The image shows Sea Level Anomaly as recorded by the RA-2 across most of Envisat’s orbital cycle 15. Local sea level can vary above and below the average by up to 100 metres – mostly due to constant factors such as tides, oceanic circulation and variations in the local gravitational field, or geoid. In this image these constant variations have been removed to make visible the remaining 1% of variations of greatest interest to researchers, such as those caused by the El Niño instability