Mid-October Deck: Weekly Niño 3.4 at 2.4 °C as CPC Holds El Niño Advisory

The NOAA Climate Prediction Center kept its ENSO Alert System at El Niño Advisory in the briefing deck it published on 13 October 2026, reporting that equatorial sea surface temperatures are above average across the east-central and eastern Pacific and that atmospheric circulation anomalies over the equatorial Pacific are consistent with El Niño. The agency said the event continues to strengthen.

Weekly index values

The CPC listed its latest weekly sea surface temperature departures as 0.2 °C in the Niño 4 region, 2.4 °C in Niño 3.4, 3.4 °C in Niño 3 and 4.4 °C in Niño 1+2, the region closest to the South American coast. These are weekly values from the deck dated 13 October 2026, not seasonal averages. Over the preceding four weeks, the agency reported, equatorial sea surface temperatures ran above average east of the Date Line and below average near and west of it, with the warm anomalies amplified east of the Date Line.

The deck's index tables and operational definitions are expressed in terms of the Relative Oceanic Niño Index (RONI), which the CPC describes as one measure of the El Niño–Southern Oscillation. RONI subtracts tropical-mean (20°S–20°N) sea surface temperature departures from the Niño 3.4 anomaly, adjusts the variance to match the original Niño 3.4 index and applies a three-month running mean. The most recent value, for July–September 2026, is 1.7 °C. The agency's published series shows the index rising through the year from −0.9 °C in December–February to 0.0 °C in March–May, 0.5 °C in April–June, 1.0 °C in May–July and 1.4 °C in June–August. The deck carries no Oceanic Niño Index figures and no other agency's classification of the event; the partial table it reproduces, which begins in 2014, peaks at 2.3 °C for November–January 2015-16.

The CPC also notes that NOAA's NCEI has discontinued ERSSTv5, so RONI values switched to ERSSTv6 as of 8 October 2026, while weekly monitoring continues on OISSTv2.1. Under the operational definitions reproduced in the deck, an episode by historical standards requires the ±0.5 °C threshold to be met for at least five consecutive overlapping three-month seasons.

Ocean and atmosphere

Below the surface, the CPC reports above-average temperatures persisting across the east-central and eastern Pacific over the past two months, basin-wide upper-ocean (0–300 m) heat content anomalies above average, and a thermocline slope index below average. Positive subsurface anomalies increased from late May to late July 2026 and have since held steady, according to the agency, which lists downwelling Kelvin waves initiated in December 2025, January 2026, March 2026 and June 2026.

In the atmosphere, the CPC found below-average outgoing longwave radiation — a signature of enhanced convection and rainfall — along the equator from near the Date Line to the eastern Pacific, and above-average OLR, indicating suppressed convection, over the Philippines, Indonesia and Papua New Guinea. Low-level 850-hPa westerly wind anomalies extended from the western to the east-central equatorial Pacific, with 200-hPa anomalies easterly over the same stretch and westerly over the far eastern equatorial Pacific.

What the CPC forecasts

The summary slide states that a strong-to-very strong El Niño is likely through January–March 2027, remaining greater than an 83% chance. In the probabilistic outlooks updated 8 October 2026, the CPC expects El Niño to persist through March–May 2027, with chances then favouring ENSO-neutral in April–June and May–July 2027.

Separately, the agency's official outlook of 8 October assesses the odds of a historic event — a three-month RONI of +2.5 °C or more, which would exceed the strength of previous El Niño events dating back to 1950 — at 54% for September–November, 83% for October–December and 70% for November–January.

Model guidance in the deck points the same way: the CPC says the NCEP CFS.v2 ensemble mean, in the forecast issued 13 October 2026, favours El Niño continuing through Northern Hemisphere winter 2026-27. Those are model outputs rather than observations. The agency notes that its U.S. seasonal outlooks combine long-term trends, soil moisture and, where appropriate, ENSO, and that the summary statements are updated monthly on the second Thursday alongside the ENSO Diagnostics Discussion.