Oceanography

Thermohaline circulation

Thermohaline circulation is generated by density differences between water masses and can result in horizontal and vertical flow. Water density increases as water becomes cooler or more saline. If density increases sufficiently, the water sinks to a level such that all water below it is more dense and above it is less dense. The water mass then flows outward in all unobstructed directions sinking below less dense and rising above more dense water masses it encounters.

Several forms of thermohaline circulation occur in the ocean. In subpolar to polar regions, cold, dense water descends and slowly flows towards the equator across the basin. These water masses may also be more saline than regular sea water because of the concentration of salts in the remaining water as sea ice forms. As the currents slowly flow, they mix with adjacent water masses and eventually lose their identity.

Excess evaporation in a restricted sea can produce dense, saline water that sinks to the appropriate density level as it flows out into the ocean. Circulation through the Strait of Gibraltar, the entrance to the Mediterranean Sea, is an example of this form of thermohaline circulation.

Thermohaline circulation also occurs in estuaries. As a river flows into the estuary, or narrow bay, the less dense fresh water forms a wedge that displaces the denser sea water. The wedge thins seaward as the fresh water spreads outward and slowly mixes with the sea water. Friction between the outflowing fresh water and the underlaying salt water establishes a form of circulation called estuarian circulation.

INTERNAL PROGRESSIVE WAVES

In contrast to surface waves, internal waves occur on boundarie(psynoclines) between water masses of different density within the ocean. Surface and internal waves are similar,but internal waves tend to be smoother, slower, more stable and can attain greater wave heights than surface waves. This is because the density difference between the water layers is less than that between air and water. Internal waves display many of the properties shown by surface waves, such as reflection, refraction and interference.

SEICHES

Standing waves that occur in natural basins are called seiches. Seiches appear as a rocking back-and-forth of the water surface or an internal boundary within the water column. The line about which the seiche rocks is called a node and it experiences no vertical motion. In contrast, the antinode is where the vertical motion is the maximum. A simple seiche has only one node in the center of the basin and two antinodes. The period of the seiche is the time required for the water surface to rock back and forth once and is dependent upon the length and depth of the basin.

Seiches display interference, refraction and reflection, but unlike other progressive waves, they do not move water in circular to elliptical orbits. In a seiche, water flows forward for a given distance, stops and then flows in the reverse direction. Maximum velocity is achieved when the surface is horizontal. No flow occurs when the surface is at a maximum tilt (antinode).

A rotary seiche is special type of seiche in which the position of the antinode rotates about the basin as the surface rocks back and forth. The node and antinodes are reduced to points.