Sea level
The sea level is the height level of the sea surface. It corresponds approached an equipotential surface of the Earth's gravity. From a long-term mean sea level, the current sea level, a distinction that is affected by tides, wind and waves, currents and salinity.
The mean sea level
In geodesy often serves a mean water level (MW) as the zero level for heights ( altitudes ). To this end, the at tide gauges ( Mareografen ) be measured water levels averaged over the decades, so that seasonal effects and the tides are virtually eliminated. Such determined in different periods means of various gauging stations are not exactly on an equipotential surface, which is why the zero level of the level of systems of various countries differ slightly. For example, the differ - Mediterranean level Trieste and Genoa to Amsterdam levels by about 30 cm.
The mean sea level corresponds largely to the geoid, and is used also as a reference surface for a geodetic earth model. The deviations from the geoid are caused mainly by ocean currents and can reach several decimetres.
The current sea level
Tide and Wind
In most places, the sea level fluctuates during the day by the tide. You have a period of an average of 12 ½ hours and depend on the state of the moon, and to a lesser extent from the sun. Flood ( often wrongly equated with flood ) prevails in the sea area over which the Moon, and on the exact opposite side, while the low tide occurs in between. A particularly strong tide, the spring tide occurs when the moon and sun are on the same side of the earth ( new moon, the conjunction of the Sun and Moon) or when they are facing each other ( full moon or opposition).
During the tidal accounts on the open sea than ± 0.5 meters, it is near the coast because of possible congestion effects often significantly larger ( values of several meters can be used by tidal power plants). If stroke reinforced by onshore winds ( winds towards the land ), a storm surge can occur. As a neap tide is referred to those considered in the Sun and the Moon from the Earth at an angle of 90 ° to each other are ( increasing or decreasing half moon) and the Flood, therefore, is less developed.
Due to the tidal water level differences and the associated shifts huge bodies of water occur in shallow waters, at the Cape and at constrictions between islands strong currents - see tidal current.
Wind over long distances ( Fetch ) produced by continuous adhesion to the water surface at the distant end of a water surface elevation of a water table. Here, surface water moves slowly in the wind direction (drift current). At the nearer end this results in a reduction. In narrow areas, such as between islands, both the water level differences and the flow to be strengthened.
Lesser, but measurable influence also regional differences in water temperature and salinity. They are the causes of most ocean currents.
Climate and uplift or subsidence of the sea level
The sea level also depends on long-term changes in climate and other influences such as Geodynamics and isostasy. Since about 1880, there was a measurable rise in sea level, which could accelerate in the future. Reasons for the increase are defrost glaciers and the thermal expansion of seawater. Evaluations of level measurements and measurements by satellite altimetry using TOPEX / Poseidon have shown that the sea level rises by currently around 18.3 cm per century the global average. This was also confirmed by the IPCC. However, the analyzes of uncertainty in the order of about 5 cm are subject. From a long-term increase particularly low-lying coastal areas and cities and islands would be threatened.
These changes are regionally very different, because each mass shift also affects the geoid and the Erdschwerpunkt. After Bretterbauer there are therefore regions with decreasing sea level.
In the last ice age, sea level was lower than today, depending on the location 80-150 m, as more water was bound as today in the much more extensive ice caps. The differences between the various types arise from the fact that the ocean floor lowers when more water is on, while he stands out, if so reduce the amount and weight of the water. If large glaciers cover the mainland, is also the country lowers the weight of the glacier.
A slow, tentative advance of water is referred to as ingression, under transgression meant a rapid advance. A gradual receding of the water is known as regression.
Regional variations in gravity
The geometric height of the water surface (which can be considered as approximated ellipsoidal ) also depends on the density of the Earth's interior: The closer the local mantle is, the higher at these sites gravity. Thus, the water level in the Indian Ocean in Sri Lanka up to 105 meters lower than average, northeast of Australia in New Guinea, the sea level is up to 80 meters higher than the average. The water level adjusts itself to the local gravitational potential of the earth. A ship which follows the route described, therefore, does no work to overcome the differences in height. This means that all points with the same gravity potential (which form an equipotential surface ) which have the same height, the height is also physically defined (see Height [ geodesy ] and geoid ).