Department of Earth, Atmosphere and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA.
Nat Commun. 2017 Mar 6;8:14197. doi: 10.1038/ncomms14197.
It is an open question whether turbulent mixing across density surfaces is sufficiently large to play a dominant role in closing the deep branch of the ocean meridional overturning circulation. The diapycnal and isopycnal mixing experiment in the Southern Ocean found the turbulent diffusivity inferred from the vertical spreading of a tracer to be an order of magnitude larger than that inferred from the microstructure profiles at the mean tracer depth of 1,500 m in the Drake Passage. Using a high-resolution ocean model, it is shown that the fast vertical spreading of tracer occurs when it comes in contact with mixing hotspots over rough topography. The sparsity of such hotspots is made up for by enhanced tracer residence time in their vicinity due to diffusion toward weak bottom flows. The increased tracer residence time may explain the large vertical fluxes of heat and salt required to close the abyssal circulation.
密度面之间的紊动混合是否足以在封闭海洋经向翻转环流的深支中起主导作用，这仍是一个悬而未决的问题。在南大洋进行的上下密度混合实验发现，从示踪剂的垂向扩展推断出的紊动扩散系数比在德雷克海峡的平均示踪剂深度 1500 米处的微结构剖面推断出的数值大一个数量级。利用高分辨率海洋模式表明，当示踪剂与粗糙地形上的混合热点接触时，它会迅速垂直扩散。由于扩散到较弱的底部流，其附近的示踪剂停留时间增加，从而弥补了这种热点的稀疏性。增加的示踪剂停留时间可能解释了为封闭深海环流所需的大量热盐垂直通量。
