It caused by the Coriolis effect; planetary vorticity along with horizontal and vertical friction, which determine the circulation patterns from the wind curl (torque).
Other ocean currents are essentially permanent and extend over large horizontal distances. On a global scale, large ocean currents are constrained by the continental masses found bordering the three oceanic basins. Continental borders cause these currents to develop an almost closed circular pattern called a gyre. On either side of the equator, in all ocean basins, there are two west flowing currents: the North and South Equatorial (Figure 8q-1) . These currents flow between 3 and 6 kilometers per day and usually extend 100 to 200 meters in depth below the ocean surface. Near the equator the water is about 8 centimeters high than in middle latitudes. This cause a very slight slope and water wants to flow down the slope. Winds blowing on the surface of the ocean push the water. Friction is the coupling between the wind and the water's surface. A wind blowing for 10 hours across the ocean will cause the surface waters to flow at about 2% of the wind speed. Water will pile up in the direction the wind is blowing. These waters move around the ocean basins by density driven forces and gravity. The density difference is a function of different temperatures and salinity. These deep waters sink into the deep ocean basins at high latitudes where the temperatures are cold enough to cause the density to increase. But the Coriolis Force intervenes and cause the water to move to the right (in the northern hemisphere) around the mound of water. These large mounds of water and the flow around them are called Gyres. The produce large circular currents in all the ocean basins. These waters move around the ocean basins by density driven forces and gravity. The density difference is a function of different temperatures and salinity. These deep waters sink into the deep ocean basins at high latitudes where the temperatures are cold enough to cause the density to increase. Surface Ocean Currents An ocean current can be defined as a horizontal movement of seawater at the ocean's surface. Ocean currents are driven by the circulation of wind above surface waters. Frictional stress at the interface between the ocean and the wind causes the water to move in the direction of the wind. Large ocean currents are a response of the atmosphere and ocean to the flow of energy from the tropics to polar regions. On either side of the equator, in all ocean basins, there are two west flowing currents: the North and South Equatorial (Figure 8q-1) . These currents flow between 3 and 6 kilometers per day and usually extend 100 to 200 meters in depth below the ocean surface. A gyre is a spiral or vortex of wind. It is the circular movement of currents in the ocean basin. Its movement is in the anti-clockwise direction in the southern hemisphere and clockwise direction in the northern hemisphere. Gyre is an important term in oceanography. Gyres can be caused by a number of reasons; one of them is the Coriolis force. The coriolis effect is caused because of the rotation of the earth. This effect causes a number of cyclones on the earth’s surface. These cause frictional surface currents towards the latitude at the center of the gyre. This build-up of water in the center creates flow towards the equator in the upper 1,000 to 2,000 m (3,300 to 6,600 ft) of the ocean, through rather complex dynamics. This flow is returned towards the pole in an intensified western boundary current. The term gyre can be used to refer to any type of vortex in the air or the sea, even one that is man-made, but it is most commonly used in oceanography to refer to the major ocean systems. A gyre in oceanography is any large system of rotating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity along with horizontal and vertical friction, which determine the circulation patterns from the wind curl (torque). The term gyre can be used to refer to any type of vortex in the air or the sea, even one that is man-made, but it is most commonly used in oceanography to refer to the major ocean systems. This is called the Coriolis effect. Winds, continents and the Coriolis effect make currents flow around the oceans in huge loops called gyres. The map shows a generalized pattern of ocean currents. In each ocean basin there is a roughly circular current flow called a gyre. Energy from the sun also causes currents to flow Ocean Currents and Climate. There are two type of Ocean Currents: 1. Surface Currents -- Surface Circulation. These waters make up about 10% of all the water in the ocean. These waters are the upper 400 meters of the ocean. 2. These waters move around the ocean basins by density driven forces and gravity. The density difference is a function of different temperatures and salinity. These deep waters sink into the deep ocean basins at high latitudes where the temperatures are cold enough to cause the density to increase.
http://www.physicalgeography.net/fundamentals/8q.html http://earth.usc.edu/~stott/Catalina/Oceans.html http://earth.usc.edu/~stott/Catalina/Oceans.html http://www.physicalgeography.net/fundamentals/8q.html http://www.qwhatis.com/what-is-a-gyre/ https://en.wikipedia.org/wiki/Ocean_gyre https://en.wikipedia.org/wiki/Ocean_gyre http://legacy.mos.org/oceans/motion/currents.html http://earth.usc.edu/~stott/Catalina/Oceans.html
what causes the gyres in each ocean basin to be circular