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首页> 外文期刊>Journal of Physical Oceanography >The Role of Stratification-Dependent Mixing for the Stability of the Atlantic Overturning in a Global Climate Model
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The Role of Stratification-Dependent Mixing for the Stability of the Atlantic Overturning in a Global Climate Model

机译：在全球气候模型中，分层依赖混合对大西洋倾覆稳定性的作用

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Using the "CLIMBER-3α" coupled climate model of intermediate complexity, the effect of a stratification-dependent vertical diffusivity on the sensitivity of the Atlantic Ocean meridional overturning circulation to perturbations in freshwater forcing is investigated. The vertical diffusivity k is calculated as k ～ N~(-α), where N is the local buoyancy frequency and the parameter α is a measure of the sensitivity of the vertical diffusivity to changes in stratification. Independent of α, the stratification of the deep ocean is weakly increased as a response to an anomalous freshwater flux in the North Atlantic in these experiments. In the region of freshwater forcing and north of it this is caused by the freshwater anomaly itself, but farther south it is associated with anomalously warm surface waters caused by a reduction of the northward oceanic heat transport. Subsequently, and in opposition to results from previous studies, the overturning is reduced by the anomalous freshwater flux, independent of the choice of α. However, the amount of reduction in overturning following a freshwater perturbation is found to depend critically on the choice of the mixing sensitivity α. If α < α_(cr), the response is similar to the model's response using constant vertical diffusivity (α = 0). For α > α_(cr), a sharp increase of the sensitivity is found. The value of α_(cr) is found to be between 0.5 and 1. A general feedback is proposed explaining this threshold behavior: if α is large, both positive and negative perturbations of stratification are amplified by associated changes in diffusivity. In the experiments presented here, this enhances the initial positive stratification anomaly in northern high latitudes, which is created by the anomalous freshwater flux. As a result, convection is strongly reduced, and the overturning is significantly weakened.
机译：使用“CLIMBER-3α”耦合复杂度中等的气候模型，研究了分层相关的垂直扩散率对大西洋经向翻转环流对淡水强迫扰动敏感性的影响。垂直扩散系数k计算为k〜N〜（-α），其中N是局部浮力频率，参数α是垂直扩散系数对分层变化敏感性的度量。在这些实验中，独立于α，深海的分层作为对北大西洋异常淡水通量的响应而微弱地增加。在淡水强迫区域及其北部，这是由淡水异常本身引起的，而在更南端，则是由于北向海洋热传输减少而引起的异常温暖的地表水。随后，与先前的研究结果相反，异常的淡水通量减少了倾覆，而与α的选择无关。然而，发现淡水扰动后倾覆的减少量主要取决于混合灵敏度α的选择。如果α<α_（cr），则响应与使用恒定垂直扩散率（α= 0）的模型响应相似。对于α>α_（cr），发现灵敏度急剧增加。发现α_（cr）的值在0.5到1之间。提出了一个一般反馈来解释此阈值行为：如果α大，则相关的扩散率变化会放大分层的正向和负向扰动。在此处介绍的实验中，这增强了北部高纬度地区的初始正分层异常，这是由异常的淡水通量造成的。结果，大大降低了对流，并且倾覆明显减弱。

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