Ledwell JR, Montgomery ET, Polzin KL, Schmitt RW, Toole JM
Woods Hole Oceanographic Institution, Massachusetts 02543, USA.
Nature. 2000 Jan 13;403(6766):179-82. doi: 10.1038/35003164.
The overturning circulation of the ocean plays an important role in modulating the Earth's climate. But whereas the mechanisms for the vertical transport of water into the deep ocean--deep water formation at high latitudes--and horizontal transport in ocean currents have been largely identified, it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished. Turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise. However, measurements of the internal wave field, the main source of energy for mixing, and of turbulent dissipation rates, have typically implied diffusivities across surfaces of equal density of only approximately 0.1 cm2 s(-1), too small to account for the return flow. Here we report measurements of tracer dispersion and turbulent energy dissipation in the Brazil basin that reveal diffusivities of 2-4 cm2 s(-1) at a depth of 500 m above abyssal hills on the flank of the Mid-Atlantic Ridge, and approximately 10 cm2 s(-1) nearer the bottom. This amount of mixing, probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry, may be large enough to close the buoyancy budget for the Brazil basin and suggests a mechanism for closing the global overturning circulation.
海洋的翻转环流在调节地球气候方面起着重要作用。虽然水垂直输送到深海的机制——高纬度地区的深水形成——以及洋流中的水平输送机制已基本明确，但目前尚不清楚从深海到表层的补偿性垂直水输送是如何实现的。跨越等密度面的湍流混合是降低水体密度并使其上升的唯一可行机制。然而，对混合的主要能量来源——内波场以及湍流耗散率的测量通常表明，跨越等密度面的扩散系数仅约为0.1平方厘米每秒（-1），太小而无法解释回流现象。在此，我们报告了在巴西盆地进行的示踪剂扩散和湍流动能耗散测量结果，这些结果显示，在大西洋中脊侧翼深海丘陵上方500米深处，扩散系数为2 - 4平方厘米每秒（-1），在更接近海底处约为10平方厘米每秒（-1）。这种混合量可能是由流经粗糙海底地形的潮流产生的内波破碎所驱动，其可能足以平衡巴西盆地的浮力收支，并为全球翻转环流的平衡提供了一种机制。
