The term "current" refers to the motion of water. Upwelling and downwelling currents are coastal and ocean phenomena that help keep ocean ecosystems going. In an upwelling current, deep water is pulled up as surface water moves away from land. The reverse action occurs in downwelling; winds blow surface water toward a coastline where the water accumulates and sinks. These processes cycle ocean nutrients by churning the nutrients and carbon dioxide in deep, cold water with the oxygen found in surface water. Downwelling keeps dissolved oxygen from feeding organic decomposition on the surface. Upwelling pulls nutrients from the deep ocean and makes them available to distant ecosystems.
There are three common types of current systems that create upwelling and downwelling in the oceans:
- Equatorial: East-west trade winds blow along the Equator. Due to the Coriolis effect, surface water gets pushed to the north and south. As the surface water moves away, water is drawn up from below to replace it.
- Coastal: As currents or winds move along a coastline, they can move the water away from the beach and draw cooler water to the surface. This phenomena partially explains why California has such cool ocean water temperatures compared to the warm local air. Its local ocean current travels down from the north pacific, so there is always a steady supply of colder water not far offshore.
- Seasonal: As land masses and their adjoining bodies of water go through seasonal changes, the prevailing wind direction will often shift 180 degrees. This change occurs as the land becomes warmer or cooler than its neighboring water. This can create an upwelling or downwelling effect, depending on the wind's direction [source: NOAA]. And this effect can be seen in lake regions as well as marine locales.
The water in rivers, marshes, ponds and swimming pools all have currents of varying strength. Oceans have an elaborate system of currents that includes rips and tides. Ocean currents greatly influence weather and the movement of marine life and nutrients. They cause temperatures in Western Europe to be warmer and support animal and plant life in the Antarctic. A disruption to ocean currents about 250 million years ago is one possible explanation for a mass extinction of marine life [source: NOAA Ocean].
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