The vast Pacific Ocean covers one third of the Earth’s surface, most of it within the tropics. Here, a confluence of factors — abundant sunlight and water vapor, a shallow oceanic thermocline, a reversal of the Coriolis effect at the equator, and an equatorial waveguide that spans 40% of the planet — spawns a rich spectrum of weather and climate variability in both the ocean and atmosphere. Much of this tropical variability teleconnects to far-flung regions, affecting weather, agriculture, fisheries, and terrestrial & marine ecosystems around the globe. Understanding and predicting this variability, in particular the El Niño / Southern Oscillation (ENSO), remains one of the great challenges of climatology and the atmospheric & oceanic sciences.