Source: Journal of Geophysical Research: Oceans
Cross-frontal secondary circulations at ocean fronts are often caused by large scale confluent and diffluent flows near the front. With confluent flows, secondary circulations near the sharpened front lead to subduction on the cold side and upwelling on the warm side of the front. Recent studies show that this upwelling and subduction can be very intense near the edge of surface mixed layer eddies with lateral scales at submesoscale (0.1-10 kilometers). However, their role in primary production has been ambiguous since upwelling and downwelling could have completely opposite influences on the primary production, i.e., upwelling keeps/brings phytoplankton and required nutrients to the sunlit surface layer whereas downwelling sends them down to the dark subsurface layers.
Using ocean Bio-Argo floats and satellite data, Mckee et al.  found that integrated phytoplankton carbon and chlorophyll within the surface mixed layer are increased along the straining front due to confluent flows. Estimated upwelling velocity coincides with the shoaling surface mixed layer and the increase of integrated phytoplankton carbon and chlorophyll, suggesting that restratification of mixed layer eddies increases primary production.
Citation: McKee, D. C., Doney, S. C., Della Penna, A., Boss, E. S., Gaube, P., & Behrenfeld, M. J. (2023). Biophysical dynamics at ocean fronts revealed by Bio-Argo floats. Journal of Geophysical Research: Oceans, 128, e2022JC019226. https://doi.org/10.1029/2022JC019226
—Takeyoshi Nagai, Editor, JGR: Oceans