The Different Nature of the Interdecadal Variability of the Thermohaline Circulation under Mixed and Flux Boundary Conditions
The Different Nature of the Interdecadal Variability of the Thermohaline Circulation under Mixed and Flux Boundary Conditions
复制标题
混合和通量边界条件下温盐环流年代际变化的不同性质
DOI：
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发表时间：
2006
期刊：
影响因子：
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通讯作者：
A. Verdière
中科院分区：
文献类型：
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作者：
O. Arzel;T. Huck;A. Verdière
在一个理想的平底单半球盆地中，利用一个高分辨率海洋模式，研究了混合和恒定通量边界条件下年代际变化的差异。确定允许区分一种类型的振荡与另一种类型的客观特征。首先，通过对恢复边界条件下得到的定常状态进行线性稳定性分析，证明了定常通量和混合边界条件下的年代际变化分别是由平均层结和环流周围的线性模不稳定性和偏离初态引起的。在密度方差收支的基础上，指出这两种振荡具有不同的能量来源：在恒定通量边界条件(热力模式)下，西边界流区向下梯度经向涡旋热通量维持年代际变化，而在混合边界条件(盐度模式)下，对流调节和恢复地表热通量之间的正反馈是年代际振荡存在的核心。此外，强迫层温度和盐度异常之间的正相关对密度变异的强迫起主导作用。此外，在常通量下，扰动的垂直结构在所有示踪场中都表现出不同深度的垂直位相滞后，而在混合边界条件下，只有温度异常表现出强烈的偶极结构。作者认为，这些差异将使人们能够确定哪种类型的振荡(如果有的话)在更详尽的气候模型中发挥作用。
The differences between the interdecadal variability under mixed and constant flux boundary conditions are investigated using a coarse-resolution ocean model in an idealized flat-bottom single-hemisphere basin. Objective features are determined that allow one type of oscillation to be distinguished versus the other. First, by performing a linear stability analysis of the steady state obtained under restoring boundary conditions, it is shown that the interdecadal variability under constant flux and mixed boundary conditions arises, respectively, from the instability of a linear mode around the mean stratification and circulation and from departure from the initial state. Based on the budgets of density variance, it is shown next that the two types of oscillations have different energy sources: Under the constant-flux boundary condition (the thermal mode), the downgradient meridional eddy heat flux in the western boundary current regions sustains interdecadal variability, whereas under mixed boundary conditions (the salinity mode), a positive feedback between convective adjustment and restoring surface heat flux is at the heart of the existence of the decadal oscillation. Furthermore, the positive correlations between temperature and salinity anomalies in the forcing layer are shown to dominate the forcing of density variance. In addition, the vertical structure of perturbations reveals vertical phase lags at different depths in all tracer fields under constant flux, while under mixed boundary conditions only the temperature anomalies show a strong dipolar structure. The authors propose that these differences will allow one to identify which type of oscillation, if any, is at play in the more exhaustive climate models.