Our planet maintains a temperature gradient with the highest temperatures occurring at the equator and the lowest temperatures at the North and South Poles. The tropics receive more heat from the Sun than they reradiate back into outer space. The poles, on the other hand, reradiate more heat into outer space than they receive from the Sun. This heat differential must be equalized to prevent a runaway thermal imbalance in which the equatorial regions of our planet would be like Venus and the polar regions like Mars. The Earth prevents a runaway thermal imbalance by dispersing tropical heat to higher latitudes through ocean currents.
As we discussed in a previous posting (Climate Change: The Role of Water), water has a high heat capacity which means it can absorb and store significant amounts of heat without raising its temperature. Ocean water absorbs heat in the tropics and this warm water is transported by ocean currents to higher latitudes. At high latitudes the heat is transferred from the water to the atmosphere and air currents disperse this heat throughout higher latitudes. Once the ocean water releases the heat to the atmosphere the water cools and moves back to the tropics where it again absorbs heat and repeats the cycle. There are six great ocean circuits that flow around ocean basins. Two of these circuits reside in the northern hemisphere and the other four in the southern hemisphere. Five of these circuits (North Atlantic Gyre, South Atlantic Gyre, North Pacific Gyre, South Pacific Gyre and the Indian Ocean Gyre) transport heat from the tropics to higher latitudes. The sixth circuit is the Antarctic Circumpolar Current which doesn’t transport heat from the tropics because it only flows around Antarctica. The strong Circumpolar Current also isolates Antarctica from the warm water flowing from the tropics which helps the Antarctica maintain a colder temperature than the Arctic.
All gyres are composed of three currents which are the western boundary currents, eastern boundary currents and transverse currents. Western boundary currents (western boundary of ocean basins) transport warm tropical water to higher latitudes. Cold water is transported from high latitudes towards the tropics by eastern boundary currents (eastern boundary of ocean basins). The water in both the western and eastern boundary currents is transported across ocean basins by transverse currents through the action of surface winds. Westerlies are surface winds centered at about 45-degrees latitude and move in a west to east direction. Trade winds are surface winds centered at about 15-degrees latitude and move east to west.
In the North Atlantic Gyre the Gulf Stream is the western boundary current and transports warm tropical water northward along the east coast of the United States. The westeriles transport the warm water of the Gulf Stream across the Atlantic Ocean in the North Atlantic Current. As this water is transported across the ocean it cools by releasing heat to the atmosphere. When the North Atlantic Current reaches the coast of Europe it becomes the Canary Current and the cold water is transported south where the trade winds transport the water across the tropical ocean in the North Equatorial Current. As the water is transported across the tropics it absorbs and stores tropical heat and enters the Gulf Stream to repeat the cycle. The four other gyres transport heat from the tropics to higher latitudes by the same mechanism described above for the North Atlantic Gyre.
Ocean currents influence the climate in several ways. In the winter Scotland, Ireland and England have a maritime climate. As the westerlies cross the Atlantic Ocean they gain heat from the warm waters of the North Atlantic Current. When these winds come ashore they baths Scotland, Ireland and England with a warm atmosphere. At lower latitudes the situation is reversed. For example, Washington DC has hot and humid summers because the Gulf Stream brings warm, moist air to Washington, DC. However, in the winter Washington DC is cold because the westerly winds approaching Washington are chilled as the air crosses the North American continent.
Ocean currents can also reverse their course which has a profound effect on the global climate. El Nino is an example of how the reversal of ocean currents impacts the climate (see Climate Change: The Role of Water). During the El Nino of 1997 to 1998 heavy rains and flooding occurred in Peru and California. Hawaii, Southwestern Africa and New Guinea experienced extreme droughts. Another example of ocean current reversal is the Younger-Dryas Event. This event occurred at the end of the last ice age and lasted about 1,300 years. The melting of glacial ice in North America triggered a massive influx of fresh water into the North Atlantic Ocean which reduced the salinity of the ocean in the North Atlantic. Scientists believe this reduced salinity shut down or reversed the course of the North Atlantic Gyre triggering a “mini ice age” in Northern Europe, New England and the east coast of Canada while parts of Eastern Europe and tropical Africa experienced increased aridity.
Global warming is responsible for the oceans absorbing more heat. This translates into more heat being transferred from the tropics to the Arctic triggering the melting of Greenland glaciers and Arctic sea ice which accelerates global warming and thus more melting (see The Arctic Feedback Factor and Climate Change). Sea ice contains only fresh water because when it freezes the salt remains in the water. The salinity of the North Atlantic Ocean will decrease because the melting of Arctic sea ice and Greenland glaciers infuse freshwater into the Arctic Ocean. Will this trigger another reversal of the North Atlantic Gyre?