End-of-September ENSO Deck: Niño 3.4 at 2.2 °C, Niño 1+2 at 3.9 °C

The NOAA Climate Prediction Center (CPC) kept its ENSO Alert System at El Niño Advisory in the slide deck it published on 28 September 2026, reporting that El Niño is present and strengthening, and that the agency sees a greater than 90% chance of a very strong event during the Northern Hemisphere fall and winter of 2026-27.

The update, prepared by CPC and NCEP, lists the latest weekly sea surface temperature (SST) departures for the four monitoring regions: 2.2 °C in Niño 3.4, 3.1 °C in Niño 3, 3.9 °C in Niño 1+2 and 0.1 °C in Niño 4. CPC states that equatorial SSTs are above average across the east-central and eastern Pacific, and that atmospheric circulation anomalies over the equatorial Pacific are consistent with El Niño.

A warm east, a cool west

The deck describes a basin split. Over the last four weeks, CPC reports, equatorial SSTs were above average east of the Date Line and below average near and west of it, with the warm anomalies east of the Date Line amplifying. Globally, the agency says above-average SSTs were prevalent over most ocean basins during the same four weeks, with equatorial values near to below average around Indonesia and above average in the Indian and Atlantic Oceans.

Below the surface, CPC reports that upper-ocean (0-300 m) heat anomalies are above average and that the thermocline slope index — the difference in anomalous depth of the 20 °C isotherm between the western Pacific (160°E-150°W) and the eastern Pacific (90°-140°W) — is below average. Positive subsurface anomalies increased from late May to late July 2026 and have since remained steady, the agency says, and have persisted across the east-central and eastern Pacific over the last two months. CPC notes downwelling oceanic Kelvin waves initiated in December 2025, January 2026, March 2026 and June 2026.

The atmospheric signature is also present in CPC's 30-day diagnostics: below-average outgoing longwave radiation, indicating enhanced convection, from near the Date Line east to the eastern Pacific, and above-average OLR — suppressed convection — over the southern Philippines, Indonesia and Papua New Guinea. Low-level 850-hPa westerly wind anomalies extended from the western to the east-central equatorial Pacific, with easterly anomalies over the eastern Pacific.

Indices and definitions

The deck reports the most recent Relative Oceanic Niño Index (RONI) value, covering June-August 2026, at 1.4 °C. RONI subtracts the tropical mean (20°S-20°N) SST departure from the Niño 3.4 (5°N-5°S, 120°-170°W) anomaly, with the variance rescaled and a three-month running mean applied; CPC presents it as a way of placing current events in historical perspective. The agency notes that from 10 August 2026 its RONI values switched from ERSSTv5 to ERSSTv6, following NCEI's discontinuation of v5, while weekly SST monitoring continues to use OISSTv2.1.

CPC's operational definitions in the deck require anomalies at or beyond ±0.5 °C, sustained across at least five consecutive overlapping three-month seasons, for a full-fledged episode to be classified. The RONI table in the deck shows 2026 running 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.

What the forecasts say

CPC's probabilistic outlooks, updated 10 September 2026 and reproduced in this deck, give a greater than 90% chance of a very strong El Niño from September-November through November-January 2026-27, and a 75% chance during October-December 2026 of a historic event that would exceed the strength of previous El Niño episodes dating back to 1950. El Niño is expected to persist through March-May 2027, CPC says, with a possible chance — 55% — of ENSO-neutral conditions in April-June 2027.

The NCEP CFS.v2 SST forecast issued 28 September 2026 is model output rather than observation; CPC reports that its ensemble mean favors El Niño continuing through Northern Hemisphere winter 2026-27.

Over North America, CPC notes that since mid-August below-average heights and temperatures have been evident over the eastern North Pacific and West Coast, while above-average heights and temperatures have persisted over the central contiguous United States. The agency says its October-December 2026 seasonal outlooks combine long-term trends, soil moisture and, where appropriate, ENSO. CPC states that its summary statements are updated once a month, on the second Thursday, alongside the ENSO Diagnostics Discussion.