When Are Eddy Tracer Fluxes Directed Downgradient
When Are Eddy Tracer Fluxes Directed Downgradient
复制标题
涡流示踪通量何时定向下降
DOI：
10.1175/jpo2841.1
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发表时间：
2006
影响因子：
3.5
通讯作者：
Richard G. Williams
中科院分区：
文献类型：
--
作者：
Chris Wilson;Richard G. Williams
利用旋流解析等流实验，研究了旋流示踪剂通量方向的控制机制。当示踪剂方差呈拉格朗日增长或示踪剂方差有较强的耗散时，涡旋通量平均呈下降梯度方向。涡流对示踪剂平均演化的影响可以通过涡流的集合来描述，每个涡流都有一个特定的生命周期。虽然累积的时间平均图像具有一致性，但对涡旋行为（如通量、涡旋动能和示踪剂方差）的局部检查似乎很复杂：涡旋优先在局部区域形成，随着下游的增长和示踪剂方差的增加而伴随下梯度涡旋示踪剂通量，而最终涡旋随着示踪剂方差和上梯度涡旋示踪剂通量的减少而衰减。在旋升期间，由于流动不稳定引起的示踪剂变形导致示踪剂方差的面积平均增加（尽管局部示踪剂方差随单个涡流生命周期的增加和减少而增加），因此隐含面积平均，下梯度示踪剂通量。在稳定状态下，涡旋通量的部分模式仅仅反映了时间平均值和涡旋流的背景示踪剂方差的平流。如果示踪剂中有强烈的沉降，则涡旋通量会偏向于向下梯度方向，这可能是在混合层中沿等等值线的涡旋热通量或沿等等值线与透光带相交的涡旋硝酸盐通量的情况。
The mechanisms controlling the direction of eddy tracer fluxes are examined using eddy-resolving isopycnic experiments for a cyclic zonal channel. Eddy fluxes are directed downgradient on average when either (i) there is a Lagrangian increase in tracer variance or (ii) there is strong dissipation of tracer variance. The effect of the eddies on the mean tracer evolution can be described through an ensemble of eddies that each have a particular life cycle. Local examination of the eddy behavior, such as fluxes, eddy kinetic energy, and tracer variance appears complex, although the cumulative time-mean picture has coherence: eddies are preferentially formed in localized regions with downstream growth and increase in tracer variance concomitant with downgradient eddy tracer fluxes, while eventually the eddies decay with a decrease in tracer variance and upgradient eddy tracer fluxes. During spinup, tracer deformation through flow instability leads to an area-average increase in tracer variance (although locally it is increasing and decreasing with the individual eddy life cycles) and therefore an implied area-average, downgradient tracer flux. At a steady state, part of the pattern in eddy fluxes simply reflects advection of background tracer variance by the time-mean and eddy flows. The eddy flux becomes biased to being directed downgradient if there is a strong sink in the tracer, which is likely to be the case for eddy heat fluxes along isopycnals outcropping in the mixed layer or for eddy nitrate fluxes along isopycnals intersecting the euphotic zone.