3: Watching the Pacific Ocean
The ENSO observing system in the tropical Pacific includes tide gauge stations (yellow circles) that measure sea level, drifting buoys (orange arrows) that measure sea-surface temperature and upper ocean currents, and moored buoys (red diamonds) that measure upper ocean temperature and currents, surface wind and energy flux. Instruments on so-called volunteer ships of opportunity measure the upper ocean temperature and pressure on various sea routes (green lines).