Fig. 21.

The El Niño forecasting algorithm. a. The red curve is the forecasting index defined in Eq. (115), the blue line stands for the ONI. The horizontal black dashed line indicates $FI=0$, and the blue shades under the ONI curve indicate the El Niño periods. True positive forecasts are marked by the green arrows, while false alarms are marked in black dashed arrows. The purple arrow is the alarm for 2018–2019 El Niño event. b. A detailed view of $FI\left(t\right)$ and the ONI since May 2012.

Reprinted figure from Ref. [66].