Abstract
Oceanic 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. Their relatively small spatio-temporal scales (of 1~10 km and a few days) have hindered a systematic observational quantification of the submesoscale ageostrophic flow variability and its impact on ocean biogeochemistry. By combining surface drifters, satellite altimetry and satellite ocean-color data, we detect that 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.
Highlights
Oceanic 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
The submesoscale ageostrophic motions are associated with a wide range of dynamical processes: these include unbalanced wave motions, such as near inertia waves and internal gravity waves, and balanced non-wave motions, such as the ageostrophic submesoscale vortices and filaments emerging from frontogenetic instability[21,22] and mixed-layer instability[23]
By combining the surface drifter and satellite remote-sensing data, we show that the Lagrangian chlorophyll variation rate depends positively on the local ageostrophic kinetic energy level and the geostrophic strain rate, as the strong strain rate enhances the local ageostrophic kinetic energy and favors an increase in near-surface chlorophyll
Summary
Oceanic 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. A large number of studies aiming to identify the missing nutrients indicate that while the upwelling by oceanic mesoscale eddies could be an important contributor, its estimated contribution still accounts for only 20–30% of the annual requirement[3,4,5,6,7,8,9,10,11] This discrepancy has stimulated the ongoing debate about what missed physical mechanisms could close the nutrient budget through additional vertical nutrient supply. With a typical sampling interval at about 6 h, the drifters are able to resolve the submesoscale ageostrophic velocity signals at the sea surface globally; this includes both the unbalanced wave motions[29] and submesoscale balanced ageostrophic motions[27,30]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
