What Drives Deep Currents?

Tidal and Gravitational Forces

• The moon's gravitational pull on the earth causes tides and currents, although it is not the sole cause. Basically, the moon's pull causes currents in medium to deep water. The effect of the moon's pulling on the water and the earth's pushing it back forms the tidal currents that affect deep water.

Thermohaline Circulation

• According to the National Atmospheric and Oceanic Administration (NOAA), thermohaline circulation has a major influence on deep ocean current formation. Thermohaline is a combination of two words: "thermo," which refers to temperature and "haline," which means salt. Thermohaline is the effect that happens when icebergs form and melt. When seawater freezes, it drives the salt out. The iceberg becomes like a giant ice cube. Because the surrounding water is saltier than deep water, the saltier water sinks because it is denser than regular seawater. In addition, the temperature difference causes cold water around the iceberg to sink. The end effect is that deep water currents form because of the temperature and salinity level differences in the ocean water. According to NOAA, this effect starts in the polar regions.

Coriolis Effect

• The Coriolis is a phenomenon that involves the spinning of the earth. To visualize it, think of holding a small flag outside a car window when the car is moving. Notice that the wind tries to take the flag because of drag. Now think of the car as being the earth and the wind as being the ocean. As the earth spins, it causes a drag on the ocean's deep water. This drag carries the water along, causing a deep current.

Ekman Transport

• The Ekman Transport phenomenon is also responsible for deep ocean currents. The top currents are formed by wind and temperature differences. As a top current flows, it drags the water below it. Each layer of water drags the layer below it. The end effect is that a deep ocean current eventually forms. To visualize this, place a deck of playing cards on a table. Press the open palm of your hand lightly on the top card. Slide the cards apart with the palm of your hand. Notice that each card is dragged by the one above it. The cards represent the ocean, your hand represents the wind and the table represents the ocean floor.

Cumulative Effect

• No single effect is 100 percent responsible for the formation of deep ocean currents. Rather, each effect contributes to the overall formation of currents. Currents tend to be stable and follow coastlines. For example, in the Atlantic Ocean north of the equator, currents start in the north polar regions because of Thermohaline effects. These currents travel south along the Canadian and North American Coastlines. Just north of the equator, the currents travel due west, eventually reaching the coastline south of Japan. They then turn north and travel along Japan's coastline, eventually forming a complete circle back to the North Pole.