Collaborative Research: Assessing the Simulated Arctic Freshwater System in CMIP5 Models, the CESM Large Ensemble, and Forced Simulations

北极淡水（FW）系统可能在未来几十年发生相当大的变化，对海洋环流、气候和生态系统产生下游影响。理解和预测这些影响的能力反过来将对渔业、能源分配和再保险行业等产生重要的社会经济影响。然而，气候预估的可靠性是不确定的，而且对在人为强迫变化中自然变率的重要性的认识也很有限。本项目旨在通过一系列的建模实验以及将模型结果与观测和再分析数据集进行比较来解决这些问题。该项目将通过在职业生涯的形成阶段为年轻、早期职业生涯的女性PI提供支持，并为研究生的培训提供支持，从而促进STEM劳动力的发展。PI将利用她与国家大气研究中心的联系，将项目结果纳入他们面向公众的外展展览。研究结果也将包括在公开演讲和大学讲座中。本项目将研究以下重大科学问题：气候模式是否显示出21世纪及以后预测的北冰洋FW变化的一致图景？模式差异是否有明显的原因（如分辨率、海冰模式复杂性、通过加拿大北极群岛的海峡数量等）？在气候模拟中，北极FW预算的不同项的内部变率的作用有多大？这将如何影响不同时间尺度下未来预测的不确定性？一个集合需要多大才能捕捉到内部变率的大小？Fram海峡和开关站地区的模拟FW组件与过去十年的观测结果一致吗？是什么因素导致了不同来源的FW变化？为了解决这些问题，首先将评估20世纪和21世纪的耦合模式比较项目第5阶段（CMIP5）模拟套件中的北冰洋淡水（FW）预算，然后分析北极淡水预算项内部变率在社区地球系统模式（CESM）模拟的大集合中的作用。最后，将在CESM中比较Fram海峡和北极开关站区域不同源FW的贡献与使用新的模拟FW示踪剂的观测结果，并研究导致不同源FW变率的北冰洋动力学。尽管与CMIP3相比，由于北极海冰模拟的改进和加拿大北极群岛增加的开放通道，可以预期北冰洋FW预算的变化，但参与CMIP5的新一代气候模式尚未对北冰洋FW预算进行分析。使用大型CESM集合提供了一个独特的机会来评估北极FW预算方面的内部变率的作用，并将其与CMIP5的跨模式分散进行比较，将使我们能够深入了解未来预估中不确定性的来源。此外，在CESM中使用FW示踪剂将允许与一组新的观测结果进行更详细的比较，并将能够分析导致在北极两个关键地区从不同来源观测到的FW年际变化的动力学。

The Arctic freshwater (FW) system is likely to undergo considerable change in coming decades with downstream impacts on ocean circulation, climate, and ecosystems. An ability to understand and project these impacts, in turn, will have important socio-economic influences on fisheries, energy distribution, and reinsurance industries, amongst others. However, the reliability of climate projections is uncertain and there is only a limited understanding of the importance of natural variability in the midst of anthropogenically forced change. This project seeks to address these issues through a series of modeling experiments and comparison of the model results with observational and reanalysis data sets. This project will contribute to STEM workforce development through provision of support to a young, early-career, female PI during the formative years of her career and support for the training of a graduate student. The PI will leverage her connections with the National Center for Atmospheric Research to include project results in their outreach exhibits for the general public. Results will also be included in public talks and university lectures.This project will investigate the following major science questions:1. Do climate models show a consistent picture of projected Arctic Ocean FW changes during the 21st century and beyond? Are there obvious reasons for the model differences (e.g., resolution, sea ice model complexity, number of straits through the Canadian Arctic Archipelago, etc.)?2. How large is the role of internal variability for different terms of the Arctic FW budget in climate simulations? How does this affect uncertainty in future projections at various timescales? How large does an ensemble need to be in order to capture the magnitude of internal variability?3. How well do simulated FW components in Fram Strait and the switchyard region agree with observations from the last decade? What are the dynamics that lead to changes in FW from different sources?To address these questions, first the Arctic Ocean freshwater (FW) budget in the suite of the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations for the 20th and 21st century will be assessed, followed by an analysis of the role of internal variability of terms of the Arctic FW budget in a large ensemble of simulations from the Community Earth System Model (CESM). Finally, the contribution of FW from different sources in Fram Strait and the Arctic switchyard region in the CESM will be compared with observations using new simulated FW tracers, and the Arctic Ocean dynamics leading to the variability of FW from different sources will be investigated. The Arctic Ocean FW budget has not been analyzed for the new generation of climate models participating in CMIP5, despite the changes that can be expected compared to CMIP3 due to the improvements to the simulation of the Arctic sea ice and the additional open passages in the Canadian Arctic Archipelago. The use of the large CESM ensemble provides an unique opportunity to assess the role of internal variability in terms of the Arctic FW budget, and comparisons to across-model scatter from CMIP5 will allow insight on the sources of uncertainty in future projections. Furthermore, the use of FW tracers in the CESM will allow a more detailed comparison with a new set of observations and will enable analysis of the dynamics that lead to the observed interannual variability of FW from different sources in two key regions in the Arctic.