After a mild La Niña late last year, temperatures, convection and rainfall rates in the equatorial Pacific Ocean returned to normal by early April of this year. An April 9 image of sea level height from the U.S./European Jason-3 satellite mission showed most of the ocean at neutral heights. But by the beginning of May, high sea levels began to build up in the Central Pacific. In the tropics, high sea levels are usually caused by a layer of warm water at or below the surface.