What are Ocean Currents and Gyres?
Currents and Gyres Explanation
According to the National Oceanic and Atmospheric Administration (NOAA) the force of “gravity, wind stress, and the apparent forces resulting from earth’s rotations” are what form currents. Steady currents, meaning those that occur over a long period of time in the same direction, occur when these forces become balanced between one another. They can also be generated by density differences where different oceans meet because of temperature and salinity differences. When currents are mixed with the friction of wind stress on the surface of the water gyres are created.
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The gyres’ movements help to drive the movement of oceans. The constant movement is what circulates ocean waters throughout the entire planet. This movement, also known as thermohaline circulation, regulates the oceans’ temperatures, saltiness, and nutrient flows.
Gyres in the Northern hemisphere move in a clockwise rotation while those in the Southern hemisphere move in a counter clockwise rotation.
There are three main types of gyres, subpolar, tropical, and subtropical. Subpolar gyres form in Polar Regions of the Earth in places with low atmospheric pressures. Tropical gyres form at the Equator, but the most common type of gyres are sub-tropical gyres. Subtropical gyres form between the two poles of the Earth and the Equator. Gyres most always contain a calm spot, also called an oligotrophic.[1] Oligotrophics are nutrient-poor because there is not a great deal of organic chemicals in or around them that are able to support producers.[2]
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Currents are still an important part of ocean flow and movement. There are two main categories that describe currents, surface currents and deep currents. Surface currents are normally created by wind and occur on the top (top meaning anywhere from 0 meters to a depth of 400 meters) of the ocean. Deep currents occur at the bottom of the ocean (meaning anywhere from 400 meters below the surface all the way to the ocean floor) and move in a straighter line at their highest point and then begin to fall towards the ocean floor. Once at the ocean floor, deep currents move across the floor and are then pushed back up off the floor to the top creating a circular motion.
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[1] Oligotrophic is an adjective that means to have little nutrients and support little growth.
[2] A producer is an organism within the food chain that produces its own energy and nutrients. They are also commonly known as autotrophs.
Looking at the ocean on a global-scale, large circulation patterns are constrained by the landmasses and continents that surround them. Oceanic gyres are formed when major currents link up with one another and neighbor or intersect with equatorial currents. There are five major oceanic gyres that sit between the seven continents and major landmasses: the North Atlantic gyre, South Atlantic gyre, Indian gyre, North Pacific gyre, and the South Pacific gyre.
Coriolis Effect
The Coriolis Effect is an essential part of both currents and gyres. The Coriolis Effect is caused when the rotation of the earth deflects the flow of currents. This causes the shallow layer on the surface of the ocean to be placed into motion by wind. In the Northern Hemisphere, currents are redirected to the right of the wind and create a clockwise rotation, while in the Southern Hemisphere currents are redirected to the left of the wind and create a counter-clockwise rotation. On the surface of the water, friction from between the wind and the Coriolis Effect create what is known as Ekman Flow that leads to the creation of gyres.[3] Gyres then cause water to move towards their center, which results in the middle of the gyre being higher than the surrounding surface. Because gravity forces water to move from a high to low-pressure area, the water at the center of the gyre is pushed outward and redirected by the Coriolis Effect. Eventually, this outward movement and force of the Coriolis Effect create a balance known as geostrophic flow.[4]
[3] Ekman Flow is a result of the Coriolis Effect. It occurs when the force of the wind moves water molecules on the surface and they catch then drag other water molecules that are deeper than them. Each layer of molecules is moved by the shallower layer and the deeper the layer the slower it moves, this goes on until the depth reaches about 100 meters below the surface.
[4] Geostrophic flow is when the horizontal movement of surface water rises and forms a balance between the pressure gradient force and the force from the Coriolis Effect.