Mean Sea Level as seen by other techniques

Since the beginning of the 1990s, altimetry is the main tool for continuous, precise and nearly-global mean sea level monitoring, with moreover regular measurements (every 10 or 35 days). However, other techniques existing a long time before, new ones have appeared that enable to validate altimetry results, and above all to better understand why mean sea level is varying.

 

Global mean sea level evolution over the 20th and 21st centuries. The red curve is based on tide gauge measurements [Church and White, 2006]. The black curve is the altimetry record (zoomed over the 1993–2009 time span). Projections for the the 21st century are also shown. The shaded light blue zone represents IPCC AR4 projections for the SRES greenhouse gas emission scenarios. Bars are semi-empirical projections. Red bar: [Rahmstorf 2007]; dark blue bar: [Vermeer and Rahmstorf, 2009]; green bar: [Grinsted et al. 2009].

Tides gauges

The longest sea level time series are provided by tide gauges. Some of them (not many) measured the sea level for more than a century. However, they support the effects of the movements of continents, and are very unevenly distributed around the globe: necessarily close to shore, but many more, and the oldest in Europe and the United States.

Mean Sea Level as measured by tide gauges since 1860 (Credits CSIRO)

In situ temperature/salinity observations

Temperature and salinity, at the surface as in depth are measured in situ by Argo floats or XBTs. Variations of the thermosteric level (i.e. due to temperature only) or steric level (due to temperature and salinity both) are thus computed over the 0-700 m layer from temperature (T) or temperature and salinity (T/S) profiles. This technique thus enable to estimate mainly the effects of varying heat in the ocean. Fresh water inputs or evaporation too have an indirect effect on this measure, by their impact on salinity (but note that this does not take into account the variations of water masses in the ocean)

Gravimetry satellites: GRACE mission

Water masses changes in the ocean (addition of water or evaporation, water mass transports) modify Earth gravity field. Grace satellite enable to observe these variations every 30 days. Thanks to this mission, the quantity of water added in the ocean (from ice melts, in particular) can be estimated.

Models

Sea level trend between 1992 and 2009, computed from Mercator Océan (1/4°) global sea surface height without assimilation (in order not to have the influence of altimetric data, which are among the assimilated data). A mean bias of +12 mm/year is removed from this trend. However, this map is similar in many aspects to the ones obtained from in situ or satellite datasets, with variations that look alike (e.g. rise of the level over the West Tropical Pacific, drop over the East). (Credit G. Garric, Mercator Ocean).

Ocean models describe the ocean in three dimensions (horizontally and vertically) as well as its evolution over time (temporal dimension or fourth dimension). A model is a mathematical description of physical phenomena. For the ocean, as for the atmosphere, the mathematical model describes the movement of fluids (water, air) on the surface of the Earth, as well as transporting of heat (temperature) and matter (salts) associated with the fluid movements. This results in equations which describe the current, the temperature and the salinity at any place in the modelled zone and as a function of time.

References

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