Lifecycle-dependent roles of mesoscale heat transport in marine heatwaves and cold-spells in the East Sea (Japan Sea)

The ocean is not a static bathtub of water; it is a churning, living system where heat moves constantly, shaping the lives of everything from microscopic plankton to massive whales. When the water gets unusually hot for a prolonged period, it is called a marine heatwave, and when it gets unusually cold, it is a marine cold spell. These events are not just temporary weather quirks; they can devastate fisheries, destroy coral reefs, and force entire ecosystems to migrate or perish. For years, scientists have known that the surface of the ocean often tells only part of the story. Just as the air above a room might feel warm while the floor remains cool, the top layer of the sea can experience extreme temperatures that do not match what is happening just a few meters below. To truly understand why these events start, how long they last, and why they end, researchers must look at the entire upper layer of the ocean and track the movement of heat as it flows through currents and swirls, rather than just watching the surface temperature rise and fall.

A team of researchers from Kyungpook National University in South Korea has taken a deep dive into these mysteries, focusing on the East Sea, also known as the Japan Sea. This body of water is a dynamic, semi-enclosed sea bordered by Korea, Japan, and Russia, known for its strong currents and sharp temperature boundaries. The scientists wanted to solve a specific puzzle: what physical forces are responsible for the birth and death of marine heatwaves and cold spells in this region? While some global studies had suggested that small, swirling eddies of water are the primary drivers of these extreme events, the researchers suspected that the story might be more complex in a smaller, more constrained sea like the East Sea. They set out to build a detailed "heat budget" for the upper fifty meters of the ocean, tracking every calorie of heat that entered or left the water over a thirty-year period from 1993 to 2022. By using advanced computer models that simulate the ocean's currents and temperature, combined with real-world data on wind and sunlight, they were able to separate the heat changes caused by large, steady currents from those caused by smaller, chaotic swirls.

The researchers began by identifying thousands of individual heatwaves and cold spells, treating each one as a unique, moving object in space and time rather than just a collection of hot or cold pixels on a map. They found a crucial surprise right at the start: the events detected at the very surface of the ocean only partially matched the events happening in the water below. About half of the heatwaves and cold spells that appeared at the surface did not have a corresponding extreme event in the deeper layer, and vice versa. This means that looking only at satellite images of the sea surface gives an incomplete picture of the ocean's true thermal state. To get the full story, the team had to analyze the entire fifty-meter layer, calculating how much heat was being pushed around by the ocean's currents.

When they looked at how these extreme events grew, the results showed that the East Sea does not follow a single, simple rule. For marine heatwaves, the warming was driven by a combination of two forces: the steady, large-scale currents and the smaller, swirling eddies. Both played significant roles in heating the water up. However, for marine cold spells, the story was different. The cooling was almost entirely dominated by the large, steady currents, with the smaller eddies playing a much smaller role. This suggests that the mechanisms that start a heatwave are not simply the mirror image of the mechanisms that start a cold spell. The location also mattered immensely; in some parts of the sea, the swirling eddies helped warm the water, while in other nearby areas, they actually worked against the warming, cooling the water instead. This regional complexity showed that a single, global explanation for how these events begin would not work for this specific sea.

The most striking discovery, however, came when the researchers looked at what happened after these events reached their peak intensity. Regardless of whether the water was extremely hot or extremely cold, and regardless of which part of the sea the event occurred in, the smaller, swirling eddies consistently acted as the "brakes" that ended the event. Once a heatwave or cold spell reached its maximum temperature, the swirling motions of the ocean began to push heat in a way that rapidly returned the water to normal conditions. In fact, the heat moved by these eddies was often strong enough to explain more than 100 percent of the temperature change needed to end the event, meaning that other processes, such as vertical mixing or heat loss to the air, were working against this cooling or warming effect. This pattern was remarkably consistent: the eddies did not necessarily start the extreme events, but they were the reliable force that terminated them, smoothing out the temperature anomalies and restoring balance to the ocean.

The study also examined how different layers of the ocean and different sizes of swirling eddies affected these results. Even when the researchers changed the depth of their analysis from fifty meters to thirty meters, or adjusted the size of the eddies they were tracking, the main finding remained the same. The swirling motions were the dominant force in ending the events, while the forces that started them were a more complicated mix of large currents and eddies that varied by location and by whether the water was getting hot or cold. This distinction between the start and the end of an event is a vital refinement to our understanding of the ocean. It suggests that while large-scale climate patterns and steady currents might set the stage for an extreme event, the chaotic, swirling nature of the ocean itself is what eventually brings the system back to equilibrium.

This research provides a clearer, more nuanced view of how the ocean handles extreme temperatures. It moves beyond the idea that one single force, like the swirling eddies, is responsible for everything. Instead, it reveals a dynamic system where different forces take the lead at different times. The large, steady currents are often the architects of the cold spells, while a partnership between currents and eddies builds the heatwaves. But when it comes time to end the drama, the swirling eddies step in as the universal reset button, working tirelessly to erase the extremes and return the sea to its normal state. By understanding these distinct roles, scientists can better predict how these events might change in the future and how they will impact the marine life that depends on a stable thermal environment. The ocean's ability to self-correct through these swirling motions is a fundamental part of its resilience, a hidden mechanism that keeps the East Sea from staying in a state of extreme temperature for too long.