When sea-surface temperatures across the equatorial Pacific Ocean measure 0.5C above average for any particular 3-month period–as they did this year for April, May, and June–it heralds the onset of the worldwide weather maker known as El Nio.
Early this year, an increase in ocean temperatures in the central Pacific led researchers at the National Oceanic and Atmospheric Administration (NOAA) to suggest that an El Nio was on the way.