Jacob Bjerknes is credited with determining the link between El Niño and and the Southern Oscillation.
Son of the world renowned meteorologist Vilhelm Bjerknes, Jacob Bjerknes is primarily recognized for his work in developing the theoretical cyclone model as part of the "Bergen School" during the 1920's. In addition to this work, it was Bjerknes who made the first link between El Niño and the Southern Oscillation in the 1960's.
Bjerknes noticed that the default state of sea surface temperatures (SSTs) at the eastern end of the Pacific are remarkably cold for such low latitudes. Since the western Pacific is relatively warm, a large SST gradient exists 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 equatorial 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. This systematic equatorial circulation associated with the zonal pressure gradient was named the "Walker Circulation" by Bjerknes. Bjerknes thought that fluctuations in this circulation initiated pulses in the Southern Oscillation and ultimately led to an ENSO event.
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 equatorial Pacific waters are explained by the horizontal advection of westward currents along the equatorial Pacific, upwelling along the equator, and upward thermocline displacement.
Bjerknes associated the feedback loop of the oceanic and atmospheric circulation over the tropical Pacific as a "chain reaction", noting that "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." Bjerkness also found 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 Niño as well as the association of the high phase with normal cold state of the eastern Pacific.
Bjerknes, J. (1966). A possible response of the atmospheric Hadley circulation to equatorial anomalies of ocean temperature. Tellus, 18(4), 820-829.
Bjerknes, J. (1969). Atmospheric teleconnections from the equatorial pacific. Journal of Physical Oceanography, 97(3), 163-172.