Physically modulated phytoplankton production and export at submesoscales in the oligotrophic South China Sea Basin

Oceanic submesoscales can significantly influence phytoplankton production and export owing to their similar timescales of days. Based on two-year Biogeochemical Argo (BGC-Argo) observations, this study investigated the development of submesoscale instabilities, particularly symmetric and mixed-layer baroclinic instabilities, and their impacts on biological production and export in the oligotrophic South China Sea basin. In the northern basin, near-surface winter blooms consistently cooccurred with seasonally deepened mixed layers. However, significantly stronger and weaker winter blooms were observed over two consecutive winters within the BGC-Argo observation period. During the first winter, symmetric-instability-induced upward nutrient entrainment played a crucial role in initiating the strong winter bloom in early December, when the mixed layer was approximately 20–30 m shallower than the nutricline. This bloom occurred approximately 20–30 days earlier than that anticipated owing to the contact between the seasonally deepened mixed layer and mesoscale-cyclone-induced uplifted nutricline. The symmetric instability also facilitated the export of fixed phytoplankton carbon from the surface to deeper layers. Conversely, during the second winter, remarkably intense mixed-layer baroclinic instability associated with an intense mesoscale anticyclone led to more significant shoaling of the mixed layer compared to the nutricline, thus increasing the vertical distance between the two layers. Under this condition, upward nutrient injection, phytoplankton bloom, and carbon export were suppressed. In contrast, the BGC-Argo float in the central basin revealed significantly inhibited seasonality of phytoplankton biomass and submesoscale instabilities compared to those in the northern basin, primarily owing to the significantly shallower winter mixed layer.