Interpreting Regional and Temporal Variability in Global Diapycnal Mixing Inferred from Argo Profiles

学术成就：湍流海洋混合的影响超越了其固有的小尺度，影响了大尺度的海洋过程，包括水团转化，分层维持和翻转环流。然而，分布的diapycnal（近垂直）混合是没有很好地描述稀疏船基观测，因为这种混合是空间斑片状和时间间歇性的。最近，技术已经发展到推断diapycnal混合率从细尺度（几十到几百米的垂直尺度）剪切或应变，使用假设的基本内部波动动力学，往往驱动湍流混合。项目？美国的研究人员已经开始将这些技术应用于全球Argo阵列测量的应变。他们利用来自海洋上层2 000米的阿尔戈浮标剖面的应变信息，对湍流能量耗散率进行了40多万次估计，表明海洋混合。虽然这些估计依赖于许多假设，并不采取直接测量方法的地方，他们已被证明是一个很好的代理分布的diapycnal混合。时间平均估计数揭示了所有海洋的参数化耗散率和扩散率分布的明确空间模式。他们证实了以前的观察，将高耗散率与粗糙地形地区联系起来。在高涡动动能地区，沿着赤道带，以及在层结较弱的高纬度地区，观测到估计耗散率升高。在西北太平洋观测到了混合的季节依赖性，这表明在上层海洋中存在风强迫的响应。在这个项目中，研究生Caitlin Whalen将继续她的博士论文的工作。她将解决这些初步结果提出的两个主要开放问题。首先，估计diapycnal混合和涡动动能场之间的相关性将被调查，通过考虑来自Argo-float和卫星海平面异常数据的复合涡的混合估计的分布。该分布将与由MIT海洋环流模型模拟的涡流进行比较，该模型受复合涡流特性的启发。第二，将把沿着赤道局部升高的纵摇混合与赤道微结构测量结果进行比较，以证实这一模式，并将观测到的沿沿着赤道的振荡置于赤道波和ENSO的背景下，其灵感来自于以前将近地表赤道混合与这些现象联系起来的结果。该项目将培训一名研究生惠伦女士，她一直并将继续参与各种外联活动，包括在桦树水族馆做志愿者，在CalEchoes学生经营的研究巡航期间协助教师和社交媒体宣传工作，参加研讨会，教育当地教师关于海洋学，并与当地电视新闻媒体讨论她的工作。科学上，拟议的工作有助于我们了解全球模式的海洋中的diapycnal混合，在全球海洋模型中的一个主要的未解决的过程。首席研究员正在领导一个由NSF资助的气候过程团队（CPT），其任务是改善全球气候模型中的横隔混合的表现。在一定程度上，无论是大小和分布的diapycnal混合是可能发生变化，在未来的气候（例如，风应力和相关的中尺度模式演变），准确预测未来或过去的气候需要发展的参数化湍流混合的基础上适当的物理。该项目的结果将纳入目前防止酷刑委员会的工作。

Intellectual Merit:The influence of turbulent ocean mixing transcends its inherently small scales to affect large scale ocean processes including water-mass transformation, stratification maintenance, and the overturning circulation. However, the distribution of diapycnal (near vertical) mixing is not well described by sparse ship-based observations since this mixing is both spatially patchy and temporally intermittent. Recently, techniques have been developed to infer diapycnal mixing rates from finescale (tens to hundreds of meter vertical scales) shear or strain, using assumptions about the underlying internal wave dynamics that often drive turbulent mixing. The project?s investigators have begun to apply these techniques to strain measured by the global Argo array. They have used strain information from Argo float profiles in the upper 2,000 m of the ocean to generate over 400,000 estimates of the turbulent energy dissipation rate, indicative of ocean mixing. While these estimates rely on numerous assumptions, and do not take the place of direct measurement methods, they have been shown to be a good proxy for the distribution of diapycnal mixing. Temporally averaged estimates reveal clear spatial patterns in the parameterized dissipation rate and diffusivity distribution across all the oceans. They corroborate previous observations linking elevated dissipation rates to regions of rough topography. Heightened estimated dissipation rates have been observed in areas of high eddy kinetic energy, along the equatorial band, as well as heightened diffusivity in high latitudes where stratification is weak. The seasonal dependence of mixing is observed the Northwest Pacific, suggesting a wind-forced response in the upper ocean.In this project, graduate student, Caitlin Whalen, will continue this work for her doctoral thesis. She will tackle two of the major open questions that these initial results pose. First, the correlation between the estimated diapycnal mixing and the eddy kinetic energy field will be investigated by considering the distribution of the mixing estimates across a composite eddy derived from Argo-float and satellite sea level anomaly data. This distribution will be compared with an eddy modeled by the MIT ocean circulation model inspired by the characteristics of the composite eddy. Second, the locally elevated diapycnal mixing along the equator will be compared to equatorial microstructure measurements to corroborate the pattern, and the observed oscillations along the equator will be placed in the context of equatorial waves and ENSO, inspired by previous results that link near-surface equatorial mixing with these phenomena.Broader Impacts:The project will train a graduate student, Ms. Whalen, who has been and will continue to be involved in a variety of outreach activities, including volunteering at the Birch Aquarium, assisting teachers and social media outreach efforts during the CalEchoes student-run research cruise, participating in workshops to educate local teachers about oceanography, and discussing her work with the local TV news media. Scientifically, the proposed work contributes to our understanding of global patterns of diapycnal mixing in the ocean, a major unresolved process in global ocean models. The principal investigator is leading a NSF-funded Climate Process Team (CPT) tasked with improving representations of diapycnal mixing in global climate models. To the extent that both the magnitude and distribution of diapycnal mixing is likely to change in a future climate (as, for example, wind stress and associated mesoscale patterns evolve), accurate prediction of future or past climate requires development of parameterizations of turbulent mixing that are based on appropriate physics. The results of the project will be incorporated into ongoing CPT efforts.