Figure 3.27. Correlations with the SOI, based on normalised Tahiti minus Darwin sea level pressures, for annual (May to April) means for sea level pressure (top left) and surface temperature (top right) for 1958 to 2004, and GPCP precipitation for 1979 to 2003 (bottom left), updated from Trenberth and Caron (2000). The Darwin-based SOI, in normalized units of standard deviation, from 1866 to 2005 (Können et al., 1998; lower right) features monthly values with an 11-point low-pass filter, which effectively removes fluctuations with periods of less than eight months (Trenberth, 1984). The smooth black curve shows decadal variations (see Appendix 3.A ). Red values indicate positive sea level pressure anomalies at Darwin and thus El Niño conditions.