What is Oceanic Upwelling?
Upwelling is simply a current with a vertical component from deep water toward the surface. It happens naturally when the wind pushes water away from an area, and the deep water comes up from below to replace it. Due to the Coriolis effect, the surface current is generally not in the same direction as the wind. If this figure depicts the west coast of California, and the wind is coming from the north, then the surface current moves away from the coast.
There are specific regions in the world where upwelling occurs, shown here in red. There is an abundance of marine life in these areas. The upwelling regions constitute about one percent of the surface of the ocean, yet they account for 50 percent of the fisheries catch worldwide! Along the west coast of South America, upwelling does not occur during the weather pattern called El Niño and the fishing is terrible. The question is why does upwelling create more fish?
Layers in the Ocean
Because the ocean is layered. Note that the y axis is ocean depth going down to 2000 meters. The x-axis is percent maximum of either nutrients or oxygen. Notice that near the surface, there is a minimum of nutrients, and an abundance of oxygen. This oxygen does not come from the air. It was created by phytoplankton, the microscopic plants of the sea. They produce more than half of the world's oxygen. Phytoplankton are constrained to the photic zone which is the the top 200 meters at best because they need light for the process of photosynthesis, and light does not penetrate water all that well. Beneath that, the deep ocean is a reservoir of nutrients such as phosphate and nitrate that plants need for food. On land when a plant or animal dies, the remains are returned to the earth, and then taken up by the roots of plants. But in the deep ocean the remains sink below where photosynthesis can use them, and nutrients remain suspended in the deep water. Upwelling brings the nutrients toward the surface. Phytoplankton grow rapidly where and when there is upwelling, then the zooplankton, the microscopic animals, eat the phytoplankton, and fish eat the zooplankton.
When photosynthesizing plants get all the ingredients that they need, growth is rapid. This satellite image demonstrates how fast plants can grow in the ocean. This is a true color photograph of Tasmania during an algal bloom. During the winter months there is no light, and algae can't grow in the ocean. And during those winter months there is natural upwelling and there also is run off from the land depositing nutrients in the water. Then along comes spring and sunshine, and the water turns from blue to green (this can happen in just days!). Most of the life in the ocean is confined to the continental shelf, and the deep ocean (which is most of the earth) is relatively barren. Photosynthesis needs light, water, carbon dioxide, and nutrients, and in the deep ocean the nutrients are in abundance 200 meters below the surface. That is why artificial upwelling has been attempted, with design ideas going back to before WWII.