CPC's 21 September ENSO Deck: Niño 3.4 at 2.1 °C, RONI at 1.4 °C

The NOAA Climate Prediction Center has kept its ENSO Alert System at El Niño Advisory in a slide update dated 21 September 2026, reporting that equatorial sea surface temperatures remain above average across the east-central and eastern Pacific and that the atmospheric circulation over the basin is consistent with El Niño. The agency describes the event as strengthening, with a greater than 90% chance of a very strong event during Northern Hemisphere fall and winter 2026-27.

The headline ocean numbers in the CPC update show a strong east-west contrast. The latest weekly sea surface temperature departures given by the CPC are 0.0 ºC in the Niño 4 region, 2.1 ºC in Niño 3.4, 3.0 ºC in Niño 3 and 3.9 ºC in Niño 1+2 — warmth concentrated toward South America and effectively absent in the far western monitoring region.

Ocean and atmosphere

The CPC's description of the past four weeks has equatorial sea surface temperatures above average east of the Date Line and below average near and west of it. Globally, the agency notes above-average temperatures over most of the world's oceans during that window, with near-average values around Indonesia and above-average values in the Indian Ocean and part of the Atlantic.

Beneath the surface, the CPC reports that basin-wide upper-ocean (0-300 m) heat anomalies are above average and that the thermocline slope index — defined in the deck as the difference in anomalous depth of the 20 ºC isotherm between the western Pacific (160ºE-150ºW) and the eastern Pacific (90º-140ºW) — is below average. Positive subsurface temperature anomalies persist east of the Date Line, while negative anomalies have strengthened to the west. The agency identifies downwelling oceanic Kelvin waves initiated in December 2025, January 2026, March 2026 and June 2026, following upwelling waves in August and October 2025.

In the atmosphere, the CPC records below-average outgoing longwave radiation — indicating enhanced convection and rainfall — along the equator from near the Date Line to the eastern Pacific over the last 30 days, and above-average OLR, indicating 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; upper-level (200-hPa) anomalies were easterly from the western to the east-central Pacific and westerly over the far-eastern equatorial Pacific.

RONI, and a dataset change

The deck devotes a section to the Relative Oceanic Niño Index, which the CPC defines as the Niño 3.4 (5ºN-5ºS, 120º-170ºW) sea surface temperature departure with the tropical mean (20ºS-20ºN) departure subtracted, variance-adjusted and smoothed with a three-month running mean. The CPC's own caption calls RONI "one measure of the El Niño-Southern Oscillation," noting that other indices can confirm whether coupled ocean-atmosphere features accompanied a given period.

The most recent value listed is 1.4 ºC for June-August 2026. The 2026 sequence in the CPC table runs -0.9 ºC in December-February, -0.4 ºC in February-April, 0.0 ºC in March-May, 0.5 ºC in April-June, 1.0 ºC in May-July and 1.4 ºC in June-August.

The agency also flags a data change: as of 10 August 2026, with NCEI discontinuing ERSSTv5, RONI values have switched to ERSST.v6. Weekly monitoring shown elsewhere in the deck uses OISSTv2.1 and is unaffected. Both index tables remain available. Under the operational definitions reproduced in the update, the CPC requires thresholds of at least +0.5 ºC or -0.5 ºC to be exceeded for five consecutive overlapping three-month seasons for a full-fledged episode.

Forecasts

The probabilistic outlooks in the deck were updated on 10 September 2026. They put the chance of a very strong El Niño at greater than 90% from September-November through November-January 2026-27, and give a 75% chance that the October-December 2026 season produces a historic event exceeding the strength of previous El Niño events dating back to 1950. The CPC expects El Niño to persist through March-May 2027, with what it calls a possible chance — 55% — of ENSO-neutral conditions in April-June 2027.

The NCEP CFS.v2 model forecast issued 21 September 2026 is model output rather than observation; the CPC says its ensemble mean favours El Niño continuing through Northern Hemisphere winter 2026-27.

Over North America, the CPC notes that since late July positive height anomalies have been present over the North Pacific, with above-average heights and temperatures dominating the central contiguous United States. The agency's October-December 2026 seasonal outlooks, it says, combine long-term trends, soil moisture and, where appropriate, ENSO.