Figure 12.18 The parallelism of fluctuations of the Southern Oscillation Index and flows of water along the Darling River in western New South Wales. Each vertical unit represents 20 SOI units. The curves show ten-year running averages.
especially in the tropical region of the southern hemisphere. The relationship is given in terms of the correlation coefficient, a statistical measure of the degree to which a change (in this case of the pressure at Darwin) is accompanied by a similar anomaly at any other place on the map. Figure 12.19 shows that when the MSLP is anomalously low in Darwin (i.e. the SOI is high and therefore Darwin is relatively wet), the MSLP tends to be high in Tahiti, where rainfall is consequently unlikely. But the correlation is slightly positive in central Africa and in the Amazon Basin, suggesting that wet periods there tend to accompany rain at Darwin.
The global relevance of the phenomenon is shown in Figure 12.20, indicating simultaneous but various changes in the rainfall at the time of negative SOI values.
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