P2C2: Hydroclimatic response of the tropical Pacific to past changes in mean state: Observations and synthesis

厄尔尼诺Niño事件将如何随着气候变化而变化？从21世纪的气候变化预测来看，目前对这个问题的答案还没有达成共识。然而，即使是El Niño事件强度、持续时间或频率的微小变化，也会对美国和国际上的水资源、野火、农业和生态系统产生重大影响。该项目将对厄尔尼诺Niño/南方涛动（ENSO）在过去两个时间间隔内的活动提供新的观测和分析，这两个时间间隔来自热带太平洋的数据很少，但在这两个时间间隔内，全球气候被认为分别相对温暖和相对寒冷。结果将与同一时期的气候模拟进行比较，以便我们能够更好地了解全球气候变化是否、如何以及为什么会导致El Niño活动的变化。该项目还将为一名博士生和一名本科生提供指导、国际、合作和多学科的培训，包括教育、推广和传播活动。它将为在一个具有丰富的前现代定居、航海和探索考古历史的地点和地区建立人、渔业和气候之间相互作用的模型提供输入。这些研究超出了本项目的范围，可能最终帮助我们了解社会如何适应过去和未来的气候变化。具体而言，配对氧同位素和Sr/Ca数据系列将从现有的和成岩筛选的珊瑚样本中开发，这些样本可追溯到小冰川期（LIA）和中世纪气候异常（MCA）时间窗口，来自南库克群岛（18.85S, 159.79W）。初步分析表明，该地区现代（PD）珊瑚氧同位素组成的年际变化主要反映了与ENSO活动相关的南太平洋辐合带位置和强度变化引起的海面盐度（SSS）变化。结合这些时间窗和PD的其他水文气候和海洋古气候数据，研究小组将测试LIA和MCA相对于PD是否存在年际事件振幅和复发间隔的差异。研究小组将把这些结果与类似采样的结果进行比较，这些结果符合pmip3的物理气候模拟，包括完整的和单一的外部辐射强迫。将在观测结果和模拟结果的不确定性范围内进行比较，以评估观测到的水文气候反应是否和在多大程度上是被迫的或非被迫的，以及通过何种机制。

How will El Niño events change as the climate changes? There is currently no consensus on the answer to this question from climate change projections for the 21st century. Yet even small changes in El Niño event strength, duration or frequency have significant effects, both in the US and internationally, on water resources, wildfires, agriculture and ecosystems. This project will contribute new observations and analysis of El Niño/Southern Oscillation (ENSO) activity during two past time intervals for which there are very few data from the tropical Pacific, yet for which the global climate is thought to have been relatively warm and relatively cold, respectively. Results will be compared to climate simulations for the same periods, so that we can better understand if, how and why global climate change can cause changes in El Niño activity. The project will also provide mentored, international, collaborative, and multidisciplinary training for a PhD student and an undergraduate researcher, including educational, outreach and dissemination activities. It will contribute inputs for modeling the interactions between people, fisheries and climate in a location and region with a rich archaeological history of pre-modern settlement, voyaging and exploration. These studies, which are beyond the scope of this project, may ultimately help us understand how societies have and may adapt to past and future climate change.Specifically, paired oxygen isotope and Sr/Ca data series will be developed from existing and diagenetically screened coral samples dating to the Little Ice Age (LIA) and Medieval Climate Anomaly (MCA) time windows from Aitutaki, southern Cook Islands (18.85S, 159.79W). Preliminary analysis suggests interannual variations in present day (PD) coral oxygen isotopic composition from the region primarily reflect sea surface salinity (SSS) variability resulting from changes in the position and strength of the South Pacific Convergence Zone associated with ENSO activity. In conjunction with other hydroclimatic and oceanographic paleoclimate data available for these time windows and the PD, the research team will test whether there were differences in interannual event amplitude and recurrence interval in the LIA and MCA relative to the PD. The team will compare the results to those obtained from similarly sampled, PMIP3-compliant physical climate simulations with both complete and single external radiative forcings. Comparison of observations and simulations, within the context of their uncertainties, will be used to assess if and the extent to which the observed hydroclimatic response is forced or unforced, and by what mechanisms.