Sea level changes observed by satellite altimeter instruments have long been recognized as a vital source for information on open ocean circulation. In a simplified manner, the ocean may be viewed as layers of fluids within which different dynamic processes dominate. Near the ocean surface (within about 30 m, though at times much deeper), fluxes of momentum from wind stress and heat are strong contributors to environmental variations. The rate at which surface fluxes mix into the ocean depends primarily on turbulence generated by wind stress and surface waves. Variations occur on relatively short time periods (on the order of hours to days) in the surface mixed layer. Aside from the mixed layer, the open ocean may be generally viewed as a warm water mass overlying a colder relatively denser water mass. Ocean variations are primarily controlled by changes in the depth of the thermocline, which is the boundary that separates the two water masses. There is a strong correlation between thermocline depth and sea level, which produces a pressure anomaly throughout the water column (see Fox et al., this issue). For example, a relatively high sea level indicates high pressure, and this pushes the thermocline downward.