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Exercise 1: Sea Level Height 
Therefore, sea levels in areas of warm water are higher than sea levels in areas of cold water. A rise in sea level of an average of 3 mm/yr was observed between 1993 and 2008. This rise, however, was not the same everywhere. In some regions it was much higher than average, while in others it was much lower.
If the height of the satellite (H) is known relative to a reference level (usually a standard regular ellipsoid-shaped surface), then the height of the sea above the reference level (h) is determined by subtracting the distance (R) measured by the altimeter: h=H-R. The surface topography (h) changes over time and place. Some physical factors contribute to it, such as the distribution of gravity over the Earth, represented by a geoid (hgeoid). A geoid is a representation of the Earth’s surface in the absence of winds, currents and Earth’s rotation. The geoid has a potato-like shape.
Other factors that affect surface topography include the instantaneous tidal height of the sea surface, the motion of the sea or ocean dynamic topography, and the local response of the ocean to the atmospheric pressure distribution over the ocean.
Some of the most important altimeter missions include ESA’s ERS-2 and Envisat, and CNES/EUMETSAT/NASA’s Jason series and Geosat Follow-on (GFO). Repetitive altimeter measures of sea level achieve an accuracy of about 1–2 cm. Anomalously high heat content causes abnormally high sea surface height. This positive anomaly is shown in red in the series of data provided (see right navigation bar). The negative anomalies appear in blue, indicating that sea level is lower than usual.
LEOWorks exercise Make an animation with the series of data provided by the altimetry sensor. There are three series of data, one from 1997-1998, one from 2005-2006, and one from 2007-2008. Compose three animations with the three sets and study them. Describe what happens in every period and make comparisions.
In the LEOWorks image processing programme, open Tools/Image Animation, which brings up the Select Files for Animation window. Choose the proper animation speed. You can see the number of the slide by activating the Active Slider.
1. Can you explain what El Niño effect is visible with respect to the mean sea level? What are the causes? 2. Can you say which of the three series of images corresponds to an El Niño year? 3. Using the latest information can you evaluate the present situation?
Last update: 17 April 2013
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