Mean Sea Level overview
Mean Sea Level is an average over all the oceans of sea surface height, with respect to a reference. However, what are really sought are the variations of this mean sea level along time. This Mean Sea Level can be global, but basin (e.g. Atlantic) or regional ones can be computed. Maps showing the trend over several years are also possible.
The variations can come from several sources (see below). Past variations can be reconstructed from several indicators. Since the 19th century, or even a bit earlier at some places, tide gauges have recorded such variations. Today, satellite altimetry (continuously since 1992), autonomous floats (Argo floats since 2003) and gravimetry data (Grace satellite) enable to measure Mean Sea Level variations, or some of their components. Ocean models are also used to understand and quantify those phenomena.
Sea level variations causes
The sea level can vary over long periods for several reasons:
- water mass variations:
water can be added to the ocean, either by increased rain over the ocean, or run-off from the rivers; glaciers melting can also add water. On the reverse, more artificial reservoirs leads to a run-off decrease, and thus to less water being brought to the ocean. Increased evaporation can also decrease the water mass (as well as glaciation, as it happened during last Ice age, when sea level was about 100 m below the nowadays level)
- temperature variations:
water dilates when it warms, which leads to higher sea level. Among other things, it leads to sea level seasonal variations, and also year-to-year variations linked to climate events (e.g. El Niño). Temperature changes over longer time scale (global warming) have, of course, also an impact.
- salinity variations:
the saltier the water, the denser it is; thus saltier water will have a lower level. Salinity variations can occur by fresh water addition (increased run-off, rain, or ice melting), which decreases salinity, or by increased evaporation, or by glaciation, which increase salinity.
- ocean circulation changes:
changes in sea level can be due to changes in the ocean circulation. Over periods of ten years or more, the currents can shift position,
Measurements and what they see
The different means of measuring sea level variations do not in fact observe exactly the same things.
|Technique||Observed part of mean sea level|
|Grace||mass+Glacial Isostatic Adjustment (GIA)|
|Temperature/Salinity profiles (Argo)||steric
or thermosteric (temperature only)
or halosteric (salinity only)
|Tide gauges||total plus ground movements (including GIA)|