Fresh water content and circulation variability in the North Atlantic: Are they related?

观测记录显示了亚极淡水储量和环流强度的大振幅、年代际到多年代际变化的明确证据，但观测记录的长度几乎不足以测量单个50年周期。直接测量大西洋经向翻转环流（AMOC）的时间甚至更短，而且肯定不够长，无法确定相关行为。大量的观测和模型研究都集中在这一主题的各个方面，但尚未对海洋系统的这一部分如何运作形成统一的图景。对观测到的和模拟的北大西洋FW含量和环流变化的原因和后果的不同解释，促使人们试图更好地对它们进行分类。一个模型吗?提出了一项资料比对研究，研究北大西洋三维环流（环流和翻转）变率和性质通量对FW含量的时间、幅度和变异性的影响。将开发一套分析指标，并一致地应用于四个耦合模型中的每一个？长期控制运行，以表征其海洋FW和热量收支，并评估变化的潜在机制。工作假设是，最近观测到的北大西洋FW含量的变化反映了这部分气候系统的自然变率。此外，FW内容有时也会起到间接或被动的作用。北大西洋的作用（即大气强迫控制海洋FW含量和环流变率）；而在其他时候，盐度直接影响环流（即高纬度FW含量增加（减少）减弱（加强）次极地海洋环流）。科学价值：通过比较几个备受推崇的耦合气候模式模拟和一致的观测结果，所提出的工作将阐明北大西洋次极区体积FW异常与全三维环流和翻转环流强度之间的联系。它将为已知表现出大波动的气候系统的一部分的内部变率的时间尺度和机制提供新的视角。它还将更新在北大西洋观测到的FW和热含量变化的时间序列，对它们的不确定性提供有意义的估计，并阐明最近观测到的波动在多大程度上落在自然变率范围内，或在多大程度上反映了某种其他强迫的后果。更广泛的影响：建议的工作旨在开发和推广一套诊断工具，这些工具将在各种耦合模式模拟中一致有效地评估属性含量和海洋环流。这些工具可以扩展到全球海洋的其他部分。它们还可以帮助识别模型中的优缺点。表示过程、参数化和反馈。具有空间误差协方差的气候学产品的构建将更好地约束估算热量和水分含量变率的不确定性，并对如何解释这些变化产生重大影响。软件的开发将应用高斯-马尔可夫技术来估计北大西洋工作中的平均和误差协方差场，然后可以很容易地应用于全球水文气候学。诸如ECCO （?）对海洋环流和气候的估计将直接受益，因为对于状态估计的目的，空间误差协方差的知识是必不可少的；然而，在大多数可用的气候学产品中，这仍然不完全（或根本没有）解决。该项目是对美国CLIVAR（气候变率和可预测性）计划和海洋研究优先计划（近期优先研究大西洋经向翻转环流）的贡献。

The observational record exhibits clear evidence of large amplitude, decadal to multi-decadal variability in sub-polar fresh water (FW) storage and gyre strength, but is barely long enough to have measured a single 50-year cycle. The Atlantic Meridional Overturning Circulation (AMOC) has been directly measured for even less time, and certainly not long enough to identify correlative behavior. A substantial number of observational and model studies have focused on various aspects of this topic, but have not yet produced a unified picture of how this part of the ocean system operates. The disparate range of interpretations for the causes and consequences of observed and modeled North Atlantic FW content and circulation variability has motivated an attempt to better sort them out. A model?data intercomparison study is proposed which will investigate the timing, amplitude and variability of FW content in relation to the full three-dimensional (gyre and overturning) circulation variability and property fluxes in the North Atlantic. A suite of analytical indices will be developed and consistently applied to each of four coupled model?s long control runs to characterize their oceanic FW and heat budgets and assess underlying mechanisms of variability. The working hypothesis is that the recently observed changes in North Atlantic FW content reflect natural variability in this part of the climate system. Moreover, there are times when FW content plays an indirect or ?passive? role in the North Atlantic (i.e. the atmospheric forcing controls the ocean FW content and circulation variability); while at other times, salinity directly influences the circulation (i.e. increased (decreased) high latitude FW content weakens (intensifies) the sub-polar ocean circulation).Scientific Merit: By comparing several well-respected coupled climate model simulations and observations in a consistent manner, the proposed work will clarify connections between volumetric FW anomalies in the sub-polar North Atlantic and strength of the full three-dimensional gyre and overturning circulations. It will provide new perspectives regarding the timescales and mechanisms of internal variability in a part of the climate system known to exhibit large fluctuations. It will also update the time series of observed FW and heat content changes in the North Atlantic, provide meaningful estimates of their uncertainties, and elucidate the degree to which the recently observed fluctuations fall within the range of natural variability or alternatively reflect a consequence of some other forcing.Broader Impacts: The proposed work aims to develop and promote a set of diagnostic tools which will consistently and efficiently evaluate property content and ocean circulation in various coupled model simulations. These tools may be extended to other parts of the global ocean. They may also help to identify strengths and weaknesses in models? representations of processes, parameterizations and feedbacks. Construction of climatology products with spatial error covariances will better constrain the uncertainty in estimates of heat and FW content variability with substantial impact on how these are interpreted. Development of software, which will apply Gauss-Markov techniques to estimating mean and error covariance fields in the proposed North Atlantic work, can then readily be applied to global hydrographic climatologies. Assimilation efforts such as ECCO (?Estimating the Circulation and Climateof the Ocean?) will benefit directly because, for the purpose of state estimation, knowledge of the spatial error covariance is essential; yet this remains incompletely (or not at all) addressed within most available climatology products.This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) Program and the Ocean Research Priorities Plan (near-term priority on the Atlantic Meridional Overturning Circulation).