Sir Gilbert Walker, Director General of the Observatory in India, assumed his post in 1904, shortly after the famine resulting from the monsoon of 1899 (an El Nino year). Walker's goal was to predict the Asian monsoon fluctuations.
Walker sorted through world weather records dating from just before the turn of the 20th century which described the sea level pressure swing between South America and India-Australia. He noticed that when pressure rises in the east, it usually falls in the west, and vice versa. Walker coined the term Southern Oscillation to describe the ups and downs in this east-west seesaw in southern Pacific pressure. The Southern Oscillation Index (SOI) is pressure difference between Tahiti and Darwin.
In the 1960's, Jacob Bjerknes pointed out the empirical relation between the atmosphere and tropical Pacific. He also proposed a two-way coupling between the ocean and atmosphere. Bjerknes' idea developed from observations of large-scale anomalies in the atmosphere and tropical Pacific Ocean during 1957-58 (an El Nino year), the International Geophysical Year.
Bjerknes observed the normal state of the sea surface temperatures (SSTs) at the eastern end are remarkably cold for such low latitudes. The western Pacific is very warm, there is a large SST radient along the equatorial Pacific. As a result, there is a direct thermal circulation in the atmosphere along the Pacific. The cool dry air above the cold eastern equitorial Pacific waters flows westward along the surface toward the warm west Pacific. There, the air is heated and supplied with moisture from the warm water. There is a zonal pressure gradient associated with this equtorial circulation and Bjerknes named this "Walker Circulation". Bjerknes felt that fluctuations in this circulation initiated pulses in Walker's Southern Oscillation.
While the surface winds are being driven westward along the equator by the zonal SST gradient, they act to create the cold upwelling ocean water in the east. The cause of the cold eastern equitorial Pacific waters are to be found in three features of wind-driven ocean dynamics:
Bjerknes refered to the oceanic and atmospheric circulation over the tropical Pacific as a "chain reaction". He wrote "An intensifing Walker Circulation also provides for an increase of east-west temperature contrast that is the cause of the Walker Circulation in the first place." Bjerknes also noted that the interaction could operate in the opposite: a decrease in the equatorial easterlies diminishes the supply of upwelling cold water and the lessened east-west temperature gradient causes the Walker Circulation to slow down. He thus provided an explanation for the association of the low phase of the Southern Oscillation with El Nino as well as the association of the high phase with normal cold state of the eastern Pacific.