365 Days of Climate Awareness 13 - Eustatic Sea Level

 

365 Days of Climate Awareness

13 - Eustatic Sea Level

Eustatic sea level is the average height of the ocean as calculated from the center of the earth. This isn't something we can measure directly so it requires some geodetic modeling. (Geodesy is defined as the study of Earth's shape, gravitational field and orientation in space, and it relies heavily on satellite data.)

To a first order of approximation, the earth is a sphere, with an equatorial radius of 6378 km. The spherical model is generally used for modeling in fluid dynamics, since that shape makes the mathematics far simpler. To a second order of approximation, the rotating planet is an oblate spheroid, bulging slightly at the equator and slightly flattened at the poles, with an equatorial radius of 6378 km and a polar radius of  6357 km, a difference of 21 km. This shape is used for highly precise global mapping. To a third order of approximation, the planet is very slightly egg-shaped, with a small bulge in the southern hemisphere and a very slightly flattened northern hemisphere, on the order of a few tens of meters. This difference is so tiny that it is not used for practical applications, because it can be accounted for by other means.

DIfference between a sphere and an oblate spheroid.

Using the oblate spheroid model, satellite altimetry measurements taken of the world ocean are processed into a time-invariant model. And this must be modeled, because conditions in reality are constantly dynamic. Tides, from the sun, moon and even planets, are active not only on the ocean but on the rocky planet itself. Weather has a number of effects, between the wind building waves, and causing larger-scale pileups of water like storm surges, and via pressure differences. High air pressure pushes the surface of the ocean down, and low air pressure allows the sea surface to rise. Furthermore subsea features like mid-ocean ridges have a gravitational pull which lead to standing mounds of water overhead.

Eustatic sea level calculations average all of those effects out. In addition to satellites, a set of tidal gauges around the world are used to calibrate the satellite measurements. One such station is on Tahiti, an island in the Pacific far from the Kermadec Trench near New Zealand and the hotspot beneath Hawaii. It is assumed this area is tectonically stable enough to provide reliable sea level records. This and similar locations are used also to reconstruct paleo sea levels, for correlation with other climatic markers.

Tomorrow: relative sea level.

Be well!