The Southern Oscillation was discovered by Sir Gilbert Walker in 1923. It is a "seesaw" of atmospheric pressure between the Pacific and Indian Oceans. There is an inverse relationship between the air pressure measured at two sites: Darwin, Australia, in the Indian Ocean and the island of Tahiti in the South Pacific. The Southern Oscillation Index (SOI) is the difference in sea-level pressure measured at Tahiti and Darwin. The Cold Tongue (CT) Index measures how much the average sea-surface temperature in the central and eastern Pacific near the equator varies from the annual cycle. The two measurements are anti-correlated, so that a negative SOI is usually together with an unusual warm ocean wind known as El Niño.