Mass flows of water, or currents, are essential to understanding how heat energy moves between the Earth’s water bodies, landmasses, and atmosphere. The ocean covers 71 percent of the planet and holds 97 percent of its water, making the ocean a key factor in the storage and transfer of heat energy across the globe. The movement of this heat through local and global ocean currents affects the regulation of local weather conditions and temperature extremes, stabilization of global climate patterns, cycling of gases, and delivery of nutrients and larva to marine ecosystems.
Ocean currents are located at the ocean surface and in deep water below 300 meters (984 feet). They can move water horizontally and vertically and occur on both local and global scales. The ocean has an interconnected current, or circulation, system powered by wind, tides, the Earth’s rotation (Coriolis effect), the sun (solar energy), and water density differences. The topography and shape of ocean basins and nearby landmasses also influence ocean currents. These forces and physical characteristics affect the size, shape, speed, and direction of ocean currents.
Surface ocean currents can occur on local and global scales and are typically wind-driven, resulting in both horizontal and vertical water movement. Horizontal surface currents that are local and typically short term include rip currents, longshore currents, and tidal currents. In upwelling currents, vertical water movement and mixing brings cold, nutrient-rich water toward the surface while pushing warmer, less dense water downward, where it condenses and sinks. This creates a cycle of upwelling and downwelling. Prevailing winds, ocean surface currents, and the associated mixing influence the physical, chemical, and biological characteristics of the ocean, as well as global climate.
Deep ocean currents are density-driven and differ from surface currents in scale, speed, and energy. Water density is affected by the temperature, salinity (saltiness), and depth of the water. The colder and saltier the ocean water, the denser it is. The greater the density differences between different layers in the water column, the greater the mixing and circulation. Density differences in ocean water contribute to a global-scale circulation system, also called the global conveyor belt.
The global conveyor belt includes both surface and deep ocean currents that circulate the globe in a 1,000-year cycle. The global conveyor belt’s circulation is the result of two simultaneous processes: warm surface currents carrying less dense water away from the Equator toward the poles, and cold deep ocean currents carrying denser water away from the poles toward the Equator. The ocean’s global circulation system plays a key role in distributing heat energy, regulating weather and climate, and cycling vital nutrients and gases.
Density differences in ocean water drive the global conveyor belt. This global circulation system is also called thermohaline circulation. When broken down into its root words, what does “thermohaline” mean?
The global conveyor belt carries water and heat energy across the globe. What is the difference in how the conveyor belt moves water in the tropics compared to the Arctic?
What forces are responsible for tidal currents and are they predictable?
- Climate change leading to increases in ocean temperatures, evaporation of seawater, and glacial and sea ice melting could create an influx of warm freshwater onto the ocean surface. This would further block the formation of sea ice and disrupt the sinking of denser cold, salty water. These events could slow or even stop the ocean conveyor belt, which would result in global climate changes that could include drastic decreases in Europe’s temperatures due to a disruption of the Gulf Stream.
- It would take a single water molecule approximately 1,000 years to complete one full cycle of the global conveyor belt. In that time, the water molecule would travel through the waters of all the major ocean basins: Pacific, Atlantic, Indian, Southern, and Arctic.
- The volume of water transported by the global conveyor belt is equal to 100 Amazon Rivers or 16 times the flow of all the world’s rivers combined.
For Further Exploration
- National Geographic Environmental Literacy Teacher Guide Series: One Ocean, Chapter One—Ocean Currents
Term Part of Speech Definition Encyclopedic Entry climate Noun
all weather conditions for a given location over a period of time.
Encyclopedic Entry: climate Coriolis effect Noun
the result of Earth's rotation on weather patterns and ocean currents. The Coriolis effect makes storms swirl clockwise in the Southern hemisphere and counterclockwise in the Northern Hemisphere.
Encyclopedic Entry: Coriolis effect current Noun
steady, predictable flow of fluid within a larger body of that fluid.
Encyclopedic Entry: current density Noun
number of things of one kind in a given area.
Encyclopedic Entry: density ocean circulation Noun
worldwide movement of water (currents) in the ocean.
ocean conveyor belt Noun
system in which water moves between the cold depths and warm surface in oceans throughout the world. Also called thermohaline circulation.
Encyclopedic Entry: ocean conveyor belt rip current Noun
a strong flow of water running from the shore to the open ocean, sea, or lake.
Encyclopedic Entry: rip current salinity Noun
solar energy Noun
radiation from the sun.
Encyclopedic Entry: solar energy thermohaline circulation Noun
ocean conveyor belt system in which water moves between the cold depths and warm surface in oceans throughout the world.
the shape of the surface features of an area.
process by which currents bring cold, nutrient-rich water to the ocean surface.
Encyclopedic Entry: upwelling weather Noun
state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.
Encyclopedic Entry: weather