How tropical instability waves impact the equatorial Pacific interannual SST variability and heat budget in 2024

The equatorial Pacific Ocean holds a secret rhythm that dictates weather patterns across the globe. At the heart of this system lies a stretch of cold water known as the cold tongue, which usually sits right along the equator. When this water gets unusually warm, it triggers El Niño, bringing heavy rains to some places and droughts to others. When it gets unusually cold, it triggers La Niña, often reversing those effects. Scientists have long known that the transition between these two states is not always smooth. Sometimes, the ocean seems to hesitate, refusing to cool down as expected after a warm spell. For years, researchers have watched the cold tongue, trying to understand what invisible forces might be pushing it back toward warmth, delaying the arrival of the next cold phase.

In 2024, a team of scientists at Nanjing University of Information Science and Technology turned their attention to a specific, puzzling moment in this cycle. Following a strong El Niño event in 2023 and early 2024, the world expected the Pacific to swing into a La Niña phase by late 2024. Instead, the cooling stalled. The water in the eastern Pacific remained warmer than anticipated, and the anticipated cold snap never fully arrived. The researchers wanted to know why. They suspected that the answer lay in the chaotic, churning motion of the ocean itself, specifically in the behavior of tropical instability waves. These are large, swirling eddies that form along the edge of the cold tongue, moving westward and mixing the water. While they had been studied before, their specific role in blocking the 2024 La Niña remained unclear.

To solve this mystery, the team built a detailed digital model of the equatorial Pacific Ocean. They did not just watch the ocean; they created two versions of it running side by side. In the first version, the model was allowed to behave naturally, letting those churning waves form and interact with the water temperature. In the second version, they used a mathematical filter to smooth out the water, effectively removing the waves while keeping everything else exactly the same. By comparing these two simulations, they could isolate the exact influence of the waves. If the version with the waves stayed warmer than the version without them, the waves were the cause of the warmth.

The results were striking. The model showed that in the summer of 2024, the tropical instability waves were exceptionally strong, far more active than the average for that time of year. In the simulation where the waves were allowed to exist, the cold tongue remained significantly warmer. In the simulation where the waves were removed, the water cooled down much more dramatically, reaching temperatures that would have signaled a clear La Niña. The researchers found that the presence of these waves reduced the cooling effect by nearly half. Specifically, in the critical Niño3 region, the waves weakened the cold temperature anomalies by 49 percent. This suggests that the waves acted as a powerful brake, stopping the ocean from cooling down as it normally would after an El Niño.

To understand how these waves managed to warm the water, the scientists broke down the heat budget of the ocean's surface layer. They looked at where the heat was coming from and where it was going. They discovered that the primary driver was not the sun or the air above the ocean, but the movement of the water itself. The waves created a massive flow of warm water from the north and south, pushing it toward the equator. This horizontal movement of water, driven by the churning waves, added heat to the cold tongue at a rate of 0.81 degrees Celsius per month. To put this in perspective, this warming effect was nearly three times stronger than the heat the ocean received from the sun and the atmosphere combined. The waves were essentially pumping warm water into the cold zone faster than the atmosphere could cool it down.

The study also revealed that this warming was not a uniform blanket but a specific, concentrated band of heat. The waves were most effective at warming the water just north and south of the equator, creating a zone where the temperature difference between the cold tongue and the surrounding water was reduced. This process, known as advection, is the physical transport of heat by moving currents. The researchers found that the swirling motion of the waves was responsible for almost all of this warming, while the steady, background currents played a very minor role. The vertical movement of water, which usually brings cold water up from the depths, was found to be negligible in this specific context, further highlighting that the horizontal churning was the dominant force at work.

These findings offer a clear explanation for why the 2024 La Niña failed to materialize as expected. The intense activity of the tropical instability waves during the summer of 2024 acted as a dynamic barrier, continuously replenishing the cold tongue with warm water. This mechanism complemented other known factors, such as wind patterns, to create a scenario where the ocean refused to cool. The research confirms that these waves are not just minor ripples on the surface but powerful agents of climate change that can alter the timing and strength of major weather events. By quantifying exactly how much heat these waves moved, the study provides a new piece of the puzzle for predicting future climate shifts, showing that the ocean's internal churning can sometimes override the signals of the atmosphere.

The work also highlights the limitations of current climate models. Many standard models use a resolution that is too coarse to see these waves clearly, which means they might miss this crucial warming effect entirely. The researchers used a high-resolution model that could capture the waves, allowing them to see the true magnitude of the heat transport. Without this level of detail, the prediction for 2024 would have likely missed the mark, assuming a cooling trend that the ocean's own dynamics were actively fighting against. This suggests that to accurately forecast the future of El Niño and La Niña, scientists must account for these mesoscale waves, which operate on a scale smaller than the big climate patterns but with a massive impact on the outcome.

Ultimately, the paper paints a picture of a dynamic, self-regulating ocean system where the churning of the water plays a decisive role in the global climate. The 2024 event was not an anomaly that broke the rules, but rather a demonstration of how the ocean's internal machinery can intervene to delay a seasonal shift. The tropical instability waves, often overlooked in broad climate discussions, proved to be the key factor that kept the equatorial Pacific warm, effectively holding back the cold waters of La Niña. This insight changes how scientists view the transition between climate phases, emphasizing that the ocean is not a passive recipient of atmospheric changes but an active participant that can reshape the climate landscape from within.