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The role of diapycnal mixing for the equilibrium response of thermohaline circulation

机译：透析混合对热盐循环平衡反应的作用

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摘要

Using a zonally averaged, one-hemispheric numerical model of the thermohaline circulation, the dependence of the overturning strength on the surface equator-to-pole density difference is investigated. It is found that the qualitative behavior of the thermohaline circulation depends crucially on the nature of the small-scale vertical mixing in the interior of the ocean. Two different representations of this process are considered: constant vertical diffusivity and the case where the rate of mixing energy supply is taken to be a fixed quantity, implying that the vertical diffusivity decreases with increasing stability of the water column. When the stability-dependent diffusivity parameterization is applied, a weaker density difference is associated with a stronger circulation, contrary to the results for a fixed diffusivity. A counterintuitive consequence of the stability-dependent mixing is that the poleward atmospheric freshwater flux, which acts to reduce the thermally imposed density contrast, strengthens the thermally dominated circulation and its attendant poleward heat transport. However, for a critical value of the freshwater forcing, the thermally dominated branch of steady states becomes unstable, and is succeeded by strongly time-dependent states that oscillate between phases of forward and partly reversed circulation. When a constant vertical diffusivity is employed, on the other hand, the thermally dominated circulation is replaced by a steady salinity-dominated state with reversed flow. Thus in this model, the features of the vertical mixing are essential for the steady-state response to freshwater forcing as well as for the character of flow that is attained when the thermally dominated circulation becomes unstable.
机译：使用热盐环流的区域平均，单半球数值模型，研究了倾覆强度对表面赤道与极点密度差的依赖性。发现热盐环流的定性行为主要取决于海洋内部小规模垂直混合的性质。考虑该过程的两种不同表示形式：垂直扩散系数恒定和混合能量供应速率为固定量的情况，这意味着垂直扩散系数随水柱稳定性的增加而降低。当应用依赖于稳定性的扩散率参数化时，与固定扩散率的结果相反，较弱的密度差与较强的循环相关。依赖于稳定性的混合的反直觉结果是，极地大气淡水通量可降低热施加的密度反差，从而增强以热为主的循环及其伴随的极向热传递。但是，对于淡水强迫的临界值，稳态的热支配分支变得不稳定，并且在正向和部分反向循环的各个阶段之间振荡的时间依赖性强的状态继之以继。另一方面，当采用恒定的垂直扩散率时，热主导的循环被具有反流的稳定盐度主导的状态所代替。因此，在该模型中，垂直混合的特征对于对淡水强迫的稳态响应以及热主导循环变得不稳定时获得的流量特性至关重要。

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