A sea-level anomaly is the difference between the total sea-level and an average sea-level for this time of year. We look at anomalies because the total sea-level measurement made by the altimeter varies from ± 100 meters. Most of this is constant, though, and is due to the Earth’s gravity and the ocean circulation. Sea-level variations caused by El Niño account for less than 1% of the total signal. If the constant part were not removed, the El Niño signal would not be observable.
El Niño is evident in the figures on the previous page as higher than normal sea-levels in the east, and lower than normal sea-levels in the west. Sea-level is beginning to drop in the east, though, and indications are that El Niño is beginning to dissipate. The sea-level is actually highest north and south of the equator. This is a sign that El Niño is dissipating, and will be discussed later. Also, note the high sea-levels in the Indian Ocean, which are nearly a mirror-image to El Niño in the Pacific. I will discuss this later in the presentation as well.
The altimeter does not directly produce the image shown on the previous slide. Instead, it measures the height of the spacecraft above the ocean surface, which can be converted into sea-level. These sea-level measurements can then be mapped to a uniform grid, color coded, and displayed. However, the conversion to the sea-level measurement is not a simple procedure.