| 摘要: |
| 南大洋（SO）经向翻转环流（MOC）是全球翻转环流的重要组成部分。长期以来，人们利用纬向平均的手段来探究南大洋MOC的空间分布特征及其与风应力、表层热强迫变化之间的关系。然而，南大洋的地形不是纬向一致的，因此许多物理学过程（诸如涡旋的分布以及MOC）也不是纬向一致的。本文采用不同地形下的循环通道模型，旨在研究南大洋MOC对于4种不同地形特征（德雷克海峡（DP）、大西洋洋中脊（MR）、克尔盖仑高原的东西两侧（LKP与RKP））的敏感性。基于变形的欧拉分解理论（TEM），在定常状态下，余流MOC被拆解为风生Ekman MOC和Eddy MOC。研究发现，地形影响了Ekman MOC的空间分布：在DP 和LKP区，ACC受狭窄通道的挤压而加速，形成急流区，经向的Ekman MOC在该急流区被冲断，分割成较小的顺时针环流；在MR 和RKP区，地形形阻影响下的底摩擦显著增强，ACC流速减弱，此处的Ekman MOC，受ACC的影响较弱，受地形的影响较大。此外，地形也改变了Eddy MOC的空间分布：对于MR区和RKP区而言，由于底摩擦较强，该Eddy MOC主要被底摩擦所耗散，涡致环流均呈“矮胖型”，进而使南极底层水（AABW）的下沉深度变浅。DP区LKP区存在急流（ACC）穿过通道，在通道两侧均存在十分强的侧摩擦，底摩擦不再起主要的耗散作用，涡致环流均呈“瘦高型”，使AABW可以到达较深的海底。统计分析表明，除MR区外，涡致环流流量均有所增加。四种地形特征影响下的Eddy MOC流量相对于无地形实验分别改变了7.6%，2.9%，-4.4%，3.3%。Residual MOC影响下的绕极深层水上涌流量的变化则更大，分别为-19%，-11%，-10.5%，-15%。因此，ACC绕过地形后下游区域涡旋活动增加，进而涡致经向输运也增大，但在地形阻挡的区域涡旋活动反而有所下降。而且，显著增加的涡致经向输运与地形影响下的Deacon Cell相互抵消，在地形的南北两侧形成了新的空间分布的MOC。 |
| 关键词: 地形、中尺度涡旋、南极绕极流、经向翻转环流、南大洋 |
| DOI： |
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| 基金项目:国家杰出青年科学基金 |
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| The Dependence of Southern Ocean Meridional Overturning Circulation on Topography. |
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deng Yaocheng,Ruijian Gou,Xiaopei Lin,Yu Huaming
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Ocean University of China
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| Abstract: |
| The meridional overturning circulation (MOC) of the Southern Ocean (SO) is an important component of the global MOC. For a long time, people have used the method of zonal mean to explore the pattern of the Southern Ocean MOC and its dependence with wind stress and surface heat flux. However, the terrain of the Southern Ocean is not zonally consistent, so many physical processes such as the distribution of eddies and MOC are also not zonally consistent. This paper adopts a reentrant-channel model under different terrains to study the dependence of the Southern Ocean MOC on four different terrains (Drake Passage(DP), Mid-Atlantic Ridge (MR), and the Left and Right sides of the Kolguerin Plateau (LKP and RKP)). Based on the Transformed Euler-mean theory (TEM), in a steady state, the residual MOC is decomposed into wind-induced Ekman MOC and eddy MOC. Research has found that terrain affects the pattern of Ekman MOC: as to the DP and LKP, ACC is accelerated by squeezing in the narrow channels, forming a jet where the Ekman MOC is cut off and divided into smaller clockwise circulation; In the MR and RKP regions, the bottom friction under the influence of terrain resistance is significantly enhanced, and the ACC flow velocity is weakened. The Ekman circulation here is less affected by ACC and more affected by terrain. In addition, the terrain has also changed the spatial distribution of vortex induced circulation: for the MR and RKP regions, due to strong bottom friction, the vortex induced circulation is mainly dissipated by the bottom friction, and both vortex induced circulations are in a "dwarf" shape, resulting in a shallower sinking depth of Antarctic formation water (AABW). There is a jet stream (ACC) passing through the channel in the LKP area of the DP area, with strong side friction on both sides of the channel. Bottom friction no longer plays a major dissipative role, and vortex-induced circulation shows a "lean high" pattern, allowing AABW to reach deeper seabed. Statistical analysis shows that except for the MR region, the vortex induced circulation flow has increased. The vortex-induced circulation flux under the influence of four terrain features has changed by 7.6%, 2.9%, -4.4%, and 3.3% compared to the non terrain experiment, respectively. The changes in the upwelling flow rate of the deep water around the pole under the influence of residual current MOC are greater, at -19%, -11%, -10.5%, and -15%, respectively. Therefore, after ACC bypasses the terrain, the vortex activity in the downstream area increases, leading to an increase in meridional transport caused by vortices. However, in areas obstructed by terrain, the vortex activity actually decreases. Moreover, the significantly increased vortex-induced meridional transport and the Deacon Cell under the influence of terrain cancel out each other, forming new spatially distributed MOCs on both sides of the terrain. |
| Key words: topography, mesoscale eddy, Antarctic Circumpolar Current, meridional overturning circulation, Southern Ocean |