What Causes Oceanic Currents?
Oceanic currents are the movement of water from one location to another. Currents are generally measured in meters per second or knots, where 1 knot equals 1.85 kilometers per hour (or 1.15 miles per hour).
Oceanic currents are important to life on Earth because they redistribute water, heat, nutrients, and oxygen around the globe. “When currents upwell, or flow up to the surface from beneath, they sweep vital nutrients back to where they’re needed most… Paleontologists have suggested that 250 million years ago, deep circulation slowed nearly to a stop, and the ocean began to stagnate. Low oxygen, sulfide and methane-rich waters filled the ocean deeps and then spread onto the continental shelves, wiping out 95% of all marine species in the greatest extinction event in Earth history” Ocean Explorer, NOAA. They also influence climate as the distribution of heat and moisture affects the atmosphere’s conditions and the weather.
Rise and Fall of the Tides
The tides are part of what causes currents in the ocean. They are stronger near the shore and bays. If the tide rises water moves inland, this is called a Flood Current. As the tide recedes the water moves sea-ward, this is called an Ebb Current. “Gravity is one major force that creates tides. In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational attraction of the sun and moon on the oceans of the earth” NOAA Ocen SErvece Education. The moon’s gravitational force pulls on water in the oceans so that it creates two tidal “bulges”, one towards the moon and the other in the opposite side due to inertia’s counterbalance.
On a global scale, currents are driven by two forces, the Wind, and the Thermohaline circulation.
Wind
Surface ocean currents are driven by the wind. Some prominent ocean currents are The Gulf Stream; this originates in the Gulf of Mexico and stretches to the tip of Florida and follows the eastern coastlines of the United States, and the Kuroshio Current located along the east coast of Japan. This is the ocean’s largest current beginning off the east coast of the Philipines, Taiwan and flows northeastward past Japan, where it merges with the easterly drift of the North Pacific Current.
Thermohaline Circulation
Below the surface, lower currents are driven by the Thermohaline circulation. “Thermo” means heat and “haline” refers to salinity. The Thermohaline circulation describes how the differences in temperature and salinity, or salt content in water play a part in the ocean currents. Cold salty water is dense and sinks to the bottom of the ocean.
“In the Earth’s polar regions ocean water gets very cold, forming sea ice. As a consequence, the surrounding seawater gets saltier, because when sea ice forms, the salt is left behind. As the seawater gets saltier, its density increases, and it starts to sink. Surface water is pulled in to replace the sinking water, which in turn eventually becomes cold and salty enough to sink. This initiates the deep-ocean currents driving the global conveyer belt” Ocean Service, NOAA.
This cold current travels south along the east coast of America and around Antartica. It branches up, one stream going towards the east coast of Africa and the other travels further east, around Australia and up north through the east of the Philippines. These two streams warm up and rise looping back around southward and westward. All these movements create the Global Conveyer Belt.