The Evaluation of Ocean Reanalyses in Their Determining Trends in Global Ocean Heat Content with a Novel Method

鉴于海洋变化和变异性是气候变化的主要证据，而且需要解释气候变异性模式，例如最近引起很大争议的全球表面变暖放缓，探测和了解海洋变化和变化性变得越来越重要。这个问题的核心是能够可靠地在全球范围内和在其全部深度上代表海洋状态。许多研究人员已经转向通过海洋再分析(ORA)来描述和理解海洋的变化和变异性，ORA是海洋模型、大气强迫和通过各种数据同化方法进行海洋观测的组合，可能提供比仅基于观测或仅基于模型的海洋估计更准确的信息。此外，ORA还提供了更多的变量和数量来解释和理解海洋的变化和变异性。目前，ORAS的估计，特别是其时间演变，仍然对观测系统的变化、动力模式的误差、大气强迫和数据同化方法很敏感。许多不确定性往往与流量有关，很难识别和估计。这项研究提出了一种新颖而简单的方法来发现ORA温度估计中可能存在的系统误差和偏差，然后利用物理上最一致的ORA来分析全球和区域OHC的变化。这项拟议的研究可能会为最近引起很大争议的全球表面变暖间歇期提供一个统一的观点，并将促进对海洋热吸收的年代际变化的理解，并对大气顶部的辐射不平衡提供有用的影响。该项目将支持一名早期职业科学家和一名研究生，为培养下一代科学家做出贡献。此外，它还将有助于开发解释海洋在调节气候变化中的作用的k-12教育材料。在这个项目中，将使用一种简单但很少使用的方法来评估现有的一大套ORA产品，并确定它们是否适合研究全球和区域海洋热含量(OHC)的气候尺度变化。这一评估是基于检查热空间和晕空间对全球空间高度的贡献随时间的变化而作出的。首先，全球光环空间组分的大小应该很小，因为它受全球海洋淡水输入的变化控制。在对现有的有机污染物进行评估后，将对被选为长期有机污染物潜在误差最小的那些进行分析，目的是更好地了解决定观测到的有机污染物变化和变异性的过程。该团队将评估表面温度的十年级轻微下降是否真实存在，以及它是否反映了额外的热量向海洋更深的层输送。研究人员还将进行分析，重点是区分等轴线运动(隆起)和沿等轴线的水团性质变化(辛辣)的贡献。如果在选定的ORA中可用，还将检查垂直热通量和相关的动力变量。

Detecting and understanding oceanic change and variability has become increasingly important, given the fact that they serve as major evidence for climate change as well as the need to interpret climate variability patterns such as the much-debated recent global surface warming slowdown. Central to this issue is the ability to reliably represent the ocean state globally and over its full depth. Many researchers have turned to describe and understand oceanic change and variability from ocean reanalyses (ORA), which are the combination of ocean models, atmospheric forcings, and ocean observations through various data assimilation methods and potentially provide more accurate information than observation-only or model-only based ocean estimations. Also, ORAs provide more variables and quantities to interpret and understand the oceanic change and variability. At present, estimates from ORAs, especially their temporal evolutions, are still sensitive to the change of the observing system, the errors of the dynamical model, atmospheric forcing, and the data assimilation methods. Many of the uncertainties are often flow dependent and are hard to identify and estimate. This research proposes a novel, yet simple way to discover the possible systematic errors and biases in the temperature estimates from ORAs and then utilizes the most physically consistent ORAs to perform analysis on the changes in global and regional OHC. The proposed study can potentially provide a unified view for the recently much-debated global surface warming hiatus, and will advance understanding of the decadal variability of ocean heat uptake as well as provide useful implications about the imbalance of radiation at the top of the atmosphere. The project will support an early career scientist and a graduate student contributing to building the next generation of scientists. In addition, it will contribute to the development of k-12 educational material explaining the role of the oceans in regulating climate change.In this project, a simple but rarely used method will be used to evaluate a large set of the existing ORA products and determine if they are suitable for examining climatic-scale changes in global and regional ocean heat content (OHC). This evaluation is based on examining the changes of the thermal-steric and halo-steric contributions to the global steric height over time. To first order, the size of the global halo-steric component should be very small, as it is controlled by changes in freshwater input into the global ocean. After evaluating existing ORAs, the ones selected as having the smallest potential errors in long-term OHC will be analyzed with a goal to better understand the processes that determine the observed OHC change and variability. The team will assess whether the decadal-scale slight reduction in surface temperature was real and whether it reflected transport of extra heat to deeper layers of the ocean. Researchers will also carry out the analyses with a focus to separate the contributions of isopycnal movement (heave) and change in water mass properties along isopycnals (spiciness). If available in the selected ORAs, the vertical heat flux and the associated dynamical variables will also be examined.