Figure 1: Time series of SSTs from N3.4 (top), and TNI (bottom) from 1871 to 2000. Both time series are normalized by the standard deviation $s$, as given. In the top panel, shading is included to show where thresholds of $\pm$0.4$^\circ$C are exceeded, indicating ENSO events. A low pass spline fit highlights interdecadal variations.