The Circulation of the Oceans Learning Objectives

2. 
   Use a rough map sketch to help explain the role that the oceans play in determining the climates of southern South America and Southern Africa, poleward 20^oS.
   

Ocean currents move warmer water toward the poles and cooler water towards the equator. The Benguela and Humboldt Currents, which are cold offshore currents, are responsible for the Namib Desert on the west coast of South Africa and the Atacama Desert on the west coast of South America, respectively. The cold ocean currents reduce evaporation and cool the air that moves over them, a combination that inhibits convection and precipitation over the adjacent coastline. The deserts that form in this way are called littoral (alongshore) deserts. In fact, one of the driest deserts in the world is the Namib Desert along the coast of southwest Africa. The desert of Baja, California (actually in Mexico), is another example.

3. 
   Water is a very unusual substance in that it reaches maximum density between the freezing point and 4°C, depending on salinity. As you cool the water surface to these temperatures it becomes denser and sinks (rather than immediately freezing). This means that you have to cool the whole water body (the lake or the surface, mixed, layer of the ocean) to this temperature before you can cool the surface layer enough to freeze, which is why some lakes can remain unfrozen even when the air temperature drops well below freezing. When you have cooled the surface layer to the freezing point, water is again unusual in that its solid form (ice) is actually less dense than the liquid, so ice floats. Consider how different the world would be if water behaved like most other substances and continued to increase in density down to the freezing point, and if ice was denser than liquid water. Speculate on what this might have meant for life on the planet.
   

Seawater already behaves in this way, i.e., it gets denser as its temperature drops from 4^oC to -2^oC (the freezing point of water with a salinity of 3.5%). However, if ice sank when it formed, then there might be a danger that the entire ocean would freeze from the bottom up. This would be countered by warming from geothermal heat. See critical problem at the end of Chapter 12. Lakes would behave differently than they do now, they would begin to freeze more quickly in winter than they do now, and they might also tend to fill with ice from the bottom up. The consequences for the life in this scenario could be quite severe-most modern organisms would struggle to survive.