The index is defined as a three-month average of sea surface temperature departures from normal for a critical region of the equatorial Pacific (Nino 3.4 region; 120W-170W, 5N-5S).
North America's operational definitions for El Nino and La Nina, based on the index, are: El Nino: A phenomenon in the equatorial Pacific Ocean characterised by a positive sea surface temperature departure from normal (for the 1971-2000 base period) in the Nino 3.4 region greater than or equal in magnitude to 0.5 deg C (0.9 deg F), averaged over three consecutive months.
La Nina: A phenomenon in the equatorial Pacific Ocean characterised by a negative sea surface temperature departure from normal (for the 1971-2000 base period) in the Nino 3.4 region greater than or equal in magnitude to 0.5 deg C (0.9 deg F), averaged over three consecutive months.
