The saltiness of seawater

  We have already know that cold water is heavier than warm and tends to sink. Salt, too, can make water heavy. On the whole, the proportion of salt in the open sea stays close to 3.5 percent. Near melting polar ice, however, the water tends to be less salty because the ice that is melting is nearly fresh. By contrast, the water near ice that is begin­ning to form will have more than an average amount of salt since the ice that is in the process of freezing leaves extra salt behind in the water. Since this kind of water—both cold and salty—will sink the deepest, the heaviest water is found at the very bottom of the sea. Heavy water of this sort, loaded with salt from beneath the antarctic ice shelf, rides the ocean floor all the way to the equator and across it into the Northern Hemisphere. The ride takes a long time. Some scientists figure that 300 years pass before a bit of cold, salty, deep water goes from the Antarctic to the equator. Others say that it takes 1,500 years. By contrast, a bit of warm, relatively unsalty water may take only a year to make the surface circuit of the North Atlantic wheel.
  There are also regions of the sea with strong up-and-down movements. Water will rise toward the surface whenever it meets heavier water. It can also rise from moder­ate depths to fill a space when surface water is carried away. Such movements are called upwellings. They are vital to the life of the sea and to man; they bring minerals with great nutritional value to the upper layers of the sea, where most marine life dwells. The world's most important fisheries are to be found in areas of upwelling.
  Although there are many mysteries about the causes of the tiny variations in temperature and saltiness that set subsurface masses of water in motion, the variations can be spotted and even turned to military advantage. Early in World War II, while conducting antisubmarine-warfare drills off Key West and in the Caribbean, the United States Navy found that its submarine detection devices, called sonar, often failed to detect submarines known to be down below. The difficulty was traced to temperature differences in the water, which bent the sonar's sound wave beams, as light beams are bent in a desert mirage. Temperature inversion— layers of cold water above warm—can bend sonar beams even more sharply.
There are other ways in which a knowledge of how salt can make deep water move has been exploited for military purposes.
  During World War II, enemy submarines traveling between the Mediterranean and the Atlantic made use of the fact that in almost completely enclosed arms of the sea, saltiness can vary considerably from the average saltiness of the open ocean. The Med­iterranean, for example, has so high a degree of saltiness (3.9 percent) that it is second only to that of the Red Sea (4 per cent).
  The average evaporation from the Mediterranean's surface is about 100,000 tons of water a second. This increases the saltiness and hence the density of the surface water. Particularly during the cool winter months, the heavy surface water sinks and flows westward through the narrow Strait of Gibraltar to spill out into the Atlantic. The wa­ter lost to the Mediterranean through this outflow must be replaced, so lighter water from the Atlantic pours back past Gibraltar on top of the salty, outgoing stream. During World War II, German and Italian subma­rines tried to use these currents to slip past the British blockades at Gibraltar. Turning off their engines to avoid giving their presence away, they hoped to drift in and out of the Mediterranean on the currents. A few actually made it.