0 km 10 20 30 40 50 60 70 80 90 100 110 120 130
Fig. 5.9 Temperature section across the Denmark Strait in latitudes 65-66° N, illustrating the southward flow of cold water from the Norwegian Sea (Worthington, 1969).
is narrow enough, the overflow will basically fill up the whole channel. The word "narrow" is defined in terms of the first baroclinic radius of deformation. Due to the relatively weak stratification and shallow water depth, the first baroclinic radius of deformation associated with overflows is quite small, on the order of a few kilometers. The Denmark Strait is very wide in this sense, so the overflow must appear in the form of a current confined to the right-side bank of the channel.
The book by Pratt and Whitehead (2007) is highly recommended for the reader who is excited about deep waterfalls in the oceans. For the general reader who needs an introduction to hydraulics, the papers by Gill (1977) and Pratt and Helfrich (2003) are a good place to start.
Deep waterfalls in the world's oceans
There are numerous deep waterfalls in the world's oceans. Typical deep waterfalls include the water exchange through the Strait of Gibraltar, the overflow through the Denmark Strait, and many others. Deep waterfalls are regulated by rotating hydraulics. Flows through these falls play a critically important role in regulating water mass transportation and transformation in the oceans. As an example, the positions of these deep waterfalls in the Atlantic Basin are shown in Figure 5.10.
The spreading of AABW in the Brazil Basin is a good example for illustrating the movement/transition of bottom water in the world's oceans. Here the coldest water with potential temperature as low as -0.4°C can be identified by the clusters of pink color in the middle of the basin in Figure 5.11. It is obvious that such cold water must come from the south, because there is otherwise no local source of such cold water in the basin. Note that the Vema Channel through which AABW enters the Brazil Basin from the southern edge is so narrow that it cannot be clearly shown in Figure 5.11.
From Figure 5.11 it can readily be seen that no AABW can escape through lateral boundaries of the basin; thus, all AABW entering through Vema and Hunter Channels in the south has to be removed from above through diapycnal mixing. The water mass budget in this basin requires a diapycnal diffusivity of 1-5(x 10-4 m2/s) (Morris et al., 2001).
The flow of bottom water through the channel can be seen clearly through a meridional section along the coast, located around 30° W over the northern half of the basin (Fig. 5.12). Four neutral density surfaces, y = 28.27,28.205,28.16, and 28.133 kg/m3, are also included as colored lines. The overflow can be readily seen on the bottom of the seafloor in Figure 5.12.
In many cases, dense water is formed in marginal seas and flows to the open ocean as outflow. Major outflows have several essential components in common (Price and Baringer, 1994):
• Air-sea exchange that produces dense water due to a combination of heat and freshwater fluxes from the ocean to atmosphere. In addition, saline rejection due to sea-ice formation can also contribute to the formation of dense water.
• Marginal sea and open ocean exchange that allows dense water formed in semi-closed marginal seas to flow into the open ocean.
• Descent and entrainment that modify the properties of the outflow water. In general, the volume of the overflow water increases more than 100%.
For the world's oceans there are four major sources of overflow: (1) Mediterranean Sea, (2) Denmark Strait, (3) Faroe Bank Channel, and (4) Filchner Ice Shelf (on the outer edge of sea ice near Antarctica).
The properties of these outflows are listed in Table 5.1. Note that although at the site of their original formation, the density of the Mediterranean outflow is the greatest of these four sources, at depths greater than 2 km, water from the Mediterranean outflow is the lightest. On the other hand, although at sea level the water mass formed off the Filchner Ice
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