Relationship to SST

It was pointed out by Hendrik Berlage in 1957, and by Jakob Bjerknes in 1966, that the Southern Oscillation is related to the occurrence of El Niño (Section. 11.2), since both occur within the Tropics, where anomalies of pressure are closely associated with alterations of sea-surface temperatures. The connection is seen in Figure 12.21. An association arises from an anomalously high SST in the central equatorial Pacific, creating atmospheric convection and hence a release of latent heat into the atmosphere which lowers its density and hence the surface pressure there, which implies a negative SOI. Another association between pressure and SST works the other way; a decreased MSLP at Tahiti reduces the force which drives the Trades, so there is less cooling of the surface by upwelling. Either way, we say that the tropical atmosphere and ocean are 'tightly coupled'.

As an example, the negative SOI from June 1982 till March 1983 coincided with an abnormally strong El Niño off Peru. Other long periods of negative SOI were from May till December 1940, and June to October 1941, again associated with periods of strong El Niños (Figure 10.16). In other words, low values of the SOI imply an El Niño. In fact, El Niños and the Southern Oscillation are simply aspects of the same global ENSO episodes, where this title is compounded from El Niño+Southern Oscillation.

The global average SST is higher when the SOI is negative, so it is sometimes referred to as a 'warm phase'. Within that period, an ENSO warm episode involves both an El Niño and a low SOI

Figure 12.19 The correlation of annual mean pressures with those at Darwin, in terms of the 'correlation coefficient'. The coefficient is +1 (shown as 10 in the diagram) if changes are the same as those at Darwin, zero if unrelated, and -1 (i.e. -10 in the diagram) if exactly opposite. Thus, -8 in the diagram, for example, indicates a strong tendency towards opposite changes.

Figure 12.19 The correlation of annual mean pressures with those at Darwin, in terms of the 'correlation coefficient'. The coefficient is +1 (shown as 10 in the diagram) if changes are the same as those at Darwin, zero if unrelated, and -1 (i.e. -10 in the diagram) if exactly opposite. Thus, -8 in the diagram, for example, indicates a strong tendency towards opposite changes.

Figure 12.20 Regions experiencing changes of rainfall during an ENSO warm episode, showing which have an increase ('wet') and which a decrease, 'dry'.
Figure 12.21 The coincidence of fluctuations of the Southern Oscillation Index and the sea-surface temperature in the central equatorial Pacific (between 150-90°W and 5°N-5°S).

as the extreme set of conditions. Tropical cyclones (Chapter 13) are more common in the South Pacific during an ENSO warm episode. Contrariwise there are 'ENSO cold episodes', which include a La Niña and a high SOI. Details are given in Note 12.N.

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