Upwelling

The rotation of the Earth, combined with the winds that end to blow toward the equator and offshore along the west coast of South America, pushes coastal surface water toward the open ocean and away from the land. Consequently, cold water is drawn up from the ocean’s depths to replace the warmer displaced surface water.  This process is referred to as coastal upwelling. Coastal upwelling processes create regions in the ocean that are biologically highly productive. These condition are favorable to both living things in the ocean and the farmers in the surrounding area.

These cold upwelling contain a copious supply of inorganic nutrients. The upwelling of ocean water brings chemicals into the sunlit layer of the ocean. They are converted to nutrients through photosynthesis for phytoplankton at the bottom of the food chain. The plants are eaten by zooplankton and fish populations, many of which are then consumed by guano birds. These guano birds live off anchoveta, a fish about 17 cm long.

To understand how El Niño affects the ocean, we first need to learn about how surface winds move the water during normal years, and how the resulting motions affect water temperatures and amounts of chemical nutrients available to the food web. We will consider two separate regions: the equatorial Pacific extending westward from the Galapagos islands to beyond the dateline, and the coastal waters off Peru and southern Ecuador.
The easterly winds that blow along the equator and the southeasterly winds that blow along the Peru and Ecuador coasts both tend to drag the surface water along with them. The Earth's rotation then deflects the resulting surface currents toward the right (northward) in the Northern Hemisphere and to the left (southward) in the Southern Hemisphere. The surface waters are therefore deflected away from the equator in both directions and away from the coastline. Where the surface water moves away, colder, nutrient-rich water comes up from below to replace it, a phenomenon known as upwelling. Both the equatorial upwelling and the coastal upwelling are concentrated in narrow regions less than 100 miles wide which show up clearly in the satellite picture to the right.

The winds that blow along the equator also affect the properties of upwelled water. In the absence of the wind, the dividing layer between the warm surface water and the deep cold water, known as the thermocline, would be nearly flat; but the winds drag the surface water westward, raising the thermocline nearly all the way up to the surface in the east and depressing it in the west, as indicated in the figure below.

The cold water below the thermocline is rich in chemical nutrients. Wherever the thermocline is shallow enough, stirring by the wind mixes the nutrient-rich water with the surface water. In the presence of sunlight, tiny plant species called phytoplankton use the nutrients to produce a greenish plant substance called chlorophyll. These explosively growing "blooms" of phytoplankton use up all the available nutrients within a week, at which time they die and sink. During their brief lifetime in the Sun they are visible in satellite images as greenish patches of water, which serve as markers for places where upwelling is bringing nutrients to the surface. The surface waters above the thermocline would soon become devoid of nutrients were they not continually being replenished by upwelling.

The newly upwelled water is colder than its surroundings. It can be tracked for several weeks using infrared satellite imagery that reveals the water temperature. Its signature in the infrared images takes the form of a distinctive "cold tongue" extending westward along the equator from the South American coast.

So it is that the winds control the upwelling and the upwelling controls the phytoplankton production. The phytoplankton production, in turn, affects the lives of the tiny sea animals called zooplankton, which "graze" on them and, ultimately, this affects all the creatures at higher levels of the marine food web. The winds are also responsible for the cold tongue in the sea-surface temperature pattern.

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