The coral was growing in a ring of tiny Pacific islands collectively known as Kiribati. Most years, warm surface water piles up on the western side of the Pacific Ocean near Australia. The easterly tropical trade winds help keep them in their place. But during El Ni o years, the winds die down and the warm pool starts spreading into the eastern Pacific and the Kiribati islands. During El Ni o years, the maximum rainfall tends to sit near the dateline and equator, which is right where our coral sits.
Warm water means more rainfall in part because heat rises.
So warm water evaporates more easily than cooler water. And air heated by the warm water rises more easily to the heights necessary to create clouds and rain. Meanwhile, rainfall decreases in other regions, such as the Pacific Northwestern United States. It's fortunate for the researchers that at their study site, rainfall tends to track higher sea surface temperatures, because the isotopic signature in the coral doesn't distinguish between increased temperatures and increased input of fresh water. But first, the scientists had to identify the annual layers in the coral.