资源环境科技发展态势分析平台

The global ocean circulation is forced by large-scale fluxes at the surface and dissipated at small scales by diffusion. To achieve a long-term equilibrium requires a dynamical route that transfers energy from large to dissipative scales. The submesoscale ageostrophic motions (1-50 km) have been hypothesized recently to play a critical role in this route by extracting energy from mesoscale eddies (50-500 km) that contain the majority of oceanic kinetic energy. By combining global surface velocity measurements by drifters and satellite altimeter-tracked eddy data set, we show that the submesoscale ageostrophic kinetic energy exhibits unexpected global mean features through the life cycle of mesoscale eddies. The ageostrophic energy is relatively small in eddy's mature phase but is large both in formation/decaying phases. Furthermore, the energy level of ageostrophic motions depends positively on the local geostrophic strain rate, suggesting that the geostrophic deformation field may dictate the energy transfer from mesoscale eddies to submesoscale motions.

Plain Language Summary Oceanic mesoscale eddies are energetic vortices with radius ranging from tens to hundreds of kilometers. These eddies contain the majority of the oceanic kinetic energy and play an important role in regulating oceanic transport of heat, salt, and other climatically important tracers. In order to understand the evolution of eddies through their life cycle, one key question that is yet to be fully answered is how the energy contained in the geostrophically balanced mesoscale eddies transfers to smaller scales where it can be dissipated irreversibly. Answering this question is fundamental to understand of the oceanic equilibration and to improve our ability to simulate/predict the climate. In this study, we combine the ocean surface drifter measurements and satellite altimeter data and detect a counterintuitive feature of energy level of submesoscale motions (1-50 km) through the life cycle of mesoscale eddies. The submesoscale energy level is low during mature phase when eddies are strong, and high in formation/decaying phases when eddies are weak. Further investigations reveal that the submesoscale energy level depends positively on the deformation strain rate around the mesoscale eddies. This latter fact points to the potential importance of mesoscale deformation field in controlling the energy transfer between the mesoscale and submesoscale motions.