Tracking Subtropical Water Mass Anomalies to the Tropics and Their Potential for Re-emergence

作为全球气候变率的强烈驱动因素的热带海洋如何受到亚热带过程的影响仍未得到解决。热带-亚热带遥相关可以通过大气发生，这可以在一个季节内建立，或者通过地下海洋通道运行数年至数十年。由于水团异常引起的副热带异常热的形成和随后的平流是一条潜在的重要海洋通道。一些理论研究表明，这一过程控制着多年至年代际气候变率。异常的产生有两种方式：在地表由风、热量和淡水通量的变化引起，或在地下由异常平流引起。利用Argo剖面浮标阵列的最新记录，许多区域研究已经确定了大多数海洋盆地中水团异常的形成和传播。现代海洋环流模式研究采用真实的海面强迫可以形成和平流类似于观测到的异常。然而，对于这些异常现象是否能够存活到赤道，然后重新出现在地面上，这两种描述并不一致。该项目将采用全球一致的观测和模拟方法，定量了解亚热带水团异常的来源、命运和可能再次出现的可能性，从而了解它们对气候的影响。作为该项目的一部分，将免费提供一份改进的全球Argo网格分析，特别侧重于水质变化和对已解决尺度和误差的现实估计。将建立一个生产系统，以确保持续更新和分发分析。麻省理工学院- whoi联合项目的研究生将学习数值建模（包括对集合的分析）和观测分析技术。由于目标过程是实现长期气候预报的主要候选过程，因此将通过合作者Peter Oke直接与美国和澳大利亚的气候预报团队讨论结果。该项目将把Argo数据的新分析与OGCM运行的一套定制集成相结合，以确定观测到的水质量变化的来源区域。基于模型的集成优化插值（OI）将首次用于绘制Argo数据，包括深度和密度表面。全球观测到的地表条件将用于强制一个专门配置和调整的数值模型，以模拟Argo期间的水质量变化。然后将这种全强迫综合与具有有限热带和亚热带/半球强迫的平行综合进行比较，在这些强迫中，温度、盐度和风分量将发生变化，以检查不同地区产生的异常的产生和命运。目标是将观测到的每个海洋盆地的赤道变率归因于当地和/或远程驱动因素，并评估是否有重新出现的证据。检查所有在源强度、平流路径、混合率和通流方面各不相同的盆地，将对亚热带变率何时何地影响远场产生更统一的看法。观察方法将包括仔细检查和消除传感器错误，并使用基于动态智能模型的集成OI框架。与通过不完整的观测或单一的全强迫模式运行来诊断形成机制不同，该研究将使用具有受控区域地表强迫的专用模拟组合。集成方法将清楚地量化模型系统中内在变异性（由于不稳定性）所起的作用，反过来，将有助于更好地理解观测到的记录。通过关注赤道上升流系统的补给流，研究小组将评估拟议中的海洋隧道对气候的重要性。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

How tropical oceans, which are strong drivers of global climate variability, are influenced by subtropical processes remains unresolved. Tropical-subtropical teleconnections occur either via the atmosphere, which can set up within a season, or through subsurface ocean pathways operating over years to decades. The formation in the subtropics and subsequent advection of anomalous heat due to water mass anomalies is a potentially important ocean pathway. Some theoretical studies suggest this process controls multiyear to decadal climate variability. Anomalies are created in two ways: at the surface by wind, heat and freshwater flux changes or in the subsurface by anomalous advection. Using the recent record from the Argo array of profiling floats, many regional studies have identified the formation and propagation of water mass anomalies in most ocean basins. Modern ocean general circulation model studies with realistic surface forcing can form and advect anomalies similar to the observed ones. However, the two descriptions do not agree on whether these anomalies can survive to reach the equator, and then re-emerge to the surface. This project will take a globally consistent observational and modelling approach to quantitatively understand the source, fate and potential re-emergence of subtropical water mass anomalies, and thus their potential to influence the climate. As part of this project, an improved gridded global Argo analysis will be made freely available with particular focus on water mass variability and realistic estimates of resolved scales and errors. A production system will be put in place to ensure ongoing updates and distribution of the analysis. A graduate student in the MIT-WHOI Joint Program will learn both numerical modelling (including the analysis of ensembles) and observational analysis techniques. Since the targeted processes are a prime candidate for enabling longer term climate forecasting, results will discussed directly with climate forecasting teams in the US and Australia via collaborator Peter Oke.This project will combine a novel analysis of Argo data with a set of tailored ensembles of OGCM runs to identify source regions of observed water mass variability. A model-based ensemble optimal interpolation (OI) will be used for the first time to map Argo data, both on depth and density surfaces. Global observed surface conditions will be used to force a numerical model specifically configured and tuned to simulate the water mass variability in the Argo period. This all-forcing ensemble will then be compared to parallel ensembles with restricted tropical and subtropical/hemispheric forcing where the temperature, salinity and wind components will be varied to examine the generation and fate of anomalies generated in different regions. The goal is to attribute observed equatorial variability in each ocean basin to local and/or remote drivers and assess if there is any evidence for re-emergence. Examining all basins, which vary in source strength, advective pathways, mixing rates and throughflows, will result in a more unified view of where and when subtropical variability impacts the far field. The observational approach will involve carefully checking and removing sensor errors and using a dynamically-intelligent model-based ensemble OI framework. Rather than diagnosing formation mechanisms through incomplete observations or from a single all-forcing model run, the study will use dedicated ensembles of simulations with controlled regional surface forcing. An ensemble approach will clearly quantify the role that intrinsic variability (due to instabilities) plays in the model system and in turn, will help better understand the observed record. By focusing on the feeder flows to the equatorial upwelling systems, the team will then assess the importance of the proposed ocean tunnel to climate.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.