Ta

70" W

Indonesian Low

Indonesian Low

Figure 5.23 Schematic perspective plan-view diagrams showing conditions in the Pacific (a) in a normal year, and (b) during an El Niño event. Note that although no El Niño can be labelled 'typical', the features shown in the diagram seem to occur in most El Niños. The pink tone indicates regions where the sea-surface temperature is higher than about 28 °C; the orange tone in (a) indicates regions that are normally dry.

Figure 5.23 Schematic perspective plan-view diagrams showing conditions in the Pacific (a) in a normal year, and (b) during an El Niño event. Note that although no El Niño can be labelled 'typical', the features shown in the diagram seem to occur in most El Niños. The pink tone indicates regions where the sea-surface temperature is higher than about 28 °C; the orange tone in (a) indicates regions that are normally dry.

? reduced upwelling

? reduced upwelling

upwelling

North-East Trades

South-East Trades

The general disruption of ecosystems and fisheries for which El Niño events are notorious is largely the result of the usually nutrient-rich surface waters of the eastern tropical Pacific being replaced by warm, nutrient-poor waters from the west. The upwelling that usually brings nutrient-rich water up from below the thermocline sometimes stops altogether. Even if it does not. the lowering of the thermocline in the eastern tropical Pacific means that any water that is upwelled comes from within the already nutrient-depleted surface layer (Figures 5.22(b) and 5.23(b)).

El Niño events are now often referred to as ENSO events, where the SO stands for 'Southern Oscillation'. 'Southern Oscillation' is the term used for the continual rise and fall of the atmospheric pressure difference at sea-level, between the Indonesian Low and the South Pacific High (cf. the North Atlantic Oscillation. Section 4.5). The two meteorological stations generally used are those at Darwin, northern Australia (for the Indonesian Low), and Tahiti (for the South Pacific High). When the pressure difference between the two is larger than average, the SO Index is described as positive; when it is smaller than average, it is described as negative. As you might expect. El Niño events occur when the SO Index is large and negative (Figure 5.24(a)). Figure 5.24(b) shows the variation of the so-called Multivariate ENSO Index for the last 50 years of the twentieth century.

Figure 5.24 (a) Monthly averages of the Southern Oscillation Index during the period 1970-1990, with El Niño events (sometimes referred to as 'warm events') indicated by red bars, and La Niña events ('cold events') indicated by blue bars, (b) The variation of the Multivariate ENSO Index during the period 1950-2000. This index is computed by the NOAA-CIRES Climate Diagnostics Center, at the University of Colorado, using weighted means for six atmospheric and oceanic parameters observed over the tropical Pacific Ocean (see text). The plot has been inverted from its usual orientation, so that it matches the Southern Oscillation Index plot in (a).

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