Tropical cyclones and internal tides contribute to deep near-inertial kinetic energy, researchers discover

Near-inertial internal waves (NIWs) are crucial energy sources for deep-sea mixing, but the origins of deep NIWs have remained largely unknown. A research team led by Prof. Wang Fan from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) has made advancements in understanding the generation mechanisms of deep near-inertial kinetic energy (NIKE). Their findings were published in Geophysical Research Letters.

The researchers utilized observation data from a full-depth mooring located at 130°E/15°N over a three-year period. They discovered that tropical cyclones can trigger NIWs that penetrate up to 3,000 meters deep, contributing over 50% to the deep NIKE.

Additionally, internal tides can generate deep NIKE through a process called parametric sub-harmonic instability, but this mechanism contributes less than 30% to the overall deep NIKE. During their observation period, four tropical cyclone events passed through the study area, each triggering different vertical propagation characteristics of near-inertial internal waves.

Dr. Zhang Zhixiang, the first author of the study, noted, “We found that, under the influence of stronger intermediate and deep warm eddies, wind-generated NIWs exhibit faster downward propagation speeds and lower energy loss rates during their descent.”

Furthermore, the presence of intermediate and deep warm eddies enhances the energy conversion efficiency of the parametric sub-harmonic instability mechanism, allowing for energy transfer from diurnal internal tides to near-inertial internal waves, even north of the critical latitude.

This study highlights the importance of accurately simulating the activity of intermediate and deep eddies, as well as the intensity of tropical cyclone wind stress, to improve the simulation accuracy of intermediate and deep NIKE.