Patterns of Ocean Circulation
Environmental scientists study ocean circulation because, along with patterns of air movement in the atmosphere, the movement of water through the oceans helps determine weather and climate conditions for different regions of the world. The three main patterns of ocean circulation are gyres, upwelling, and thermohaline circulation.
Patterns of ocean circulation: Gyres
As the prevailing winds in earth’s atmosphere blow across the surface of the oceans, the winds push water in the direction that they’re blowing. As a result, the surface water of the oceans moves in concert with the air above it.
This dual movement creates large circular patterns, or gyres, in each of the planet’s oceans. The ocean gyres move clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Ocean gyre circulation moves cold surface water from the poles to the equator, where the water is warmed before the gyres send it back toward the poles. The water’s temperature influences the temperature of the air: Cold currents bring cooler air to the coastline as they move toward the equator, and they bring warmer air to the continents they pass on their way back toward the poles.
Patterns of ocean circulation: Upwelling
Sometimes the movement of surface currents along a coastline leads to a circulation process called upwelling. As a result of the Coriolis effect, upwelling commonly occurs on the west coast of continents, where the surface waters moving toward the equator are replaced by deeper cold water that moves up to the surface.
The deep water brings with it nutrients from the bottom of the ocean. These nutrients support the growth of primary producers, which support the entire food web in the ocean.
Regions of the world where deep ocean upwelling occurs are often very productive with high numbers of many different types of organisms living in them.
Patterns of ocean circulation: Thermohaline circulation
The largest circulation of water on the planet is a direct result of changes in temperature and salinity. Salinity is the measure of dissolved salt in water. The pattern of ocean currents related to salinity and temperature is called the thermohaline circulation (thermo = heat; haline = salt). This figure gives you a general idea of what this pattern looks like.
Sometimes called the thermohaline conveyor belt, this circulation pattern moves cold water around the globe in deep water currents and warmer water in surface currents. A single molecule of water being transported by thermohaline circulation may take a thousand years to move completely throughout the Earth’s oceans.
The conveyor is driven by changes in the density of water as a result of changes in both temperature and salinity. Here’s how this circulation pattern works:
Warm water in a shallow current near the surface moves toward the North Pole near Iceland. As this water reaches the colder polar region, some of it freezes or evaporates, leaving behind the salt that was dissolved in it. The resulting water is colder and has more salt per volume than it did before (and thus is more dense).
The cold, dense, salty water sinks deeper into the ocean and moves to the south, as far as Antarctica. After it makes its way near Antarctica, the cold, deep current splits, one branch moving up toward India into the Indian Ocean and the other continuing along Antarctica into the Pacific Ocean.
Each branch of the cold, deep current is eventually warmed in the Indian Ocean or the northern part of the Pacific Ocean. Although the water still contains the same amount of salt, it’s a little less dense because it’s warmer than the cold water surrounding it; as a result, it moves upward, becoming a surface current.
The warm, shallow, less dense surface current moves to the west, across the Pacific Ocean, and into the Indian Ocean, where it rejoins the Indian Ocean branch. Both branches then continue into the Atlantic Ocean and head back toward the North Pole.
Environmental scientists who study global climate change are interested in how increased ice melting in the Arctic and Greenland will affect the thermohaline circulation. The addition of large amounts of fresh water will reduce the salinity and density and may change the pattern of global ocean circulation.