Eddy Memory Mode of Multidecadal Variability in Residual-Mean Ocean Circulations with Application to the Beaufort Gyre
Eddy Memory Mode of Multidecadal Variability in Residual-Mean Ocean Circulations with Application to the Beaufort Gyre
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DOI：
10.1175/jpo-d-16-0194.1
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发表时间：
2017-04-01
影响因子：
3.5
通讯作者：
Spall, Michael A.
中科院分区：
文献类型：
--
作者：
Manucharyan, Georgy E.;Thompson, Andrew F.;Spall, Michael A.
中尺度涡流形成了波弗特环流对埃克曼泵浦的响应，但人们对它们的瞬态动力学知之甚少。气候模型通常使用 Gent-McWilliams (GM) 参数化，将涡流函数 psi* 与等密度斜率 s 和涡流扩散率 K 成正比。这种局部时间参数化导致水流指数平衡。这里，使用理想化的涡旋解析波弗特环流模型来证明 psi* 具有过去海洋状态的有限记忆，这违反了关键的 GM 假设。因此，平衡环流遵循螺旋下沉轨迹，这意味着存在可变性的阻尼模式 - 涡流记忆 (EM) 模式。 EM 模式在旋转过程中表现为等重斜率超调 15%（2000 km(3) 淡水含量超调），并且不能用 GM 参数化来解释。开发了一种改进的参数化，使得 psi* 与有效等重斜率 s* 成比例，并带有过去斜率的有限记忆伽玛。引入涡流记忆解释了模型结果，并揭示了周期为 2 pi 根 T-E gamma 近似于 50 年的振荡，其中涡流扩散时标 T-E 近似于 10 年，gamma 近似于 6 年，从涡旋解析模型中诊断出来。 EM 模式将 Ekman 驱动的环流方差增加了 gamma/T-E 大约 50% +/- 15%，尽管两个时间尺度都随着平均强迫的增加而减小，但该分数保持相对恒定。这项研究表明，电磁模式是旋转湍流的一般属性，并强调需要对瞬态涡流场特性进行更好的观测约束。
Mesoscale eddies shape the Beaufort Gyre response to Ekman pumping, but their transient dynamics are poorly understood. Climate models commonly use the Gent-McWilliams (GM) parameterization, taking the eddy streamfunction psi* to be proportional to an isopycnal slope s and an eddy diffusivity K. This local-in-time parameterization leads to exponential equilibration of currents. Here, an idealized, eddy-resolving Beaufort Gyre model is used to demonstrate that psi* carries a finite memory of past ocean states, violating a key GM assumption. As a consequence, an equilibrating gyre follows a spiral sink trajectory implying the existence of a damped mode of variability-the eddy memory (EM) mode. The EM mode manifests during the spinup as a 15% overshoot in isopycnal slope (2000 km(3) freshwater content overshoot) and cannot be explained by the GM parameterization. An improved parameterization is developed, such that psi* is proportional to an effective isopycnal slope s*, carrying a finite memory gamma of past slopes. Introducing eddy memory explains the model results and brings to light an oscillation with a period 2 pi root T-E gamma approximate to 50 yr, where the eddy diffusion time scale T-E similar to 10 yr and gamma approximate to 6 yr are diagnosed from the eddy-resolving model. The EM mode increases the Ekman-driven gyre variance by gamma/T-E approximate to 50% +/- 15%, a fraction that stays relatively constant despite both time scales decreasing with increased mean forcing. This study suggests that the EM mode is a general property of rotating turbulent flows and highlights the need for better observational constraints on transient eddy field characteristics.