spatial pattern of interannual climate variations with identifiable centers of action. He was able to eliminate solar variability as a major contributor to the oscillation. Walker strongly suspected that oceanic processes were responsible for the oscillation, but was unable explore his ideas with the available data and thus missed the connection to El Niño (Walker 1924, Walker and Bliss 1932).

The first major breakthrough in understanding the mechanism of Walker's Southern Oscillation was to come forty years later. In the 1960s, Jacob Bjerknes began an examination of the meteorological conditions associated with El Niño, an interannually varying intrusion of warm equatorial waters along the western coast of South America. The warm intrusions had major economic impacts on the tuna fishing industry in the affected region. Bjerknes (1966) quickly recognized that El Niño was connected with large-scale fluctuations in trade-wind circulations in both the northern and southern hemispheres of the Pacific sector. He soon connected these trade-wind fluctuations to the Southern Oscillation (Bjerknes 1969).

Bjerknes's most important contribution was the reasoning he used in explaining the coupling between the oceanic and atmospheric circulations. On the basis of empirical evidence, Bjerknes hypothesized that El Niño and the Southern Oscillation are the result of the coupling between the east-west atmospheric circulation in the Pacific sector and also a coupling between the current and thermal structure of the upper ocean in the eastern equatorial Pacific Ocean. He observed that when the trade winds are strong (the “normal” condition), relatively cool equatorial water extends from the South American coast to the central Pacific. Bjerknes attributed the cool waters to equatorial upwelling caused by easterly wind stress acting on the ocean surface. He reasoned that the resulting pattern of sea surface temperature reinforces the strength of the trade winds by favoring large-scale atmospheric cooling, descent, and cloud-free conditions over the equatorial eastern Pacific, accompanied by large-scale ascent with relatively large amounts of precipitation, convective clouds, and atmospheric heating over the central and western equatorial Pacific. In the equatorial region, the east-west atmospheric heating differences would be expected to drive an east-west atmospheric overturning, which Bjerknes named the Walker circulation.

Bjerknes was able to link major decreases in the strength of the east-west gradient in equatorial sea surface temperature to decreases in the strength of the Walker circulation. He also linked changes in the gradient of sea surface temperature to disturbances in the planetary-scale atmospheric wave pattern over the sector covering the North Pacific Ocean and North America. This new information justified the cautious use of equatorial oceanic and atmospheric conditions for experimental climate forecasts (Bjerknes 1966, 1969). Bjerknes concluded that the variations in atmospheric heat input from the equatorial



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