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Grade level: 9-12
Theme: ocean circulation
Video: density_effects.flv

I think most everybody can understand that if there is light fluid on top of heavy fluid then we have what's called a "stable system". Because things that are heavier are perfectly "happy" to be underneath something that's light. And what we say is that there is something that's less dense on top of something that's more dense.

But in the ocean it often happens that the reverse happens: so we might get something that's heavy, heavier (like) more dense waters on top of lighter water. And when that happens, we say that the ocean is unstable and we have "overturning". That means that the waters that are heavy come down to the bottom part of the ocean and those that are light need to go up. And that happens when waters are cooled and it also happens when the upper waters get saltier because of evaporation, etc.

So those density differences mean that the ocean is going to circulate: it's going to move. And those density differences can come about because of temperature changes or salinity changes.

Grade level: 9-12
Theme: ocean circulation
Video: density_reverse_direction.flv

The average density of sea surface water can be calculated from the average sea surface temperature and salinity using the state equation for seawater. This animation shows the long term average sea surface density, with light blue regions having the least density and dark blue regions having the greatest density. The sea surface density variations are actually very small, less than 3 percent overall, but the variation is very important.

There are three stable, dense regions in the ocean's surface, one in the sea around Iceland, Greenland, and Scandinavia and the other two near or under major Antarctic ice shelves. In these regions, the surface water becomes dense enough to sink and join the deep ocean currents. In fact, this sinking is thought to drive these deep currents as part of a system called the Thermohaline Circulation. This circulation has a strong effect on the Earth's climate, influencing the Gulf Stream, El Niño events, and both past and future climate shifts. (source)

Grade level: 9-12
Theme: ocean circulation
Video: sss_reverse_direction.flv

The heat of the sun forces evaporation at the ocean's surface, which puts water vapor into the atmosphere but leaves minerals and salts behind, keeping the ocean salty. The salinity of the ocean also varies from place to place, because evaporation varies based on the sea surface temperature and wind, rivers and rain storms inject fresh water into the ocean, and melting or freezing sea ice affects the salinity of polar waters.

This animation shows the long term average sea surface salinity, where white regions have the highest salinity and dark regions the lowest. Notice the higher salinity in the Atlantic, due partly to salty water coming from the Mediterranean, and the lower salinity at the mouths of major rivers. (source)

Grade level: 9-12
Theme: ocean circulation
Video: thermohaline_conveyor_iPod.m4v.flv

Surface ocean currents are driven mostly 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 mostly 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 degrees south is the the only part of the Earth where the ocean can flow all the way around the world with no land in the way. 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 and the surface circulation to close with the northward flow in the Atlantic. (source)

Grade level: 9-12
Theme: ocean circulation
Video: thermohaline_rev.flv

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 color on the world's ocean's at the beginning of this animation represents surface water density, with dark regions being most dense and light regions being least dense (see the animation Sea Surface Temperature, Salinity and Density). The depths of the oceans are highly exaggerated (100x in oceans, 20x on land) to better illustrate the differences between the surface flows and deep water flows. The actual flows in this model are based on current theories of the thermohaline circulation rather than actual data. 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 or simulate.

This version of the visualization combines the Earth look of the original thermohaline visualization with the new thermohaline flow field generated for the Science on a Sphere production, "Loop". (source)