In January 1992, a container ship near the International Date Line, headed to Tacoma, Washington, from Hong Kong, lost 12 containers during severe storm conditions. One of these containers held a shipment of 29,000 bathtub toys. Ten months later, the first of these plastic toys began to wash up onto the coast of Alaska. Driven by the wind and ocean currents, these toys continue to wash ashore during the next several years, and some even drifted into the Atlantic Ocean.
The ultimate reason for the world's surface ocean currents is the sun. The heating of the earth by the sun has produced semi-permanent pressure centers near the surface. When wind blows over the ocean around these pressure centers, surface waves are generated by transferring some of the wind's energy, in the form of momentum, from the air to the water. This constant push on the surface of the ocean is the force that forms the surface currents.
Learning Lesson: How it is Currently Done
Around the world, there are some similarities in the currents. For example, along the west coasts of the continents, the currents flow toward the equator in both hemispheres.
These are called cold currents as they bring cool water from the polar regions into the tropical regions. The cold current off the west coast of the United States is called the California Current.
Likewise, the opposite is true as well. Along the east coasts of the continents, the currents flow from the equator toward the poles. There are called warm current as they bring the warm tropical water north. The Gulf Stream, off the southeast United States coast, is one of the strongest currents known anywhere in the world, with water speeds up to 3 mph (5 kph).
Ocean currents will have a huge impact on long-term weather any location will experience. For example, due to the Gulf Stream, the overall temperature of Norway and the British Isle is about 18°F (10°C) higher in the winter than other regions located at the same latitude.
Take it to the MAX! Keeping Current
While ocean currents are shallow-level circulations, there is global circulation which extends to the depths of the sea called the Great Ocean Conveyor. Also called the thermohaline circulation, it is driven by differences in the density of the sea water which is controlled by temperature (thermal) and salinity (haline).
In the northern Atlantic Ocean, as surface water flows north, it cools considerably. When the water cools to a point where sea ice forms, the salts are extracted (meaning sea ice is fresh-water ice). The extracted salts make the water beneath the sea ice more dense, causing it to sink to the ocean floor.
Learning Lesson: That Sinking Feeling
This motion drives a slowly southward flowing deep-ocean current. The route of the current is through the Atlantic Basin around South Africa and into the Indian Ocean and on past Australia into the Pacific Ocean Basin.
If the water is sinking in the North Atlantic Ocean, then it must rise somewhere else. This upwelling is relatively widespread. However, water samples taken around the world indicate that most of the upwelling takes place in the North Pacific Ocean.
It is estimated that once the water sinks in the North Atlantic Ocean, it takes 1,000-1,200 years before that deep, salty bottom water rises back to the upper levels of the ocean again.