ONI (Oceanic Nino Index)

Indices

The ONI is NOAA's long-standing El Nino index. Each value is the average anomaly over three consecutive months, labelled by their initials: JJA = June-July-August (see Season codes (DJF, JJA, SON...)).

• It uses the ERSST ocean dataset (version 6 since 2026) and centred 30-year base periods updated every 5 years.
• Historically, an El Nino "episode" is counted when the ONI is at or above +0.5 °C for at least 5 consecutive overlapping seasons.
• Recent values can be revised for up to two months after they are first posted.

Since 1 February 2026, NOAA uses the relative version, RONI, for official monitoring. The ONI is still published and still shown here, because it is the index most people and older articles refer to.

How to read it

One point per month on the chart, dated at the centre month (JJA 2026 is plotted at mid-July 2026). The band from -0.5 to +0.5 is neutral.

Why it matters for Koh Samui

The app's ENSO factor for Samui is based on the ONI (plus the weekly Nino 3.4 value): >= +0.5 gives Watch, >= +1.5 gives Prepare.

Example

CPC values for 2026: AMJ +0.95, MJJ +1.39, JJA +1.80 °C.

Sources

• NOAA CPC -- Oceanic Nino Index table (ERSSTv6)
• NOAA CPC -- index data files

Related

• RONI (Relative Oceanic Nino Index)
• Nino 3.4
• Season codes (DJF, JJA, SON...)
• Weak, moderate, strong and "super" El Nino
• Factor: El Nino (event strength)

ENSO status now

ONI (NOAA CPC)

+1.80 °C, JJA 2026 (previous +1.39 °C)

RONI (NOAA CPC)

+1.36 °C, JJA 2026

Nino 3.4 weekly (NOAA CPC / OISST)

+3.0 °C anomaly, week of 16 Sep 2026 (SST 29.6 °C)