“The height (or "relief") of the sea surface is caused by both gravity (which doesn't change much over 100's of years), and the active (always changing) ocean circulation. The normal slow, regular circulation (ocean current) patterns of sea-surface height move up and down (warming and cooling and wind forcing) with the normal progression of the seasons ... winter to spring to summer to fall. Using theory of ocean dynamics, TOPEX/Poseidon and Jason sea-surface heights can be used to calculate how much heat is stored in the ocean below. The year-to-year and, even, decade-to-decade changes in the ocean that indicate climate events such as the El Niño, La Niña and Pacific Decadal Oscillation are dramatically visualized by these data. Sea-surface height is the most modern and powerful tool for taking the "pulse" of the global oceans.” (text from the JPL website).

Other Related Links of Interest:
An Overview
Very cool viewer of sea surface height