Mean sea level

The long term changes in "mean" sea level provide basic information on the climatic role of oceans and general ocean circulation. The international scientific community is conducting studies to detect long-term trends and link them to climate change. But the global network of tide stations has significant shortcomings: the stations are not evenly distributed (concentrated in the northern hemisphere) and they are ill-suited to monitoring the general dynamics of the oceans. To achieve this goal, specific tide gauges must be installed and used in conjunction with highly precise satellite positioning systems (GPS, DORIS), which will monitor the geodesic stability of the base that the gauges are based on.

 

Îlot à proximité de Saint-Malo avec les traces de différents niveaux de la mer (crédit SHOM 2012) Cliquez sur la figure pour l'agrandir

 

It is generally accepted that the global sea level is rising by about 1 to 2 mm per year, but estimates of the rate of increase differ significantly according to the authors. This is because the exact causes of the rise are not known, and in particular, what portion can be attributed to the greenhouse effect, due to the increase of industrial carbon dioxide in the atmosphere. The Topex Poseidon and Jason satellites measured a mean rise of around 2 mm per year between 1993 and 2000, but this increase is not uniform throughout the world's oceans: there are regional differences of ± 20 mm / year.

To address this issues, tides gauges must be positioned in a unique reference system, the reference ellipsoid for example. This is why tide stations are increasingly equipped with geodetic positioning systems using satellite technology.

 

Observations

Tidal observations

 

For the purposes of climate study, long-term tidal observations are valuable, especially those of Brest, which began in 1807. With nearly 200 years of measurements, the Brest tide station has the long-term data required to determine long-term variations in sea level.

This example shows that local fluctuations of annual mean sea level are relatively significant, reaching or exceeding ± 5 cm from one year to another. This is why accurate trend assessments require observation periods on the order of 100 years.

In Brest, the mean sea level has risen some 20 centimetres since 1846. But this example is not representative of all long-term observations available across the globe. In most cases, these observations show an average increase of about 1 to 2 mm / year, with a spread around these values of the same order as that of Brest. However, there are sites, especially in Scandinavia, where the trend is reversed.

 

Evolution du niveau moyen annuel à Brest de 1846 à nos jours. Chaque point représente le niveau moyen calculé sur une année avec le filtre de Demerliac.

 

The available tidal observations are not a good indicator of the overall trend due to the uneven distribution of tide stations, located mostly in the temperate regions of the northern hemisphere. The variability of the trend from one site to another is mainly due to vertical movements of the earth's crust, which the gauges obviously cannot detect (measurement of the relative level). These tectonic movements can be measured using satellite technology that can locate various levels (marine or terrestrial) compared to an absolute reference.

 

Altimetry observations

 

The data acquired by numerous satellites equipped with radar altimeters providing near-centimetre accuracy (Topex-Poseidon since October 1992, and Jason since 2003 and Jason 2 since 2008), indicate a rise in sea levels of the same order of magnitude as those mentioned above. However, because of interannual fluctuations, many years of altimetric data across the globe are needed to reduce the uncertainty of this estimate. The available satellite data show a high spatial variability of the trends over periods on the order of a decade. However, the reliability of the radar altimeters on board satellites has been demonstrated and the quality of the data is improving. Moreover, space geodesy technology can position specific points on the surface of the Earth with centimetre accuracy in the geocentric reference system International Terrestrial Reference System (IRTS), which has been adopted by the International Union for Geodesy and Geophysics (IUGG). Operational systems such as GPS and Doris can reference the tide gauge datums to the ITRS.

 

Évolution du niveau moyen des mers vu par les altimètres depuis janvier 1993

 

 

This makes it possible to monitor changes in sea level against an absolute reference. International and national programs to improve the accuracy of reference systems and geoid models from these satellite technologies are currently underway. For the same reasons as for altimetry data, exploitable results (for the study of long term sea level variations) cannot be expected for several years.

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