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, precipitation, freezing and thawing are all factors that affect the sea level because they vary the water density (salinity and temperature). These are slow and wide ranging phenomena.

 

Exceptional levels

Ocean circulation, evaporation, precipitation, freezing and thawing are all factors that affect the sea level because they vary the water density (salinity and temperature). These are slow and wide ranging phenomena.

Exceptional levels usually occur when extreme non-tidal disturbances coincide with extreme tide levels. Tide forecasts generally are not off by more than 0.3 m to 0.4 m for height and 10 to 15 minutes for time, but during the hurricane that hit Brittany on 15 and 16 October 1987, the positive surge was 1.60 m to 2.50 m depending on the area.

  Surcote observée lors de la tempête du siècle en Bretagne (octobre 1987)

 

Marégramme du conquet lors de la "tempête du siècle" en Bretagne le 16 octobre 1987

Reproduction of a tide curve from the Brest Tide Station during the storm of the century in Brittany (16 October 1987) Tide curve from the Conquet Tide Station during the storm of the century in Brittany (16 October 1987)
   

Though they are impossible to predict, these phenomena can still be described statistically (average return period for negative or positive surges of given height).

 

Prévimer

Prévimer is an operational coastal oceanography demonstrator. The PREVIMER system is aimed at a broad audience seeking short term coastal environment forecasts for the three mainland coastlines: English Channel, Atlantic, and Mediterranean. A set of observations and digital modelling tools for providing 4-day (96h) forecasts on the state of the sea, currents, sea levels, temperatures, and primary production.

Ifremer, in partnership with SHOM, Météo France, Institut de Recherche pour le Développement, Institut Universitaire Européen de la Mer and Technopôle Brest Iroise, implements the technologies to generate this valuable information, which is posted daily on the Internet and archived at the Operational Coastal Oceanography data centre.

 

Meteotsunami

A meteotsunami or meterological tsunami is a tsumani-like phenomenon that can occur in certain bays or harbours. It can be generated in shallow water due to atmospheric pressure fluctuations and resonance.

In general, there are three main mechanisms for the creation of a meteotsunami:

  1. A meteorological disturbance;
  2. Resonance between a weather disturbance speed and the wave speed in deep water;
  3. Amplification of the event by a harbour, bay or creek configuration.
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