seawater delta O-18 will be diagnosed in both the global and regional models, by constructing budgets of delta O-18. This will determine, as a function of spatial scale, which processes are most important for delta O-18 anomalies near sites used for proxy collection. The results can then be used during future efforts to validate GCMs against proxy data: is running an isotope-enabled model necessary, and if so at what resolution? The delta O-18 budgets will also be used to assess how different properties of El Nino/La Nina events (i.e. amplitude, warm/cold asymmetry) might be recorded by fossil corals, which can eventually provide guidance for improvements to physical parameterizations in GCMs. Broader Impacts: This project represents a fundamentally new and highly interdisciplinary approach to quantitative paleoclimate reconstruction, and the results will be useful for a wide variety of applications; all model output will be made publicly available. Collaboration with the NOAA Pacific Islands Fisheries Science Center will ensure that the results are relevant for ecological applications, and outreach work with underrepresented student populations around Oahu in collaboration with the Pacific Center for Ocean Science Education Excellence (COSEE-Pacific) will help to raise public awareness of coral reef conservation.Sponsoring Scientists: Brian Powell (U. Hawaii; lead sponsor), Russell Brainard (NOAA PIFSC), Kim Cobb (Georgia Tech), Mark Merrifield (U. Hawaii), David Noone (CU-Boulder)