How Remote Sensing Enables NOAA to Monitor El Niño

Earlier this year, the National Oceanic and Atmospheric Administration (NOAA) announced that El Niño conditions had initiated in the tropical Pacific Ocean. Determining whether El Niño is truly underway is a highly complex scientific task. It requires the continuous monitoring of ocean temperatures, wind speeds, atmospheric pressure, rainfall, and cloud patterns across thousands of miles of open water. No single map, satellite image, or isolated temperature measurement can provide the answer on its own. Scientists must synthesize many different pieces of data to construct the full picture.

Much of this vital information originates from remote sensing systems. These systems monitor the Pacific Ocean from space. They are supplemented by ocean buoys and other direct measurements taken in the water, known as in-situ measurements. Together, these observations allow scientists to track changes occurring at the ocean surface and in the atmosphere above it. This combination of data sources is essential for understanding such a large and remote area.

For many decades, NOAA relied on the Oceanic Niño Index (ONI) as its primary tool for identifying El Niño and La Niña conditions. The ONI tracks sea surface temperature anomalies within a specific area called the Niño 3.4 region. This area is located in the tropical Pacific and is centered on the equator. Scientists have used this region for a long time to monitor the El Niño-Southern Oscillation (ENSO), which is the climate pattern that causes El Niño and La Niña.

However, in 2026, NOAA’s Climate Prediction Center adopted a new tool called the Relative Oceanic Niño Index (RONI) for monitoring ENSO. Unlike the traditional ONI, which compares temperatures in the Niño 3.4 region to a historical average, RONI compares conditions in that region to temperatures across the broader tropical oceans. According to NOAA, this new approach helps account for long-term ocean warming. It also preserves the ability to accurately identify El Niño and La Niña events. Both indices depend on accurate measurements of sea surface temperature. This makes satellite observations a critical part of ENSO monitoring.

Satellite observations make it possible to monitor ocean temperatures across the entire Pacific basin. This is a vast improvement over monitoring only a limited number of locations. By repeatedly measuring the temperature of the ocean surface, satellites reveal where warm water is accumulating. They also show how those patterns change over time. This continuous view is necessary because the ocean is too large for ships or buoys to cover completely.

Persistent warming across the central and eastern tropical Pacific remains one of the defining characteristics of El Niño. Whether these temperatures are incorporated into the ONI, the RONI, or other monitoring tools, sea surface temperature measurements provide a foundation for understanding changing conditions. They give scientists a baseline to compare other data against.

Some of the most important changes associated with El Niño occur below the ocean’s surface. The warming that affects weather patterns does not start at the top. Months before the characteristic warming of El Niño appears across the Pacific surface, large pools of warm water can accumulate at depth. These deeper pools store heat that can later influence the surface climate. Measuring those temperatures helps scientists estimate how much heat is available to fuel a developing event.

Information about these deeper waters comes from a combination of observing systems. This includes moored buoys that stay in one place and satellites that scan from above. Because changes at depth often precede changes at the surface, these measurements can provide an early indication that El Niño conditions are developing. This early warning is crucial for predicting future weather patterns.

Satellite measurements of sea surface height provide another clue about how much heat is stored within the ocean. This might seem unrelated to temperature, but it is directly connected. As water warms, it expands. This expansion causes areas containing unusually warm water to occupy slightly more volume than the surrounding cooler waters. The result is a subtle rise in sea level. This rise can be measured from space with great precision.

Satellite radar altimeters aboard missions such as Sentinel-6 Michael Freilich can detect these small differences in height. Recent NASA imagery has shown elevated sea surface heights across portions of the equatorial Pacific. This elevation indicates the presence of large pools of warm water stored within the upper ocean. Most people associate El Niño with warmer ocean temperatures, but the connection between ocean heat and sea level is less obvious. Research has shown that sea surface height can serve as an additional indicator of developing El Niño conditions. Ocean warming causes the ocean surface to rise, while cooler water contracts. This physical change provides scientists with another valuable data point.

Equatorial Kelvin waves are pulses of warm water that move eastward beneath the surface of the Pacific Ocean. These waves are not visible from the surface, but their effect on sea level is detectable. As they travel across the basin, they raise sea levels along their path. This allows satellite altimeters to track the movement of warm water from the western Pacific toward the central and eastern Pacific.

In visualizations created by NOAA and NASA, Kelvin waves often appear as bands of elevated sea surface height moving eastward. The arrival of these warm-water pulses is often among the earliest signs that El Niño conditions may strengthen. By tracking these waves, scientists can predict how the event will evolve in the coming months. This helps them anticipate changes in global weather patterns.

El Niño is not simply an ocean phenomenon. It is the result of complex interactions between the ocean and the atmosphere. For that reason, NOAA monitors more than just ocean temperatures and sea level. Scientists track atmospheric pressure patterns, trade winds, rainfall, cloud cover, and atmospheric water vapor across the tropical Pacific. All of these factors are connected.

The Southern Oscillation Index (SOI) is one of the longest-running indicators associated with ENSO conditions. It measures differences in atmospheric pressure between Tahiti and Darwin, Australia. Persistent changes in those pressure patterns are closely associated with the development of El Niño and La Niña events. When the pressure differences shift, it signals a change in the atmospheric circulation that drives the climate pattern.

Satellites also measure outgoing longwave radiation (OLR). This is the heat energy emitted from Earth back into space. Thick cloud systems block some of that radiation from escaping. Therefore, OLR provides an indirect way to monitor cloud cover and atmospheric convection. During El Niño, increased cloud cover and thunderstorm activity often shift eastward into the central Pacific. This shift reduces outgoing longwave radiation and provides another indication that the atmosphere is responding to warming ocean conditions. Satellites provide a basin-wide view of these atmospheric conditions, even in remote areas where direct measurements are limited.

No single dataset determines whether El Niño has arrived. Each type of data provides a different piece of the puzzle. Sea surface temperatures reveal whether the Pacific is warming. Subsurface temperatures and sea surface height measurements indicate how much heat is stored within the ocean. Kelvin waves show where that heat is moving. Atmospheric observations reveal whether the atmosphere is responding to those changes.

By combining observations of ocean temperatures, ocean heat content, sea level, winds, rainfall, atmospheric pressure, and outgoing longwave radiation, scientists can build a complete model. This is done in conjunction with measurements from ocean buoys and atmospheric monitoring networks. This comprehensive approach allows scientists to track the development of El Niño across thousands of miles of the tropical Pacific. It turns a complex set of oceanic and atmospheric measurements into a clearer picture of changing conditions.

Together, these observations allow scientists to monitor the evolution of El Niño across an ocean basin that covers millions of square miles. Remote sensing from space is the key to this effort. It provides the continuous, wide-angle view that ground-based instruments cannot. By integrating data from satellites, buoys, and atmospheric sensors, NOAA can provide accurate forecasts. This helps societies prepare for the weather impacts that El Niño brings. The technology allows us to understand a climate phenomenon that is too vast and dynamic to measure in any other way.