Pacific Warm-Water Volume Just Reached Its Third-Highest Month Since 1980.

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Subsurface coverage of an El Niño almost always arrives as a Kelvin wave story. A pulse of warm water crosses the Pacific, the thermocline deepens in the east, sea surface temperatures rise a few weeks later. That is a real mechanism and CPC named it in the 9 July discussion: "a recent downwelling Kelvin wave deepened the thermocline and raised temperatures in the eastern Pacific."

But a Kelvin wave and the basin-wide recharge state are two different measurements answering two different questions, and they are routinely reported as if they were one. Kelvin waves redistribute thermocline depth and warm water zonally on relatively short timescales, and they matter dynamically: they change SST, currents and the ocean-atmosphere feedbacks that follow. Basin-wide warm water volume describes something slower and wider. Right now it is the more interesting number.

+3.00

June 2026 warm water volume anomaly, ×10¹⁴ m³ (PMEL/TAO)

2nd

Highest June on record since 1980, behind June 1997 at +3.08

+1.25

Peak reached in 2015, a year that still produced a very strong event

Two Different Things, Routinely Confused

These are not the same measurement and they answer different questions.

A downwelling Kelvin wave is a propagating disturbance. It moves east along the equator over a period of weeks, depresses the thermocline where it arrives, and raises sea surface temperatures in the eastern Pacific. It is not passive: by changing thermocline depth it feeds back into currents, SST and the coupled system. It speaks to the next month or two.
Warm water volume is a basin-wide state variable, not a wave. NOAA's Pacific Marine Environmental Laboratory computes it as the volume of water warmer than 20°C between 5°N and 5°S, from 120°E to 80°W. It is a proxy for equatorial upper-ocean heat content and thermocline depth, not a measure of total ocean heat energy. It changes over months to seasons, and it is the quantity in the recharge oscillator, the standard framework for why ENSO oscillates at all.

The two are related but not interchangeable. CPC's subsurface index in the July discussion covers 180° to 100°W, the central and eastern part of the band, which is why it responds visibly to an arriving wave. The PMEL measure spans the whole equatorial Pacific and responds to the broader recharge state instead. Their divergence can help separate regional thermocline changes from changes in the basin-wide state. Neither measure alone determines the seasonal outcome.

Where the Reservoir Actually Is

Monthly warm water volume anomalies, in units of 10¹⁴ m³, from the PMEL/TAO project. The record begins in January 1980 and currently ends with June 2026.

Month 2026	Jan	Feb	Mar	Apr	May	Jun
WWV anomaly	+1.02	+1.49	+1.87	+2.84	+2.70	+3.00

IAMElNino.com analysis. Ranked against every month in the record, June 2026 is third, behind April 1997 at +3.12 and June 1997 at +3.08. Ranked against every June, it is second, again behind 1997. The basin has gone from a modest deficit at the start of 2025 to near the top of its 46-year range in about eighteen months.

How That Compares With the Events People Reach For

The peak monthly anomaly reached during each event's development year, with the month it occurred.

Event	Peak WWV anomaly	Month	That January	That June
1982-83	+2.88	Oct 1982	+1.15	+1.72
1997-98	+3.12	Apr 1997	+1.63	+3.08
2015-16	+1.25	May 2015	+0.31	+1.12
2023-24	+2.48	Apr 2023	+2.20	+2.33
2026-27	+3.00 so far	Jun 2026	+1.02	+3.00

Two things stand out, and the second is the one that should stop anyone from reading this as a forecast.

The first is that at the June checkpoint, 2026's basin-wide warm water volume anomaly is close to June 1997, and well above 1982-83, 2015-16 and 2023-24 at the same point in the calendar. That is one metric at one calendar month. It is not an event analogue, and the full trajectories are not identical.

The second is 2015-16. It peaked at +1.25, the lowest of the five, less than half of where 2026 sits now. It then produced one of the strongest El Niño events in the record: seasonal RONI reached +2.25 in November-January. 2015 demonstrates that warm water volume alone does not uniquely determine the eventual surface-event magnitude. The research literature treats it as a genuine ENSO precursor whose lead relationship and predictive value vary over time, not as a one-to-one predictor.

Cases Where High Warm Water Volume Did Not Produce an Immediate El Niño

IAMElNino.com analysis. Search the record for high warm water volume outside the famous events and the picture gets harder still.

May 2008 reached +2.73, higher than the peak of the 2023-24 event. Seasonal RONI for October-December 2008 was −0.60 and November-January was −0.86. The Pacific went the other way. El Niño conditions did eventually arrive, but not until late 2009, more than a year later.
June 2022 reached +2.02 in the middle of a multi-year La Niña, with RONI at −0.9 or below through that autumn. El Niño followed in 2023, again on a lag of roughly a year.

So a full reservoir is not a schedule. In both of those cases the recharge was real and the surface response came much later than a naive reading would suggest, or in a different order entirely.

What is different in August 2026 is that the surface has already responded. RONI for May-June-July is +0.98°C and CPC has an El Niño Advisory in force. The 2008 and 2022 cases were recharge without an event underway. This is recharge with one underway. That is a materially different situation, and it is also why these historical cases limit what the number can promise rather than settling anything.

