User:StevePny/Global Ocean Circulation

=Global Ocean Circulation=

Introduction
Ocean circulation is the large scale movement of waters in the ocean basins. Winds drive surface circulation, and the cooling and sinking of waters in the polar regions drive deep circulation. (http://www.tsgc.utexas.edu/topex/ocean.html)

The general circulation of the atmosphere is closely related to the general circulation of the ocean. Winds blowing over the sea surface produce ocean currents. Winds also transport evaporated water, which precipitates elsewhere as rain. A large amount of heat is transferred to the atmosphere in this process. Evaporation and precipitation also affect the patterns of water density, which are inseparably tied to ocean circulation. (http://earthguide.ucsd.edu/virtualmuseum/climatechange1/10_1.shtml)

Dynamics
Surface circulation carries the warm upper waters poleward from the tropics. Heat is disbursed along the way from the waters to the atmosphere. At the poles, the water is further cooled during winter, and sinks to the deep ocean. This is especially true in the North Atlantic and along Antarctica. Deep ocean water gradually returns to the surface nearly everywhere in the ocean. Once at the surface it is carried back to the tropics, and the cycle begins again. The more efficient the cycle, the more heat is transferred, and the warmer the climate. (http://www.tsgc.utexas.edu/topex/ocean.html)

Seawater continuously moves around the globe as if it is on a huge conveyor belt, moving from the surface to the deep waters and back. Because the distance the water has to travel is so large, it takes about 1000 years for seawater to go all the way around the Earth. (http://www.atmosphere.mpg.de/enid/1vc.html)

The movement of water around the oceans has two parts which are strongly linked:

1. a density driven circulation which is driven by the differences in the density of seawater at different locations. The density of seawater depends on its temperature and salinity. As a result, this movement is known as the thermohaline circulation (Greek: thermo = heat, háls = salt). 2. a wind driven circulation that results in huge surface currents, for example the Gulf Stream.

http://www.atmosphere.mpg.de/media/archive/11915.gif

In the Northern Hemisphere
Ocean circulation transports surface seawater to the polar region where it cools. This cooling releases heat which warms the air and makes the water cold and, therefore, dense enough to sink to the bottom of the ocean. This results in the formation of new deep water which displaces existing deep water pushing it towards the equator. The major regions for this deep water formation are the Labrador and Greenland Seas in the northern North Atlantic Ocean. This North Atlantic Deep Water then flows south along the ocean floor allowing more warm surface water to flow into the region to replace it. Strong cooling also occurs in the Bering Sea in the North Pacific, but the structure of the ocean floor here prevents the deep water that forms from entering the ocean circulation. (http://www.atmosphere.mpg.de/enid/1vc.html)

Antarctica
Deep water formation also occurs around Antarctica during the production of sea ice. This ice contains very little salt and so, as the ice forms, the surrounding water becomes saltier and more dense. This very dense water slides down the edge of the Antarctic continent to form Antarctic Bottom Water. This water then spreads out and moves around most of the ocean floor. For some time Oceanographers thought that the deep waters that formed at the poles moved towards the equator, slowly warming and rising to the surface over the whole ocean, and that this water then returned to the poles in warm surface currents to complete the cycle. However, recent studies have shown that this gradual upwelling process is too slow to explain the age of seawater.

We now think that as deep water circulates around the bottom of the ocean, it meets the mid ocean ridges which are mountainous areas on the sea floor. The roughness of these causes strong mixing which forces the deep water to rise to the surface. The wind also causes strong mixing in the Southern Ocean and this also brings the deep water back to the surface. Once at the surface, the water returns to the poles in wind driven surface currents to complete its cycle. (http://www.atmosphere.mpg.de/enid/1vc.html)

Antarctica's role in global circulation:

Predicting near-term global climate changes depends on how well the many processes that make up Earth's climate system are understood, how realistically these can be expressed in global change models, and how effectively the evolution of the system can be extracted with powerful supercomputers.

One of the most important processes-one that is uniquely related to the Antarctic-is ocean ventilation, the process by which the deep ocean affects the atmosphere on the time scale of decades to centuries.

