There is a pattern of zonal flows over the equator across the Pacific Ocean named the Walker circulation after Gilbert Walker, who discovered it in the 1920s (Figure 12.17). The strength and direction of the circulation are measured by the difference between sea-level pressures (in hPa) at Papeete (17°S in Tahiti in the central Pacific) and Darwin (12°S in northern Australia), 8,500 km away. Tahiti is



90°W 0° 90°E 180°E 90°W

Figure 12.17 The Walker circulation in terms of zonal winds in the tropical troposphere during a normal and during an ENSO year, respectively.

Figure 12.17 The Walker circulation in terms of zonal winds in the tropical troposphere during a normal and during an ENSO year, respectively.

under the influence of the South Pacific high (Figure 12.1) and experiences easterly Trades most of the year, while Darwin is near the west equatorial Pacific warm pool (Section 11.2) and has a distinctly monsoonal climate. The mean difference in the present month between the MSLP values at the two places is compared with the long-term average difference at this time of year, to obtain the current MSLP anomaly. (An 'anomaly' is a difference from the normal.) The month's anomalies are calculated for all past years to derive the standard deviation (a measure of the scatter— Note 10.L) for that month of the year. Dividing the relevant standard deviation into the current MSLP anomaly, and then multiplying by ten, gives the Southern Oscillation Index (SOI) for the present month. Values either over 20 or below -20 are found to occur only 4.6 per cent of the time, although the SOI has varied between +33 and -39 during the last century. It is usually smoothed by taking a five-month running mean (Note 10.H), to remove some of the short-term scatter.

The SOI value is connected with the rainfall, as shown in the following examples:

1 The greater the SOI value, the stronger the westwards Trade wind (Figure 12.17), since winds near the equator blow directly from places of high to those of low pressure, unaffected by the weak Coriolis effect at low latitudes. A strong westward flow (i.e. a highly positive SOI) implies more uplift over Darwin, for instance, and hence heavier rainfall over Australia (Table 12.1).

2 Fluctuations of the SOI match differences in the flow of the Burdekin River in Queensland and hence in the thickness of annual coral deposits in the sea at the mouth of the river.

3 Figure 12.18 shows a connection between SOI and rainfall in terms of the flow along a river draining western New South Wales.

4 There has been a correspondence between Australian annual wheat yields and the JuneAugust SOI during the period 1948-91. The coefficient of correlation was 0.47, which is impressive but insufficient to be useful in predicting yield.

5 A negative SOI value implies possible drought over northern Australia (Figure 10.16).

The SOI fluctuates every few years, due to a seesawing of pressures between Papeete and Darwin.

A rise at one place is accompanied by a fall of pressure at the other, and so an enhanced change of the difference between the two. This fluctuation is called the Southern Oscillation, from which we derive the name of the SOI.

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