Sea surface temperature maps are not the most important ones at a NOAA operations center during an intense El Niño event. The vivid red patches that are visible enough to be replicated in news articles and weather broadcasts as they travel across the equatorial Pacific in satellite images are the ones that receive media attention. The cross-sections of the equatorial Pacific at depth, color-coded by temperature anomaly, illustrate where heat has accumulated in the water column in comparison to historical averages. These maps are what oceanographers pay closer attention to. These maps currently depict something that hasn’t been seen in a long time. The anomalies are strong, deep, and were somewhat predicted by NOAA’s forecasting tools years before the present event started.
The subsoil is where El Niño’s physics begin. Warm surface water is pushed into the western Pacific by trade winds that sweep westward over the tropical Pacific during neutral conditions, allowing colder water to upwell along the coast of South America. The layer where the temperature abruptly declines with depth, known as the thermocline, is located deeper in the west and shallower in the east.
El Niño circumstances cause the thermocline to flatten over the basin, the trade winds to decrease, and warm water to slosh back eastward. However, the heat is traveling beneath the surface before any of this is apparent. At depths of several hundred meters, the warm water that will eventually surface first manifests as a subsurface thermal anomaly, commonly referred to as a Kelvin wave, that is moving eastward through the ocean’s interior.
NOAA has constant visibility into the location and strength of these subsurface anomalies because to the TAO/TRITON buoy array, which spans the tropical Pacific and is equipped with devices that detect temperature at various depths down the water column. As they drift through the basin and regularly profile the temperature structure from the surface to 2,000 meters, Argo floats offer extra data coverage. When combined, these technologies enable forecasters to identify impending El Niño conditions months before they completely materialize at the surface, which is precisely what happened during the current event’s development. Long before the surface manifestation that eventually resulted in record sea surface temperatures, the subsurface heat signal was observable and quantifiable.
The current anomalies reveal heat at depths close to 300 meters, or about 1,000 feet, which is much higher than historical criteria for this stage of the El Niño cycle. As the warm water continues its eastward journey and climbs toward the surface, the temperatures in these deep layers indicate stored energy that will eventually be released to the atmosphere. This release is responsible for the atmospheric reactions that make El Niño significant far beyond the tropical Pacific: jet stream disruption, modified precipitation patterns in North and South America, rainfall suppression in parts of Australia and Southeast Asia, and modifications to hurricane formation conditions in the Atlantic and Pacific. These consequences are all downstream of the current water column building.
Without going into too much detail, the relationship to NOAA’s predictive modeling is worth discussing. Because of the chaotic dynamics of the climate system, NOAA is unable to accurately predict certain subsurface conditions years in advance. The statistical likelihood of various El Niño scenarios, including the risk of extreme events and the predicted range of subsurface heat anomalies given those scenarios, is projected across seasonal to annual timescales by NOAA’s Climate Forecast System and related models. The simulations found that in high-intensity El Niño scenarios, the current extreme heat conditions fall within that range. This is an observation that the situations that are currently occurring were included in the models’ distribution of potential futures, not a retrospective assertion that anyone could have forecast exactly what is happening.
When it comes to what the underlying heat buildup ultimately creates, forecasters have been paying close attention to the consequences for global teleconnection. The atmospheric circulation responses will spread throughout the world when the energy that is currently held in the equatorial Pacific subsurface releases itself into the atmosphere. Weather patterns already show some of such reactions. As the El Niño event develops and the heat that has been building in the Pacific water column continues to move toward the surface and the air above it, others will become evident in the upcoming months.
