Mean rise in sea level is only part of the story

The rise in the level of the oceans is far from uniform. In fact, while in certain ocean regions the sea level has indeed risen (by up to 20 millimetres a year in places), in others it has fallen an equivalent amount. These regional differences, observed by Topex/Poseidon since 1993, mostly reflect sea level fluctuations over several years., estimates of the rise in sea level-now running at 2.5 millimetres a year-have gained in accuracy.

Combined map of regional patterns of observed sea level (in mm/year). This map can be obtained using gridded, multi-mission Ssalto/Duacs data since 1993, which enable the local slopes to be estimated with a very high resolution (1/4 of a degree on a Cartesian projection). Isolated variations in MSL are thus revealed, mainly in the major ocean currents and ENSO events (Credits EU Copernicus Marine Service, CLS, Cnes, Legos).
undefinedDownload the data (NetCDF).





El Niño is behind rise in sea level

Global Mean Sea Level (MSL, blue line) and Sea Surface Temperature (SST, red dotted line) from early 1993 to mid-1998. In 1997 there was a rise of 15 mm. (Credits CNRS/Legos).

The meteorological effects of El Niño 1997-1998 were felt worldwide, but it also contributed to variations in mean sea level. Indeed, sea level anomalies measured by Topex/Poseidon were over 20 centimeters in the equatorial Pacific when the phenomenon was at its height (and as much as 30 centimeters off the coast of Peru). These anomalies obviously had an effect on the global mean of sea levels.




Mediterranean Sea

Mean sea level evolution in Mediterranean Sea

Sea level trend (mm/year) in the Mediterranean for January 1993 to June 2014 (Credits Cnes/Legos/CLS).

The sea level in the Eastern Mediterranean basin has risen significantly in recent years, apparently due to warmer water temperatures (observed by in-situ measurements). But if we look at the Ionian Sea off the tip of Italy, data acquired by Topex/Poseidon show that sea level in fact fell.