Sea surface height might sound like an odd thing to track for an ENSO story, but it's one of the most reliable physical proxies available: when ocean water warms, it expands and the surface literally rises. NASA's Jet Propulsion Laboratory uses exactly this signal, measured by the Sentinel-6 Michael Freilich satellite, to track the buildup of heat ahead of El Niño's surface arrival.
What Researchers Actually Said
According to JPL sea level researcher Severine Fournier, conditions in the western Pacific on June 8 "looked similar to those from the same time in 1997", the year of one of the strongest El Niño events on record. That's the headline-grabbing part.
But the more careful read is the second half of the finding: warm conditions in the eastern Pacific have lagged behind 1997's pace, with fewer Kelvin waves, the slow-moving subsurface heat pulses that eventually surface and drive El Niño, having arrived there by the same date. More are reportedly on the way, which is consistent with the broader Kelvin wave activity feeding this year's event.
Why the Distinction Matters
It's tempting to round "looks like 1997 in the west" up to "this will be as strong as 1997." That's not what the data says yet. Fournier's own assessment was careful: "For now, it looks like it's going to be a big one, more so than I would have said last week, but we still need more observations to know what's going to happen."
The honest takeaway: the setup resembles a major event. Whether the eastern Pacific catches up to that western Pacific signal over the coming weeks is the open question that will determine whether 2026 ends up rivaling 1997-98 or settling into a strong-but-not-historic event.
What Sea Surface Height Is Actually Measuring
The physics here are simpler than the instrument suggests, and worth stating because it explains both the power and the limits of the signal.
Water expands as it warms. A column of ocean that has absorbed heat is physically taller than the same column was before, by an amount that is small in absolute terms and precisely measurable from orbit. Sea surface height is therefore a proxy for heat integrated through the water column, not just at the surface. That is exactly why it leads: it registers heat that has not yet reached the surface where SST sensors would see it.
The limitation follows from the same fact. Sea level also responds to salinity, to winds pushing water around, and to the long-term global rise from a warming ocean and melting land ice. A regional sea surface height anomaly is not a thermometer reading, and separating the ENSO signal from the background requires exactly the kind of care that a striking side-by-side image invites you to skip.
The Trap in Side-by-Side Images
Comparison maps of 2026 against 1997 are genuinely informative and also genuinely easy to misread, for two reasons that are rarely stated alongside them.
First, the colour scale is a choice. Two maps drawn on different scales, or on the same scale against different baselines, can look far more or less alike than the underlying numbers are. A visual resemblance is a claim about a rendering, not a measurement.
Second, and more substantially, the baseline has moved. Global mean sea level in 2026 is materially higher than in 1997 for reasons that have nothing to do with ENSO. Anomaly maps handle this by referencing a modern climatology, but the comparison still involves two eras with different background states. This is the same problem RONI exists to solve on the temperature side, which we cover in the RONI vs ONI explainer.
Fournier's quoted caution, that more observations were needed, is the right posture and it is the part most likely to be dropped when an image travels.
How This Aged
This piece was written on a June 8 snapshot, and the open question it posed has since partly resolved. The eastern Pacific did not stay behind. By late July, weekly Niño 1+2 anomalies had climbed sharply and the event developed a pronounced east-weighted structure, which we track in the east-west gradient piece.
Worth noting what that does and does not vindicate. The sea surface height signal did lead the surface signal, which is the mechanism this page describes working as advertised. It does not establish that 2026 will match 1997-98, and the amplitude question remains open until the October to December season resolves. For current status see where things stand, and for the subsurface mechanism itself, our explainer on heat content.