High sea levels are beginning to build up in the Central Pacific, according to NASA’s latest images from the Jason-3 satellite mission. These maps show a patch of high sea level slowly traveling eastward along the equator and through the tropical Pacific Ocean, a pattern that often leads to an El Niño event.

Last year, the Pacific Ocean saw only a mild La Niña, or cooling of the water at irregular intervals. The shared US/European Jason-3 mission had shown most of the ocean to be at neutral heights as on April, but at the beginning of May, high sea levels began to rapidly emerge. On Wednesday, NASA’s Jet Propulsion Laboratory, which manages Jason-3 in partnership with NOAA, the French space agency (CNES), and the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), released startling images of the Pacific’s sea levels in May compared to just a month prior.

Left, in April, the Jason-3 satellite shows most of the Pacific Ocean at neutral heights (green). In May, a Kelvin wave (red) appears on the equator.
Left, in April, the Jason-3 satellite shows most of the Pacific Ocean at neutral heights (green). In May, a Kelvin wave (red) appears on the equator. 

In the tropics, rapidly heightened sea levels are usually caused by a layer of warm water at or below the surface. The high sea level patch that is currently moving east is known as a downwelling Kelvin wave, which, according to NASA scientists, is “often a precursor to an El Niño event,” potentially creating disastrous weather conditions and influencing rainfall as far away as Africa and Europe. This Kelvin wave is the result of westerly wind bursts that are moving the heightened sea levels east along the equator, spreading warm water layers that are normally confined to the western Pacific Ocean.

Large El Niño events can affect weather and climate across the globe, including the west coast of the United States. At the very least, El Niños usually mean above-average winter rainfall in California. But in other regions, some of the harshest El Niño events have been linked to flooding, damaged infrastructure, and outbreaks of diseases transmitted by mosquitoes, such as malaria and dengue.

NASA’s maps show that warm water is currently confined to the subsurface, with no warming at the ocean surface, which is the first indicator of an upcoming El Niño event. In response, forecasters at NOAA say the agency will be on high alert for more Kelvin waves as summer approaches.

While El Niño events are thought to have occurred for centuries, they have become stronger and more frequent since the 1970s, which many scientists link to climate change. There were roughly 30 El Niño events in the 1900s, but since 2000, such events have been observed in 2002-2003, 2004-2005, 2006-2007, 2009-2010, and 2014-2016, with the most recent El Niño being one of the strongest on record.