The Seasonality of Physically Driven Export at Submesoscales in the Northeast Atlantic Ocean

Abstract

Submesoscale dynamics O(1–100 km) are associated with enhanced vertical velocities and evolve on a time scale similar to that of biological production (hours to days). Here we consider an annual cycle of submesoscale dynamics and their relation to productivity and export in a small (20 × 20 km) region of the northeast Atlantic Ocean. In this region, a springtime bloom is initiated by restratification of the mixed layer in June, although intermittent shoaling of the mixed layer maintains phytoplankton populations throughout the year. An optical community index suggests a dominance of large species (e.g., diatoms) during spring and picophytoplankton during the winter. We review three types of submesoscale instabilities—mixed layer (baroclinic), gravitational, and symmetric—and consider the impact of each on export of fixed carbon out of the surface layer. Mixed layer instabilities can potentially export material out of the mixed layer during winter, although the vertical velocity across the base of the mixed layer is sensitive to the parameterization scheme. Symmetric instabilities, in contrast, provide a clear mechanism for rapid export out of the mixed layer. A crucial factor determining export potential is the strength of the pycnocline at the base of the mixed layer. Export production is sensitive to the degree of overlap that exists between intense submesoscale activity associated with deep mixed layers in the winter and high productivity associated with the spring restratification, meaning that physically driven export of fixed carbon will likely happen over a short time window during spring.