Thermohaline Circulation on a Flat Map

The oceans are mostly composed of warm salty water near the surface over cold, less salty water in the ocean depths. These two regions don't mix except in certain special areas. The ocean currents, the movement of the ocean in the surface layer, are driven primarily by the wind. In certain areas near the polar oceans, the colder surface water also gets saltier due to evaporation or sea ice formation. In these regions, the surface water becomes dense enough to sink to the ocean depths. This pumping of surface water into the deep ocean forces the deep water to move horizontally until it can find an area on the world where it can rise back to the surface and close the current loop. This usually occurs in the equatorial ocean, mostly in the Pacific and Indian Oceans. This very large, slow current is called the thermohaline circulation because it is caused by temperature and salinity (haline) variations.

This animation shows one of the major regions where this pumping occurs, the North Atlantic Ocean around Greenland, Iceland, and the North Sea. The surface ocean current brings new water to this region from the South Atlantic via the Gulf Stream and the water returns to the South Atlantic via the North Atlantic Deep Water current. The continual influx of warm water into the North Atlantic polar ocean keeps the regions around Iceland and southern Greenland generally free of sea ice year round.

The animation also shows another feature of the global ocean circulation: the Antarctic Circumpolar Current. The region around latitude 60 south is the only part of the Earth where the ocean can flow all the way around the world with no obstruction by land. As a result, both the surface and deep waters flow from west to east around Antarctica. This circumpolar motion links the world's oceans and allows the deep water circulation from the Atlantic to rise in the Indian and Pacific Oceans, thereby closing the surface circulation with the northward flow in the Atlantic.

The flows in this visualization are based on current theories of the thermohaline circulation rather than actual data or computational model runs. The thermohaline circulation is a very slow moving current that can be difficult to distinguish from general ocean circulation. Therefore, it is difficult to measure and simulate.

This visualization was produced for the Science On a Sphere production "Loop". It is intended to be over-layed on a world map background. Below are 3 sets of 4 sequences. The first set of 4 sequences are all composited over a world map background with a limited number of frames that make them loopable (with a very slight jump at the point where the looping happens). This is primarily provided for real-time displays such as hyperwall systems. The 4 sequences are: all depth layers combined, shallow depths, middle depths, and deep depths.

The second set is the same as the first set except that the layers are not composited over the background and instead include and alpha channel. The third layer is actually the frames that were used in the film "Loop" and consist of a large number of continuous, seamless frames. Each sequence is as before, all layers, shallow, middle, and deep layers all with alpha channels.

The depth layers nominally correspond to the following ranges below sea level: shallow (0m - 600m), middle (1875m - 2500m), and deep (3000m - 4000m). These depths do vary with bathymetry. So, in areas where the sea floor is not very deep, these depths are scaled so that the flows do not interesct the sea floor or each other.