If we could mark a cubic meter of sea water and follow its global meanderings through the various oceanic current systems, we would find that it spends most of its time isolated in the deep ocean, where it is dark and cold. Only occasionally-once every 600 years on the average-would it appear on the surface, and then only in high southern latitudes. In the tropics and sub-tropics, a thin surface layer of warm (hence lighter) water prevents deep water from coming to the surface. South of the Antarctic Circumpolar Current, this warm layer disappears and no longer inhibits the upward movement of water. This process is called overturning or ventilation of the ocean. Typically, when deep water reaches the surface, it gives up heat to the much colder atmosphere and picks up dissolved atmospheric gases, including carbon dioxide.

The ventilation or overturning of the ocean can affect climate change in two important ways. The delivery of heat to the atmosphere raises the temperature at high southern latitudes, and the removal of carbon dioxide into the deep ocean for long-term storage reduces the greenhouse effect. The removal of carbon dioxide is crucial for global change.

We have learned much about how carbon dioxide and water vapor-two greenhouse gases-act as an insulating blanket in the atmosphere and that the surface temperature will increase as the atmospheric level of carbon dioxide increases. For nearly 40 years the rise of carbon-dioxide levels in response to fossil fuel and biomass burning, as well as fluctuations due to natural effects, has been observed.

However, to express effectively how the atmosphere reacts to the production of greenhouse gases in a global climate model, we have to understand how the atmosphere can shut off the overturning of the southern ocean. We can think of at least two ways: one is to create less dense water at the surface through warming or increased precipitation at high latitudes; the other is to slow down the Antarctic Circumpolar Current. The study of these processes is a continuing high priority in Antarctic oceanography. (http://www.nsf.gov/pubs/1996/nstc96rp/sb6.htm)

The Gulf Stream
The Gulf Stream is one of the most important wind driven currents. It transports very warm tropical water from the Caribbean Sea and the Gulf of Mexico across the North Atlantic to northern Europe. The warmth of the water heats the air above and the movement of this warm air is a very important way by which heat is transported northwards. As a result of this heat transport, northern Europe is very much warmer than corresponding latitudes in North America and countries around the Pacific Ocean.

For example, the yearly average temperature at Iqaluit (64oN, 068oW) in the Northwest Territories of Canada is -9.1 oC. This compares with an average for Trondheim (63oN, 010oE) in Norway of +4.8 oC. Long term records suggest that, as a result of the Gulf Stream, average temperatures in Northern Europe are 9 oC higher than the average temperatures for the same latitude elsewhere.

The Gulf Stream is an example of a western boundary current, a current which flows along the western side of a major ocean basin. The corresponding current in the Pacific Ocean is the Kuroshio Current, and in the Indian Ocean, the Aghulas Current. They result from an interaction between the shape of the ocean basin, the general direction of the wind and the rotation of the earth. They all have a high velocity (the Gulf Stream has an average velocity of 1 m s-1, thats 3.6 km h-1) they are all quite narrow (between 100 and 200 km wide) and all have a very important influence on the climate of the region. Eastern boundary currents also occur; these transport cold surface waters from the poles to the equator. They tend to be weaker than their western counterparts. (http://www.atmosphere.mpg.de/enid/1vc.html)

Heat Transport
Energy from the Sun doesn't fall equally all over the Earth. Most of the Sun's energy enters the Earth at the equator. This leads to large temperature gradients between the equator and the Poles. Movement of both the air and the oceans is controlled by these temperature differences and the result is a transfer of heat from the equator to the poles. About half the heat transport around the planet is by the oceans so the oceans are an extremely important part of the Earth's climate control system. If ocean circulation is changed by global warming, major changes in climate are therefore likely. Ocean circulation also transports oxygen from the air into the ocean making marine life possible. (http://www.atmosphere.mpg.de/enid/1vc.html)

Tracers
A number of tracers allow oceanographers to track the circulation of the global oceans. Examples of tracers include temperature, salinity, CFCs. By measuring the values of these tracers, one can estimate general flows of water masses in the ocean.