Source: WeatherSTEM
Movement of water in the ocean is driven by two main factors: Frictional drag of wind over the ocean surface and the difference in the densities of water within the ocean. Most ocean circulation is driven by a combination of these two mechanisms. However, in general terms, frictional drag from wind drives shallow, surface currents with depths up to 100m or 300 ft. and density-driven currents are found deeper in the ocean. Let’s look at the first type of movement. Because of the frictional drag in water, ocean currents move more slowly than the prevailing winds that drive them. Because of this, as well as the presence of land masses, the motion of the air and water are not identical. Major ocean surface currents move in semi-closed circular patterns called gyres. Instead of going around the land mass, the current deflects backward, in a circular pattern. This phenomenon is called continental blocking and leads to the formation of Earth’s 5 main gyres.