Climate centres use several standard regional averages of sea surface temperature anomalies as indices to monitor the state of the equatorial Pacific ocean, especially the anomalous warming and cooling associated with El Niño and La Niña.
The diagrams below illustrate these indices as observed in recent months (in black), and the evolution predicted by the Met Office dynamical long-range ensemble forecast system (in red).
By using several regions placed along the equatorial strip, variations in individual events can be quantified.
Niño3 lies between 150°W and 90°W, 5°N and 5°S Niño4 lies between 160°E and 150°W, 5°N and 5°S Niño3.4 lies between 170°W and 120°W, 5°N and 5°S.
The Met Office contributes to the World Meteorological Organization's (WMO) statement on the state of the tropical Pacific.