English: Sea level data from the U.S.-European satellite Sentinel-6 Michael Freilich shows early signs of a developing El Niño along the equatorial Pacific Ocean. The measurements show Kelvin waves, low waves that can be tens of miles (hundreds of kilometers) wide, moving from west to east at the equator towards the west coast of Ecuador. When they form at the equator, Kelvin waves can move warm water – associated with higher sea levels – from the western Pacific to the eastern Pacific.

El Niño is a periodic climate phenomenon that can affect weather patterns around the world. The condition can bring cooler, wetter conditions to the U.S. Southwest and drought to countries in the western Pacific such as Indonesia and Australia. El Niño is characterized by higher sea levels and warmer-than-average ocean temperatures along the western coasts of North and South America. Water expands as it warms, so sea levels tend to be higher in places with warmer water. The climate phenomenon is also associated with a weakening of the trade winds.

The Sentinel-6 Michael Freilich satellite data shown here covers a seven-week period between the beginning of March and the end of April 2023. By April 24, Figure A, Kelvin waves had piled up warmer water and higher sea levels (shown in red and white) off the coasts of Peru, Ecuador, and Columbia. A series of Kelvin waves starting in spring is a well-known precursor to an El Niño. The waves seen during March and April 2023 are similar to the ones that preceded the 1997-1998 El Niño, one of the most powerful in recorded history.

Satellites like Sentinel-6 Michael Freilich can detect Kelvin waves with an instrument called an altimeter that uses radar signals to measure the height of the ocean's surface. Warmer areas show up as higher sea levels.

The satellite, named after former NASA Earth Science Division Director Michael Freilich, is one of two that compose the Copernicus Sentinel-6/Jason-CS (Continuity of Service) mission.

Sentinel-6/Jason-CS was jointly developed by ESA (European Space Agency), the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), NASA, and the National Oceanic and Atmospheric Administration, with funding support from the European Commission and technical support on performance from the French space agency CNES (Centre National d'Études Spatiales). Spacecraft monitoring and control, as well as the processing of all the altimeter science data, is carried out by EUMETSAT on behalf of the European Union's Copernicus programme, with the support of all partner agencies.

NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, contributed three science instruments for each Sentinel-6 satellite: the Advanced Microwave Radiometer, the Global Navigation Satellite System - Radio Occultation, and the Laser Retroreflector Array. NASA also contributed launch services, ground systems supporting operation of the NASA science instruments, the science data processors for two of these instruments, and support for the U.S. members of the international Ocean Surface Topography Science Team.