The international Sentinel-6 Michael Freilich sea level satellite observed a swell of warm water, called a Kelvin wave, moving eastward in the equatorial Pacific Ocean, arriving off the South American coast in May. Warm Kelvin waves often precede El Niño events. Credit: NASA/JPL-Caltech NASA and its European partners are tracking a major buildup of warm water in the Pacific Ocean that could signal the arrival of El Niño later this year. Data collected by the Sentinel-6 Michael Freilich