On co-existing diffusive and anti-diffusive tracer transport by oceanic mesoscale eddies
On co-existing diffusive and anti-diffusive tracer transport by oceanic mesoscale eddies
复制标题
DOI：
10.1016/j.ocemod.2021.101909
复制
发表时间：
2021-10
期刊：
影响因子：
3.2
通讯作者：
M. Haigh;P. Berloff
中科院分区：
文献类型：
--
作者：
M. Haigh;P. Berloff
利用海洋中尺度涡旋对被动示踪物的涡旋输送进行参数化的一种常见方法是调用输送张量。这个张量的对称部分，扩散张量，量化了扩散的涡旋示踪输运。最近的研究已经从涡旋解析模拟中诊断出扩散张量的对号本征值(扩散系数)，而所有当前的参数化只实现正扩散系数。对于相反符号的特征值，相关的扩散涡旋示踪通量不一定是向下梯度的，因此可能不会通过将方差传递到小尺度来混合示踪剂。在这项研究中，我们利用涡旋分辨模拟了具有松弛(源/汇)强迫的被动示踪剂，以探索这种扩散涡通量。在确认了不同示踪对的扩散张量具有相反符号的本征值之后，我们证明了相应的扩散涡旋示踪通量驱动了方差的净向下梯度转移，当扩散张量本征值都为正时，这是可以保证的。局部上升梯度通量是常见的，其频率强烈地依赖于松弛剖面。对于不同的示踪剂，减弱或加强弛豫对下降梯度通量频率的影响是不同的。然而，对于所有被考虑的示踪剂，净向下梯度转移的幅度随着松弛的加强而减弱，这是均匀扩散耗散较小涡动变化的结果。我们的结果表明，对于有源/汇的海洋示踪剂，参数化的扩散涡旋示踪剂通量不应该是全局向下梯度的。
A common approach for parameterising eddy transport of passive tracers by mesoscale eddies in the ocean is by invoking a transport tensor. The symmetric part of this tensor, the diffusion tensor, quantifies diffusive eddy tracer transport. Recent studies have diagnosed opposite-signed eigenvalues (diffusivities) of the diffusion tensor from eddy-resolving simulations, while all current parameterisations implement only positive diffusivities. For opposite-signed eigenvalues the associated diffusive eddy tracer flux is not necessarily down-gradient and therefore may not mix the tracer by transferring variance to the small scales. In this study we explore such diffusive eddy fluxes by using an eddy-resolving simulation of passive tracers with a relaxation (source/sink) forcing. After confirming that the diffusion tensors for different tracer pairs have opposite-signed eigenvalues, we show that the corresponding diffusive eddy tracer flux drives a net down-gradient transfer of variance, as would be guaranteed when the diffusion tensor eigenvalues are both positive. Locally up-gradient fluxes are common, with their frequency strongly dependent on the relaxation profile. The effects of weakening/strengthening the relaxation on the frequency of down-gradient fluxes is different for each tracer. However, for all tracers considered the amplitude of the net down-gradient transfer weakens as the relaxation strengthens, a consequence of the homogeneous diffusion dissipating less eddy variance. Our results indicate that for oceanic tracers with sources/sinks the parameterised diffusive eddy tracer fluxes should not be globally down-gradient.