Satellite Altimetry
Continuous satellite measurements of sea surface height track changes in the ocean
What changed
NASA uses a continuous record of satellite altimetry to compare ocean conditions in 1997, 2015, and 2026. The value of this comparison lies in placing the location and distribution of anomalies within a shared observational framework, beyond a single label of intensity.
Key readingAI-assisted analysis
The continuous succession of satellite missions turns individual observations into a comparable record, allowing researchers to track both the intensity and spatial structure of anomalies. This long-term comparability is an important foundation for understanding different El Niño events.
In focus35 Years of Satellite Altimetry Put El Niño in Context
This breakdown starts with a story focused on this topic. It is not a complete assessment.
The facts
01Comparing Sea Surface Height Anomalies Across Years
Material published by NASA on October 23 compares maps of sea surface height anomalies for October 13, 1997, October 12, 2015, and October 12, 2026. The data come from TOPEX/Poseidon, Jason-2, and Sentinel-6 Michael Freilich, respectively.
The seasonal cycle and long-term trends have been removed from the maps to highlight short-term natural variations such as El Niño. The article notes that, in 2015, waters with elevated sea surface height along the equator reached the South American coast less extensively than in 1997 and 2026.
02Successive Missions Build a Long-Term Reference
The article describes an altimetry record spanning 35 years: similar satellites observe along nearly the same ground tracks, with overlapping periods of operation between successive missions. Warm water expands and raises the sea surface, so sea surface height anomalies in the central and eastern equatorial Pacific can offer clues to the heat accumulation associated with El Niño.
H/ACC insights
Continuous Measurements Make Event Patterns Comparable
H/ACC editorial interpretation: Progress in this kind of observational infrastructure means that questions about different years can be examined against a comparable record. Overlapping observations between successive missions make it possible to check for differences as instruments hand over the work. The data accumulated through this succession of missions can then help place the location, extent, and magnitude of a given year’s anomalies in a long-term context.
This also allows events of similar intensity to be examined in greater detail. Where waters with elevated sea surface height remain, and whether they extend to the coast, are details that a single intensity label cannot capture. These observations can inform subsequent research and forecasts; their actual impacts still need to be assessed alongside atmospheric conditions and further observations.
What to watchAI-assisted analysis
- In subsequent altimetry maps, will areas of elevated sea surface height continue to extend toward the South American coast, and how will their extent and magnitude change from month to month?
- Do public data descriptions provide cross-mission bias corrections, uncertainty estimates, and comparative validation from overlapping observation periods to support the comparability of this long-term record?
Limits of this assessment
- This is a comparison of an existing observational record. The latest map represents October 12, rather than real-time ocean conditions on October 23.
- Sea surface height anomalies provide clues to ocean conditions and cannot be treated directly as a complete measure of ocean heat content. Similar sea surface patterns alone also cannot determine rainfall in a particular region.
- For this article, H/ACC has not independently processed raw satellite measurements, validated calibration across missions, or reproduced weather forecasts.