Meteoean studies

At each point in the ocean, offshore and near the coast, the sea level is affected by closely interrelated physical processes such as the water density (depending on temperature and salinity), local currents other than the tide, fluctuations in ocean circulation, waves, atmospheric pressure and wind. Sea level measurements therefore bear the signature of each of these phenomena and can be used to study them. These sea level variations not caused by the tide components (called "noise") exist in the high frequencies (swell, seiche, for example) or low frequencies (atmospheric pressure). The noise can seriously impede or prevent the detection of low amplitude components.

 

Coucher de Soleil à la baie d'Yves, Charente-Maritime (Crédits SHOM - Nicolas Pouvreau, mai 2010) Cliquez sur l'image pour l'agrandir

 

The most important components are separable if one year of observations are available. But by making certain assumptions, consisting of assuming that components whose periods are close behave similarly vis-à-vis the generative force, it is possible to perform usable analyses with only one month of observations. At the extreme limit, if the observations are of excellent quality and not too disrupted by meteorological-related height variations, it is possible to perform an analysis with only 15 days. In this case, it is better not to use winter observations. The noise can seriously impede or prevent the detection of low amplitude components.

 

Meteorology

Atmospheric pressure and wind affect water levels: currents and height variations due to wind, storm surges and seiches.

In general, the observed heights are higher than those seen when the barometric pressure is low or during onshore winds. They are lower with high pressures and during offshore winds.

 

Seiches

 

A seiche is a free oscillation in a basin, that is to say an oscillation whose frequency is specific to the basin.

Seiches are generally associated with periods of bad weather. They can be caused by a storm surge residue that generates and maintains an oscillation due to resonance from the geographical configuration of the basin.

This oscillation is observed in enclosed or semi-enclosed basins and in shallow waters, for example between Lorient and Ile de Groix in France.

Periods of seiches ranges from a few minutes to several hours. They disappear by energy dissipation when their cause has disappeared.

 

Oceanographic and climatic processes

Ocean circulation, evaporation, prei1 oscilla" > Temp_hore winds.

 

Seiches

 

Ocean circulation,> Temp_hore winds.

 

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