Relying upon the data of 1957, Bjerknes went further than Walker in stating his thesis on El Niño. Only a thin sheet of water separates the atmosphere from the ocean depths. Thus, the difference in temperature between that sea surface and the air above it is what causes heat to flow and water vapor to be exchanged. Bjerknes proposed in 1966 that the relaxed trade winds of 1957 not only caused the appearance of unusually warm surface waters over the tropical Pacific, but were in turn caused by the increase in sea surface temperatures (SST) during that year. This amazing circular argument implied that this phenomenon was neither strictly atmospheric nor oceanic but a product of interactions of the two.

Dr. Richard T. Barber beautifully summed it up when he said, "The ocean is clearly driving the atmosphere."

Analysis of His Findings
The differences between the two distant pressure systems (Tahiti minus Darwin) have been converted into an index called the Southern Oscillation Index (SOI). Usually, there is a low pressure system in the region of Indonesia and northern Australia, centered near Darwin. This system brings storminess to the region, providing some parts of Australia, the driest inhabited continent for sustaining its settlements, wildlife, and ecosystems. At the same time, there is a high pressure system in the southeastern Pacific, centered near Tahiti. The sea level pressure at each of these two points is related to each other. When one is increasing, the other is usually decreasing. Therefore, the difference in seal level pressure (Tahiti minus Darwin) is used as an index that characterizes ENSO.

Thus, El Niño is related to the negative phase of SOI and the cold event to the positive phase. This index, the SOI, has been reliable associated with a number of climate related events: when the sea level pressure at Darwin increases, the likelihood of drought in the Australian-Indonesian region also increases; when the pressure at Tahiti decreases, the likelihood of more rainfall in the equatorial central Pacific region increases. These particular sea level pressure changes appear to set the stage for a possible onset of El Nino. Along with pressure changes come changes in wind speed and direction, shifts in the location of pools of warm and cold oceanic surface water, changes in the strength of coastal upwelling, and shifts in the location of biological productivity in the ocean which, in turn, alters the location of various fish populations. It is important to realize that although there is no one-to-one correlation between observed El Nino events and the SOI, this relationship is very strong.

Continue and see the 3D animations showing the normal conditions in the Pacific Oceans.