Comparing isopycnal eddy diffusivities in the Southern Ocean with predictions from linear theory
Comparing isopycnal eddy diffusivities in the Southern Ocean with predictions from linear theory
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DOI：
10.1016/j.ocemod.2015.08.001
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发表时间：
2015-10
期刊：
影响因子：
3.2
通讯作者：
A. Griesel;C. Eden;Nikolaus Koopmann;E. Yulaeva
中科院分区：
文献类型：
--
作者：
A. Griesel;C. Eden;Nikolaus Koopmann;E. Yulaeva
我们表明，预测的位涡扩散系数的线性稳定性分析（LSA），是相同的拉格朗日横流等密扩散系数的线性化版本。两者都可以用相同的表达式表示--涡动动能（EKE）与积分时间尺度的乘积，积分时间尺度涉及拉格朗日衰减尺度γ或最不稳定波的增长率ω i，频率与平均流速和不稳定波的相速度的真实的部分之差有关。扩散系数从LSA相比，拉格朗日等密涡扩散系数估计从70多万数值粒子在南大洋的涡旋模型。它们表现出不同的空间依赖性。LSA预测的涡流扩散系数，增强在转向水平的平均流速等于相速度的不稳定波。相反，拉格朗日扩散系数没有表现出明显的转向水平最大值，而是在许多地方表面强化。从LSA得到的拉格朗日和扩散系数之间的差异可以理解，因为从LSA预测的EKE与模拟的EKE不同，并且因为估计的衰减尺度γ平均比最大线性增长率大4倍。诊断的拉格朗日积分时间尺度具有最大值的平均流速等于最不稳定波的相速度的深度，但扩散率的最大值向表面移动，因为模拟的EKE随深度迅速下降。讨论了扩散系数简单参数化的可能性。
We show that the potential vorticity diffusivity predicted by linear stability analysis (LSA), is the same as a linearized version of Lagrangian cross-stream isopycnal diffusivity. Both can be written in terms of the same expression− the product of the eddy kinetic energy (EKE) and the integral time scale that involves the Lagrangian decay scale γ or the growth rate ω i of the most unstable wave, and a frequency that is related to the difference of the mean flow speed and real part of the phase speed of the unstable waves. Diffusivities from LSA are compared to Lagrangian isopycnal eddy diffusivities estimated from more than 700,000 numerical particles in the Southern Ocean of an eddying model. They show different spatial dependency. LSA predicts eddy diffusivities that are enhanced at the steering level where the mean flow speed equals the phase speed of the unstable waves. In contrast, Lagrangian diffusivities exhibit no clear steering level maxima, but are instead surface intensified in many places. The differences between the Lagrangian and diffusivities from LSA can be understood because EKE predicted from LSA differs from the simulated one, and because the estimated decay scale γ is on average about 4 times larger than the largest linear growth rate. The diagnosed Lagrangian integral time scale has maxima at the depth where the mean flow speed equals the phase speed of the most unstable wave, but the diffusivity maxima are shifted towards the surface because the simulated EKE decreases rapidly with depth. Possibilities for a simple parameterization for the diffusivity are discussed.