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Water cycles between ocean, atmosphere and glaciers. Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time long enough that fluctuations caused by waves and tides are smoothed out, typically a year or more.
Tides are the regular rise and fall in water level experienced by seas and oceans in response to the gravitational influences of the Moon and the Sun, and the effects of the Earth's rotation. During each tidal cycle, at any given place the water rises to a maximum height known as "high tide" before ebbing away again to the minimum "low tide" level.
Mean high water (MHW) is the average of all the daily tidal high water levels observed over a period of several years. It is not the same as the normal tidal limit. In the United States this period spans 19 years and is referred to as the National Tidal Datum Epoch. [7]
Sea level is affected not only by the volume of the ocean basin, but also by the volume of water in them. Factors that influence the volume of the ocean basins are: Plate tectonics and the volume of mid-ocean ridges: the depth of the seafloor increases with distance to a ridge, as the oceanic lithosphere cools and thickens.
Tidal range is the difference in height between high tide and low tide. Tides are the rise and fall of sea levels caused by gravitational forces exerted by the Moon and Sun, by Earth's rotation and by centrifugal force caused by Earth's progression around the Earth-Moon barycenter. Tidal range depends on time and location.
The cold water moves back towards the equator as a deep sea current, driven by changes in the temperature and density of the water, before eventually welling up again towards the surface. Deep ocean water has a temperature between −2 °C (28 °F) and 5 °C (41 °F) in all parts of the globe. [14]
Because the vast majority of the world ocean's volume is deep water, the mean temperature of seawater is low; roughly 75% of the ocean's volume has a temperature from 0° – 5 °C (Pinet 1996). The same percentage falls in a salinity range between 34 and 35 ppt (3.4–3.5%) (Pinet 1996). There is still quite a bit of variation, however.
The eustatic sea level (from Greek εὖ eû, "good" and στάσις stásis, "standing") is the distance from the center of the Earth to the sea surface. [1] [2] An increase of the eustatic sea level can be generated by decreasing glaciation, increasing spreading rates of the mid-ocean ridges or increasing the number of mid-oceanic ridges.