What a Falling Reservoir Would Mean

This is the part most subsurface coverage leaves out, and it is easy to get backward. In the recharge oscillator framework, an El Niño does not simply run out of steam: the equatorial band discharges heat poleward and warm water volume falls. A number does not end an El Niño. The transition involves coupled winds, currents, thermocline adjustment, meridional heat transport and surface fluxes. What declining equatorial warm water volume provides is evidence that the discharge phase has begun, an important precursor to the eventual transition rather than a solitary cause of it.

1997-98 shows the order of events, and it is the opposite of what people often assume. Warm water volume peaked at +3.12 in April 1997. It was down to +0.66 by November and turned negative in December, at −0.77. The surface event did not peak until then: seasonal RONI reached its maximum of +2.28 in November-January, with the ocean already discharging underneath it. By June 1998 warm water volume had reached −4.52, and 1998-99 was a substantial La Niña.

So the sequence in 1997-98 ran: maximum reservoir, then decline, then a surface event that kept strengthening anyway, then the surface peak, then a strongly negative reservoir as the event transitioned. A sustained decline from the warm water volume maximum can indicate that the equatorial discharge phase has begun, and it can begin before the surface El Niño reaches its maximum. It does not show that the surface peak is already behind us.

What This Does Not Tell You

It is not a magnitude forecast. 2015-16 peaked at less than half the current value and still became a very strong event. Warm water volume alone does not uniquely determine event magnitude.
It is not a timing forecast. The 2008 and 2022 highs were followed by El Niño roughly a year later, not that season.
June is not necessarily the peak. The series currently ends at June 2026. Whether June was the maximum or the reservoir is still filling is not yet knowable, and 1982-83 did not peak until October.
It says nothing about regional impacts. Warm water volume is a basin-scale ocean quantity. It does not forecast rainfall, drought or storms anywhere, and we do not convert it into one. See our methodology.
This site does not currently ingest it. The coupling checklist on our own dashboard lists subsurface heat content as having no independent input, because we do not fetch it. That remains true. This post is written against the PMEL file directly, not against anything on our homepage.

The testable version: June 2026 basin-wide warm water volume was near the top of the 1980 to 2026 record. The next question is whether that value continues rising or begins a sustained decline. A July value below June would make June the local maximum to date and give the first monthly indication that discharge may have begun, but a single month settles nothing: several declining months would be needed to establish a trend. That trend can then be compared with the timing of the seasonal RONI maximum, and alongside it we will watch whether zonal-mean thermocline depth declines and whether the equatorial band eventually reaches a discharged state. In past events, including 1997-98, warm water volume began falling before the surface El Niño peaked. This is a test of whether the expected recharge-discharge sequence shows up in 2026. It is not a forecast of the eventual RONI magnitude, and nothing here predicts one.

What to Watch

When PMEL publishes its July 2026 value. The direction of change, sustained over several months, matters more than any single level.
CPC's 13 August discussion and whether it again describes an arriving downwelling Kelvin wave, or begins describing upwelling.
The gap between CPC's 180° to 100°W subsurface index and basin-wide warm water volume. Their divergence can help distinguish regional thermocline changes from changes in the basin-wide recharge state. Neither measure alone determines the seasonal outcome.
Whether coverage keeps treating the Kelvin wave and the basin-wide recharge state as the same signal. They are related measurements on different timescales, and they answer different questions.

The Short Version

CPC is right that a downwelling Kelvin wave deepened the thermocline and warmed the eastern Pacific. That is the faster, more visible signal. The slower one is that basin-wide warm water volume reached +3.00 × 10¹⁴ m³ in June, the third-highest month in 46 years and within 0.12 of the highest ever recorded.

That is not a prediction of a record event. 2015-16 reached a very strong El Niño on less than half that, and 2008 and 2022 produced no immediate event on +2.73 and +2.02. What a near-record value does suggest is that a shortage of basin-wide warm water volume is not the obvious limiting factor at this stage. The eventual strength still depends on coupled winds, heat redistribution, surface fluxes and other feedbacks. The number worth watching from here is not how high it goes, but whether it begins a sustained decline, and when that decline sits relative to the surface peak.

Data vintage. Figures in this article are those published as of 6 August 2026, and the article has not been rewritten to match later data. NOAA revises weekly Niño-region values each Monday and can revise recent seasonal RONI and ONI values for up to two months after first posting. Where a number here differs from the live dashboard, the dashboard carries the newer figure.

Written and meteorologically reviewed by Christopher W. Corwin, B.S. Meteorology, IAMElNino.com · Drafted with AI research assistance, built from the NOAA PMEL/TAO warm water volume file and CPC's July 2026 ENSO Diagnostic Discussion, with every value taken from the primary data and every ranking recomputed against the full 1980 to 2026 record, then independently fact-checked before publication. That check corrected the mechanism in this piece: an earlier draft said a falling reservoir showed the surface peak was already behind us, which the 1997-98 record disproves, since warm water volume began falling months before the surface peaked. Warm water volume is a proxy for equatorial upper-ocean heat content derived from a moored array and gridded analyses, and recent months are subject to revision.