On the eddy transfer of tracers: Advective or diffusive?
On the eddy transfer of tracers: Advective or diffusive?
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DOI：
10.1357/0022240973224346
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发表时间：
1997-05-01
影响因子：
0.5
通讯作者：
Williams, RG
中科院分区：
文献类型：
--
作者：
Lee, MM;Marshall, DP;Williams, RG
在海洋学中，地转涡旋传统上被视为以下降梯度的方式扩散水团和示踪剂。然而，涡旋也具有平流作用，可能导致示踪剂的梯度上升转移，正如大气示踪剂研究和最近为海洋开发的涡旋参数化所认识到的那样，涡旋通过斜压不稳定中密度表面的下降形成的二次环流提供平流转移或“大”速度。在这里，我们使用涡旋解析等环流海洋模式来研究纬向急流的经向转移。射流经历斜压不稳定，形成一个充满活力的涡旋场，并诱导子午速度。发现球速度与位涡梯度有关，而与厚度无关。一种瞬态示踪剂在南边界和北边界分别释放出高值和低值。在最初的几年里，示踪剂以向下梯度的方式扩散。被动示踪剂的隐含涡流扩散率与动态示踪剂的势涡率惊人地相似。在年代际尺度上，涡旋平流占主导地位，上层示踪剂向极地扩散，下层示踪剂向赤道扩散。在时间平均经向流较弱的地方，这种涡旋诱导的平流可能在控制水质量分布方面起重要作用。在观测上，输运速度很难直接测量，但我们认为可以从瞬态示踪剂（如CFCs）在达到统计稳定状态之前的扩散推断出来。
Geostrophic eddies have traditionally been viewed within oceanography as diffusing water masses and tracers in a down-gradient manner. However, eddies also have an advective role that may lead to an up-gradient transfer of tracers, as has been recognized in atmospheric tracer studies and recent eddy parameterizations developed for the ocean, Eddies provide an advective transfer or ''bolus'' velocity through the secondary circulation formed by the slumping of density surfaces in baroclinic instability.Here we use an eddy-resolving isopycnal ocean model to investigate the meridional transfer across a zonal jet. The jet undergoes baroclinic instability, forming a vibrant eddy field and inducing a meridional bolus velocity. The bolus velocity is found to be correlated with gradients of potential vorticity rather than thickness.A transient tracer is released with high and low values at the southern and northern boundaries respectively. Over the first few years, the tracer spreads diffusively in a down-gradient manner. The implied eddy diffusivity of the passive tracer is found to be reassuringly similar to that of the dynamic tracer, potential vorticity. On the decadal time scale, however, the eddy-induced advection dominates and leads to a poleward spreading of tracer in the upper layer, and equatorward spreading of tracer in the lower layer. This eddy-induced advection is likely to be important in controlling the water-mass distribution wherever the time-mean meridional flow is weak. Observationally, the transport velocity is difficult to measure directly, but we argue might be inferred from the spreading of transient tracers, such as CFCs, before they reach a statistically-steady state.