Diapycnal displacement, diffusion, and distortion of tracers in the ocean
Diapycnal displacement, diffusion, and distortion of tracers in the ocean
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海洋中示踪剂的斜轴位移、扩散和扭曲
DOI：
10.1175/jpo-d-22-0010.1
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发表时间：
2022
影响因子：
3.5
通讯作者：
Ferrari, Raffaele
中科院分区：
文献类型：
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作者：
Drake, Henri F.;Ruan, Xiaozhou;Ferrari, Raffaele
小规模混合驱动非绝热上升流，关闭深海翻转环流。原位湍流的间接微观结构测量表明，在粗糙的地形上，混合是底部增强的，这意味着内部的下行和倾斜的底部边界层的上升流更强。在示踪剂释放实验（TREs）中，惰性示踪剂被有目的地释放，并随着时间的推移被调查其分散情况，已被用于独立推断湍流扩散，但通常提供的估计超过了微观结构的估计。为了调和这些差异，阮和法拉利推导了精确的示踪剂加权浮力力矩诊断，我们在这里将其应用于准现实模拟。示踪剂的底流驱替率正好是示踪剂平均浮力速度的两倍，浮力速度本身就是不对称上升流/下升流偶极子的卷积。然而，示踪剂的底波扩散速率既包括示踪剂平均原位扩散的预期正贡献，也包括一个额外的非线性底波畸变项，这是由浮力和浮力速度之间的相关性引起的，可以是任何一种符号。由于层状内部的底部增强混合，扭曲通常是正的（拉伸），但在底部附近是负的（收缩）。我们的模拟表明，这两种影响在巴西盆地示踪剂释放实验中恰好抵消，导致净畸变可以忽略不计。相比之下，接近底部的示踪剂经历了随时间变化的导序扭曲。由于实际稀疏采样导致的示踪矩误差通常很小（<20%），特别是与逆模型中由于忽略失真效应而导致的结构误差相比。这些结果表明，尽管TREs是必不可少的，但不应将其视为“明确的”约束。
Small-scale mixing drives the diabatic upwelling that closes the abyssal ocean overturning circulation. Indirect microstructure measurements of in situ turbulence suggest that mixing is bottom enhanced over rough topography, implying downwelling in the interior and stronger upwelling in a sloping bottom boundary layer. Tracer release experiments (TREs), in which inert tracers are purposefully released and their dispersion is surveyed over time, have been used to independently infer turbulent diffusivities—but typically provide estimates in excess of microstructure ones. In an attempt to reconcile these differences, Ruan and Ferrari derived exact tracer-weighted buoyancy moment diagnostics, which we here apply to quasi-realistic simulations. A tracer’s diapycnal displacement rate is exactly twice the tracer-averaged buoyancy velocity, itself a convolution of an asymmetric upwelling/downwelling dipole. The tracer’s diapycnal spreading rate, however, involves both the expected positive contribution from the tracer-averaged in situ diffusion as well as an additional nonlinear diapycnal distortion term, which is caused by correlations between buoyancy and the buoyancy velocity, and can be of either sign. Distortion is generally positive (stretching) due to bottom-enhanced mixing in the stratified interior but negative (contraction) near the bottom. Our simulations suggest that these two effects coincidentally cancel for the Brazil Basin Tracer Release Experiment, resulting in negligible net distortion. By contrast, near-bottom tracers experience leading-order distortion that varies in time. Errors in tracer moments due to realistically sparse sampling are generally small (<20%), especially compared to thestructural errors due to the omission of distortion effects in inverse models. These results suggest that TREs, although indispensable, should not be treated as “unambiguous” constraints on diapycnal mixing.