Extreme conditions have made deep-sea exploration nearly as challenging as space exploration, and nearly as infrequent. The deep waters of the ocean, surprisingly, do not provide a uniform environment where marine organisms are uniformly distributed. They are affected by a number of topographical features that are much more dramatic than any found on land.
Examples of these large-scale topographical features, include mid-ocean ridges, sea mounts, ocean canyons and trenches. Water movement in the ocean is also affected by winds and the Corioliís forces of the spinning earth at the surface while density effects due to temperature and salinity are more important for watermass movements at deeper levels.
Surface circulation in the North Atlantic Ocean
The advent of satellite imagery measuring sea surface temperatures has made it easier to follow surface current patterns.
These patterns are strongly influenced by winds and the Corioliís forces of the spinning earth. In the North Atlantic Ocean, the surface circulation forms two large gyres or giant, circular ocean currents; the subpolar gyre and the subtropical gyre. In the subtropical gyre water from the subtropics is transported northward along the coast of North America in a fast-moving, warm current; the Gulf Stream.
The current waters can be easily identified on satellite images showing sea surface temperatures. This picture is a composite for several days in March, 2004. You can see the thin yellow-orange line moving along the coast of North America. At around the Grand Banks (~45 degrees N), the Gulf Stream divides with some of the water being re-circulated to the west, some moving directly east across the Atlantic as the Azores current and some continuing north-east as the North Atlantic Current.
Many people confuse the Gulf Stream and the North Atlantic Current. Unlike the Gulf Stream, the North Atlantic Current is not a well defined, fast-moving current. This next image is also a different composite image of March sea surface temperature data. While you can still see a pattern of movement of the warmer water, the current is much less focused than the Gulf Stream was along the coast of North America. The boundary between the warm water of the subtropical gyre and the cooler, less saline water of the subpolar gyre is called the Sub Polar Front. Frontal zones are often highly productive border areas separating ecosystems of markedly different characteristics.
Paulo Polito and Ken Casey, graduate students at the University of Rhode Island, made the Sea Surface Temperature composite picture using four years of SST data from the satelite Pathfinder for the month of March.
The easterly moving waters cross the mid-Atlantic Ridge between ~45 degrees N and the Charlie Gibbs Fracture Zone at ~52 degrees N. After crossing the mid-Atlantic Ridge, the North Atlantic Current and Sub Polar Front waters turn north dividing into a number of branches. The branch that passes Europe and up into the North and Norwegian Seas is responsible for making northern Europe, and especially Scandinavia, a more hospitable place to live than other places of similar latitude. This warmer current, a branch of the North Atlantic Current, is commonly referred to as a continuation of the Gulf Stream, but technically the fast-moving, narrow stream is only found along the eastern coast of North American. Together, the warmer waters of the Gulf Stream, the North Atlantic Current and at the Sub Polar Front are clearly discernible in sea surface temperature satellite images.
Deep-water circulation in the North Atlantic Ocean
Differences in density due to temperature (colder is heavier) and salinity are the driving forces behind deep-water circulation. Most of the North Atlantic Deep Water originally comes from the Norwegian Sea. This cold water flows south over the sills between Iceland and Scotland as well as over sills between Iceland and Greenland and into the eastern and central parts of the North Atlantic. The deep cold water movement pattern is largely determined by sea-floor topography, moving along the basin walls defined by the mid-Atlantic Ridge and the continental shelves.
At the Charlie Gibbs Fracture Zone, a major fracture feature cutting through the mid-Atlantic Ridge at around 50?N, there are actually three levels of current movement. At the surface, warmer waters of the North Atlantic Current move across the ridge in this area from west to east. At mid-depths a low salinity intermediate depth current from the Labrador Sea also moves through the Fracture Zone from west to east. While at depth the cold, heavy waters, which originated in the Norwegian Sea, move from east to west.
Sometimes, when ocean current systems interact, surface water masses move toward each other and sink along the meeting line -- concentrating phytoplankton and the nutrients that they need for growth. These zones of enhanced productivity are often visible in satellite ocean colour data recording chlorophyll production, for example.
Ocean currents can be measured with various flow meters that can be lowered into the sea from ships. While useful, such ship-based measurements provide only snap-shots of the water movement situation. Longer-term measurements have been obtained from unmanned floats either attached at specific locations or freely drifting in the sea.
Recently information from satellite images relating to sea surface temperatures or chlorophyll levels have also given valuable overviews of surface current patterns.