The Oceanic Niño Index — the temperature anomaly in the central
equatorial Pacific that NOAA uses to declare an El Niño or a La Niña — for every
overlapping season since 1950, wrapped one decade to a row. Press play to weave
it: rust above the line is warm, blue below is cool. The pale band on every year
boundary is the three seasons that straddle New Year, and the ocean has no
calendar, yet 37 of the
42 episodes here crested inside one. Switch to the
water off Peru and 1983 grows a bulge the official index never had. Tap or hover
any season.
Data & source
Data: NOAA
Climate Prediction Center — Oceanic Niño Index and monthly
Niño-region SST anomalies, both from the ERSSTv5 analysis · public domain
(US Government / NOAA) · fetched —.
Scope and gaps, plainly. The ONI is a three-month running mean
of the sea surface temperature anomaly in one box of ocean — Niño 3.4, 5°N–5°S
and 170°W–120°W. It is the official yardstick NOAA declares episodes with, and
it is not the whole of ENSO: the atmosphere's half of the oscillation is not in
this file at all. Seasons overlap, so consecutive readings share two of their
three months and the curve is smoother than the ocean was. Episodes here follow
NOAA's own rule and nothing cleverer — five or more consecutive overlapping
seasons at or beyond ±0.5 °C — which has two consequences worth naming: the
record opens part-way through a La Niña in DJF 1950, so that episode's start is
cut off by the record rather than by the ocean, and the warm run at the end has
not lasted five seasons yet, so it is drawn and labelled as a run in progress
and is not counted among the episodes. "Off Peru" is Niño 1+2 (0°–10°S,
90°W–80°W) put through the same three-month mean; it is measured against a fixed
1991–2020 base while the ONI uses centred 30-year bases that shift through the
record, so the two curves are comparable in shape and not in absolute anomaly.
The first and last seasons of the Niño 1+2 curve would need a month from outside
the file, so they are left blank rather than averaged from two months. Only
episodes peaking at 1.5 °C or beyond are named on the chart, and on a narrow
screen none are, because a name is a quarter of a row there; every episode is
in the table below, and tapping any season names the one it belongs to.