Close Window

Downwelling

Question

  1. What causes downwelling?
    Check Answer

    Downwelling occurs when the water on the surface of the sea becomes denser than the water beneath it and so it sinks. Seawater gets denser when it gets colder or saltier.

  2. Where does most downwelling occur?
    Check Answer

    Most downwelling happens at the poles. There, cold air chills the water. The water brought in by the surface gyres is pretty salty already, because it comes from the tropics, where evaporation increased salinity. And once it gets to high latitudes, the water becomes even more saline as ice forms and further concentrates sea salts.

  3.  

    Life on Earth nearly died out at the end of the Paleozoic Era 250 million years ago. At that time, all the continents had just come together to form a supercontinent called Pangaea, and a single superocean called Panthalassa. The global climate had warmed by several degrees, especially at the poles.

  4. How do you think these conditions might have affected ocean circulation, and led to low oxygen levels in the sea?
    Check Answer

    The arrangement of the continents would certainly make circulation patterns different than they are today. For example, it seems like on the surface, there wouldn't be so many gyres, since there was only one giant ocean basin. There might have been more east-west movement and perhaps fewer north-south currents, since there were fewer continental barriers to flows driven by the prevailing winds.

    If the air was warmer at the poles, then the water would be too. And without cold water and ice formation at high latitudes, there would be little downwelling and not much of a global conveyer belt in the deep sea. So all in all ocean circulation would probably have been much weaker than it is today. Oxygen in the deep sea would have been used up, and without strong downwelling, there was no way to replenish it.

Close Window