Modern sea surface temperatures are monitored by NASA's TOPEX/Poseidon and newer
Jason-1 satellites. Using radar altimetry, they measure sea surface elevations as they orbit the Earth. Sea surface elevations are controlled by gravity, ocean dynamics, and temperature. Variations in elevation, especially over relatively short time periods, are mostly due to changes in water temperature because gravity is constant and ocean dynamics can be approximated as constant (on these short time scales). Above 4 C, water will expand when warmed and shrink when cooled. Sea surface elevations rise and fall as a result of changing temperatures. Elevations can therefore be used to determine sea surface temperatures and the corresponding PDO Index (NASA, 2003).

An example of sea surface elevation measurements is shown in the following figure. Elevation highs and lows are red and blue, respectively. For more information and images, please visit NASA's Jet Propulsion Laboratory website.

Source: Image courtesy NASA JPL, based on data from Jason-1