Meridional density gradients do not control the Atlantic overturning circulation
Meridional density gradients do not control the Atlantic overturning circulation
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DOI：
10.1175/2009jpo4200.1
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发表时间：
2010-02
影响因子：
3.5
通讯作者：
A. M. Boer;A. Gnanadesikan;N. Edwards;A. Watson
中科院分区：
文献类型：
--
作者：
A. M. Boer;A. Gnanadesikan;N. Edwards;A. Watson
大量的建模和理论标度研究支持这样一个概念，即大西洋经向翻转环流（AMOC）的变化，无论是由风还是浮力通量造成的，都可以用AMOC与适当定义的经向密度梯度（MDG）之间的简单因果关系来理解。千年发展目标应该直接转化为子午压力梯度。本文采用模块化海洋模式与能量-水分平衡模式进行了两组实验，其中AMOC - MDG的正关系被打破。在第一套7个模式整合中发现，南大洋风的增加导致了倾覆的增加，而赤道和北大西洋之间的表面密度差减小。在第二组8个模型积分中，对状态方程进行了操作，以便在模型温度加上238至98℃范围内的人工增量DT时计算密度。（DT的增加导致密度对温度梯度的敏感性增加。）无论密度差是在表面计算还是在上层海洋上平均计算，这些模式积分中的AMOC都会随着千年发展目标的增加而下降。传统的尺度分析只能产生这种较弱的AMOC，如果尺度深度减少到足以补偿较强的MDG。对五种深度尺度的估计进行了评价，发现当使用最大翻转环流深度或其估计深度时，AMOC的变化可以从尺度分析中得到，但不能从斜倾角深度中得到。在AMOC的理论模型中，通常假定这两个深度尺度是相同的。这表明，在这些模式整合中，千年发展目标与AMOC之间的相关性被打破，因为AMOC的深度和强度受到南大洋风和南极底水形成等远程强迫的强烈影响。
A wide body of modeling and theoretical scaling studies support the concept that changes to the Atlantic meridional overturning circulation (AMOC), whether forced by winds or buoyancy fluxes, can be understood in terms of a simple causative relation between the AMOC and an appropriately defined meridional density gradient (MDG). The MDG is supposed to translate directly into a meridional pressure gradient. Here two sets of experiments are performed using a modular ocean model coupled to an energy‐moisture balance model in which the positive AMOC‐MDG relation breaks down. In the first suite of seven model integrations it is found that increasing winds in the Southern Ocean cause an increase in overturning while the surface density difference between the equator and North Atlantic drops. In the second suite of eight model integrations the equation of state is manipulated so that the density is calculated at the model temperature plus an artificial increment DT that ranges from 238 to 98C. (An increase in DT results in increased sensitivity of density to temperature gradients.) The AMOC in these model integrations drops as the MDG increases regardless of whether the density difference is computed at the surface or averaged over the upper ocean. Traditional scaling analysis can only produce this weaker AMOC if the scale depth decreases enough to compensate for the stronger MDG. Five estimates of the depth scale are evaluated and it is found that the changes in the AMOC can be derived from scaling analysis when using the depth of the maximum overturning circulation or estimates thereof but not from the pycnocline depth. These two depth scales are commonly assumed to be the same in theoretical models of the AMOC. It is suggested that the correlation between the MDG and AMOC breaks down in these model integrations because the depth and strength of the AMOC is influenced strongly by remote forcing such as Southern Ocean winds and Antarctic Bottom Water formation.