Wind Damping of Internal Tides in the Global Ocean
This study demonstrates that wind forcing exerts a systematic damping effect on internal tides globally, contributing a net energy loss of 15 GW and accounting for approximately 4% of their total energy sinks, thereby necessitating the inclusion of wind work in internal tide energy budgets.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The ocean is a vast, churning engine that drives the Earth's climate, moving heat from the equator toward the poles and cycling nutrients that sustain life. To keep this engine running, the water must constantly mix, blending warm surface layers with cold, deep waters. This mixing is largely powered by internal tides, which are waves that travel beneath the surface, generated when the regular rise and fall of the ocean floor pushes against the water above. As these waves move across the globe, they eventually break and turn their energy into turbulence, much like surface waves crashing on a beach. Scientists have long known that the shape of the ocean floor plays a major role in where and how this energy is lost. However, a crucial piece of the puzzle has remained hidden: the role of the wind. While wind is usually thought of as a force that pushes the ocean and adds energy to it, new research suggests that on the specific scale of these internal waves, the wind acts differently, quietly draining energy away.
A team of researchers led by Zhiyu Liu and Yuxun Liu at Xiamen University has now brought this hidden process into focus. By combining a highly detailed computer simulation of the global ocean with real-world observations from satellites and drifting buoys, they discovered that the wind systematically dampens, or slows down, internal tides across the entire planet. This finding challenges the prevailing view that the ocean floor is the primary sink for tidal energy. Instead, the study reveals that the air above is actively participating in the ocean's energy budget, acting as a brake that removes a significant amount of energy from the internal wave system.
The researchers began by running a sophisticated simulation of the global ocean, one so detailed that it could resolve the complex, small-scale movements of these internal waves. They calculated the work done by the wind on the surface currents associated with these tides. In physics, "work" describes how a force transfers energy to an object. If the wind blows in the same direction as the current, it adds energy; if it blows against the current, it removes energy. The team found that, globally, the wind almost always removes energy. This happens because the wind stress depends on the speed of the wind relative to the water. When the internal tide flows against the wind, the relative speed is higher, creating a stronger drag that removes more energy than the wind adds when the tide flows with the wind. Over a full tidal cycle, this imbalance results in a net loss of energy for the wave.
In their simulation, the team calculated that this wind damping removes approximately 29.8 gigawatts of energy from the global ocean. To ensure this result was not just an artifact of their computer model, they developed a simpler mathematical framework to estimate the effect using real-world data. This approach required only the average speed of the wind and the speed of the internal tides, along with how well their directions aligned. When they applied this method to satellite wind data and measurements from thousands of drifting buoys, the results held up. The observation-based estimate showed a global energy loss of about 15.4 gigawatts. While this number is lower than the simulation, it confirms that wind damping is a real, widespread phenomenon, not just a theoretical possibility.
The study also mapped out where this effect is strongest. The damping is most intense in regions where both the winds are strong and the internal tides are energetic, such as the Luzon Strait in the South China Sea, the Aleutian Ridge, and downstream of the Gulf Stream. Unlike the dissipation caused by the ocean floor, which is concentrated in narrow bands along steep underwater mountains, wind damping is spread broadly across the open ocean. On a global scale, this wind-driven energy loss accounts for about 2% of the total energy sinks for internal tides. However, this average masks its local importance. In specific mid-latitude regions, wind damping can account for up to 10% of the energy lost by internal tides, a contribution that is too large to ignore.
This discovery extends our understanding of the ocean's energy budget beyond the water itself. It shows that the atmosphere and the ocean are linked in a more complex way than previously thought, with the wind acting as a constant, global brake on internal waves. While the exact amount of energy removed depends on how we measure the speed of the tides and how the atmosphere and ocean interact, the direction of the effect is clear: the wind consistently drains energy from these waves. This process helps explain how tidal energy is transferred to smaller scales, eventually driving the mixing that sustains the global ocean circulation. By identifying wind damping as a key player, the researchers have filled a critical gap in our knowledge of how the Earth's climate system functions, revealing that even the air above the waves plays a vital role in the ocean's hidden currents.
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