The El Niño Briefing #3: Four boxes, one verdict: this is the classic, east-loaded El Niño

Take the four Niño boxes NOAA measures every week and lay them side by side. Reading east to west, this is where the ocean stood on the week centred 12 August:

• Niño 1+2 (the Peruvian coast): +4.0°C
• Niño 3 (eastern Pacific): +3.2°C
• Niño 3.4 (the headline box): +2.7°C
• Niño 4 (western Pacific): +1.0°C

Now the same four, six weeks earlier, on 1 July: +3.3, +2.0, +1.7, +1.1.

Three of the four boxes climbed hard. The fourth did not move at all. Niño 3 gained +1.2°C in six weeks; Niño 4 is a tenth of a degree cooler than it was.

That is not a detail. It is the difference between the two kinds of El Niño.

When the warmth concentrates east of the Date Line, you get the classic, textbook event — the one named on Peru's coast, where the warm water piles against South America and the rains come with it. When the warmth centres on the Date Line instead, with Niño 4 leading, you get the other flavour, and its effects land in noticeably different places. The 2026–27 event has now spent six weeks declaring which one it is, and it is not the second one.

Two honest caveats, because this is where forecasts oversell. The pattern can still shift — events have migrated west before, especially through the peak. And "classic" describes the ocean's shape, not a guarantee about any particular country's weather; the atmosphere still gets a vote everywhere the signal travels. What the split does buy you is a better prior about which of our regional pages to read first.

The rest of the board, as of this week:

• Weekly Niño 3.4: +2.7°C (NOAA, week centred 12 August) — the sixth consecutive weekly rise, from +1.7°C at the start of July. The next weekly reading lands Monday.
• BOM's relative Niño 3.4: +2.2°C — Australia's own warming-baseline correction, on the same weekly cadence.
• RONI, the index NOAA classifies by: +1.0°C for May–July. Unchanged since the last issue; NOAA updates it monthly.
• ONI, the conventional seasonal index: +1.39°C for May–July.
• SOI: −2.4 for July — and this one is worth a second look. April was −0.6, May −0.9, June −1.4, July −2.4. A negative Southern Oscillation Index means the Walker circulation is running in its El Niño configuration, and the reading has deepened every month since spring. Following the record June–July MEI we covered last issue, it is a second independent line of evidence that the atmosphere is fully engaged, not just the ocean.

What to watch next: Monday's weekly SST update, whether Niño 4 finally moves, and the July–August MEI in early September.

— The El Niño Briefing