Mean Sea Level as seen by other techniques

The satellites altimetry observing system allows to monitor nearly all oceans on earth since the 1990's, in a continuous and regular manner. This observing system has revolutionnised the ocean science but this is not the only observing system developed to monitor sea level. Other tehcniques, such as tide gauges, existed since a long time ago. Others like ARGO flaot and GRACE have been developped to complete and/or validate satellite altimetry data. The combination of allavailable data from the different observing system is a powerfull tool to understand better why and how sea level is varying.

Compilation of paleo sea level data (purple), tide gauge data (blue, red and green), altimeter data (light blue), and central estimates and likely ranges for projections of global-mean sea level rise from the combination of CMIP5 and process-based models for RCP2.6 (blue) and RCP8.5 (red) scenarios, all relative to pre-industrial values. Source IPCC 5th Assessment Report, 2013, Chapter 13.

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
Altimetry	total (steric+mass)
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)

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 1993 and 2016, computed from an experiment performed by Mercator Océan (global 1/4°) without assimilation (in order not to have the influence of altimetric data, which are among the assimilated data). The rise in this simulated mean sea level is 1.74 mm/year and no bias is removed from this trend. Apart the Southern Ocean, 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.