Speaker：Zhang Zhengguang，Associate Professor，Ocean University of China
Invited by: Zhou Lei
Place：IOO Meeting Room, Xuhui Campus
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, at the same time, submesoscale ageostrophic processes have been progressively recognized as an important upwelling mechanism to close the nutrient budget and sustain the observed primary production of phytoplankton in the euphotic layer. By combining surface drifters, satellite altimetry and satellite ocean-color data, 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, when the strain rate of mesoscale surface geostrophic flow is strong, it favors a higher ageostrophic kinetic energy level and an increase in surface chlorophyll concentration. The strain-induced frontal processes are characterized by a surface chlorophyll increase and secondary ageostrophic upwelling along the light side of the oceanic density front. Further analysis indicates that the balanced ageostrophic motions with longer time scales are more effective in inducing chlorophyll increase than the unbalanced shorter time-scale wave motions.