Surface Currents

Ocean water moves in predictable ways along the ocean surface. Surface currents can flow for thousands of kilometers and can reach depths of hundreds of meters. These surface currents do not depend on the weather; they remain unchanged even in large storms because they depend on factors that do not change. (2)

Surface currents are created by three things: global wind patterns, the rotation of the earth, and ocean basins’ shape. Surface currents are significant because they distribute heat around the planet and are a significant factor influencing climate around the globe.

Global Wind Currents

Winds on Earth are either global or local. Global winds blow in the same directions all the time and are related to the unequal heating of Earth by the Sun, that is that more solar radiation strikes the equator than the polar regions, and the rotation of the Earth called the Coriolis effect. The causes of the global wind patterns will be described later when we look at the atmosphere. (2) Water in the surface currents is pushed in the direction of the significant wind belts:

• Trade winds are consistent winds that flow east to west between the equator and 30 degrees North and 30 degrees South latitude.
• Westerlies are the prevailing winds that blow from the west in the middle latitudes.
• Polar easterlies are winds that flow from the east between 50 degrees and 60 degrees north and south latitude, as well as the north and south poles.

Rotation of the Earth

The wind is not the only factor that affects ocean currents. The Coriolis effect describes how Earth’s rotation steers winds and surface ocean currents. The Coriolis effect causes freely moving objects to appear to move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The objects themselves are moving straight, but the Earth is rotating beneath them, so they seem to bend. (2)

An example might make the Coriolis effect easier to visualize. If an airplane flies five hundred miles due north, it will not arrive at the city that was due north of it when it began its journey. Over the time it takes for the airplane to fly five hundred miles, that city moved, along with the Earth it sits on. Therefore, the airplane will arrive at a city to the west of the original city (in the Northern Hemisphere) unless the pilot has compensated for the change. So, to reach his intended destination, the pilot must also veer right while flying north.

As wind or ocean currents move, the Earth spins underneath it. As a result, an object moving north or south along the Earth will appear to move in a curve, instead of in a straight line. Wind or water that travels toward the poles from the equator is deflected to the east, while wind or water that travels toward the equator from the poles gets bent to the west. The Coriolis effect bends the direction of surface currents to the right in the Northern Hemisphere and left in the Southern Hemisphere. (2)

Deep Currents

Thermohaline circulation drives deep ocean circulation. Thermo means heat, and haline refers to salinity. Differences in temperature and salinity change the density of seawater. Thermohaline circulation is the result of density differences in water masses because of their different temperature and salinity. (5)

Lower temperature and higher salinity yield the densest water. When a volume of water is cooled, the molecules move less vigorously, so the same number of molecules takes up less space, and the water is denser. If salt is added to a volume of water, there are more molecules in the same volume, so it is denser. Changes in temperature and salinity of seawater take place at the surface. Water becomes dense near the poles. Cold polar air cools the water and lowers its temperature, increasing its salinity. Freshwater freezes out of seawater to become sea ice, which also increases the salinity of the remaining water. This frigid, very saline water is very dense and sinks, a process called downwelling. (1)

Two things then happen. The dense water pushes deeper water out of its way, and that water moves along the bottom of the ocean. This deep-water mixes with less dense water as it flows. Surface currents move water into the space vacated at the surface where the dense water sank. Water also sinks into the deep ocean off Antarctica. Since unlimited amounts of water cannot sink to the ocean’s bottom, water must rise from the deep ocean to the surface somewhere. This process is called upwelling. (1)

Upwelling occurs along the coast when the wind blows water strongly away from the shore. This leaves a void that is filled with deep water that rises to the surface. Upwelling is significant where it occurs. During its time on the bottom, the cold deep water has collected nutrients that have fallen through the water column. Upwelling brings those nutrients to the surface. That nutrient supports the growth of plankton and forms the base of a vibrant ecosystem. California, South America, South Africa, and the Arabian Sea all benefit from offshore upwelling. Upwelling also takes place along the equator between the North and South Equatorial Currents. Winds blow the surface water north and south of the equator, so deep water undergoes upwelling. The nutrients rise to the surface and support a great deal of life in the equatorial oceans. (2)