Currents in shallow water, such as that near beaches, are considered surface currents.
The ocean has both surface currents and deep currents. Surface currents are the ones that exist above 1,300 feet deep. This includes well known currents such as the gulf stream in the Gulf of Mexico. Other ocean currents, those that exist below 1,300 feet are known as deep ocean currents. These currents account for almost 90 percent of all of the water in a given ocean. Deep ocean currents may also be referred to as Thermohaline circulation. Ocean currents are measured in Sverdrup. One Sv is equal to a flow rate of 1 million meters cubed per second.
Temperature is a large factor in the cause of deep currents. Temperature affects how fast the currents are allowed to flow by increasing or decreasing the density of the water. Surface currents move at different speeds than deep currents because the water at the surface can be heated or cooled more easily by the sun. Because temperature varies widely across the surface of the Earth due to relative distance from the sun, it also varies greatly in deep ocean waters, allowing deep water currents to travel at different speeds as they change latitude.
The density of deep water in the oceans helps to determine the speed at which the water can move when being shifted along the current line. The density of the water is calculated utilizing both the temperature of the water and the salinity of the water, both of which can make deep waters heavier or lighter, increasing or decreasing the current flow of the water. Colder water temperatures will cause the density of the water to increase, making it harder for the current to push the water through, effectively slowing the current. The same thing happens with deep ocean water that has a high salinity. The added salt in the water adds weight, causing the water to move more slowly. Removing the salt content and heating the water causes the current to speed up.
Freshwater fluxes have some affect on the movement of deep water ocean currents. These fluxes occur when a freshwater source that deposits in the ocean changes the rate at which water flows into the ocean. These changes can occur due to rain, drought, dams, and movement of boats or other devices across the surface of the freshwater source. A sharp increase in the amount of freshwater deposited into the ocean, depending on the density of the water, can increase the speed of a deep water current, while a sharp decrease in freshwater can slow the current.
Gravity is one of the main causes of deep currents in the ocean. The Earth’s gravitational pull affects currents, as does the moon’s positioning at any given time. This gravitational force helps to keep deep ocean currents constantly moving in their given direction. Gravity also helps to determine the way in which currents travel across the ocean basin. Currents are pulled in the direction of the gravitational pull. Shifts in gravitational pull that cause currents to travel faster or to change direction can cause flooding on land.