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11.5: Sea Level

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    • Contributed by Miracosta Oceanography 101
    • Sourced from Miracosta)

    What is Sea Level?

    "Sea level” is generally used to refer to mean sea level (MSL). A common accepted definition of mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location.

    Sea level is an average level for the surface of one or more of Earth's oceans from which heights such as elevations may be measured. However, sea level varies for place to place due to gravitational differences in the solid earth, and variations in sea water characteristics (water density) and atmospheric pressure effects. For instance, Figure 11.10 shows topography of the ocean surface one specific day, however, it is constantly changing day by day, season to season. MSL is a standardized geodetic reference point for geographic locations.

    Figure 11.10. Sea level height map on a particular day (departure from mean sea level).

    Mean Sea Level (MSL) is not really level...

    • Sea levels are different for each ocean basin. Sea level is about 20 cm higher on the Pacific side of North America than the Atlantic due to the water being less dense (on average) than on the Pacific side. Variations in sea level are due to the prevailing weather and ocean conditions.
    • Differences in MSL are also related to the gravity variation cause by different densities rocks in the lithosphere and depth of the ocean basins (Figure 11.11). For instance mid-ocean ridges (MORs) tend to be low gravity areas.

    Sea level is influenced gravitational acceleration. A boat on sea level region near the North Pole in the Arctic Ocean has the highest gravitational acceleration of the planet: 9.8337 m/s2. The lowest is on Mount Huascaran in Peru on the equator where the gravitational acceleration is only 9.7639 m/s2 (a difference of 0.7% - which you would not feel).

    Figure 11.11. Gravity map of the Earth exaggerated: highs are red, lows are blue.