Carbon export through submesoscale instabilities: Combining in situ and satellite products

Submesoscale processes are important mediators of carbon export, as they lead to vertical velocities of $O$(100 m/day) in the ocean that are large enough to substantially contribute to the vertical flux of fixed carbon out of the surface ocean. These vertical velocities are caused by transient instabilities that act on spatial scales of less than 10 km, making them difficult to characterize from sparse ship-based measurements. Here we use Seaglider measurements from a small, 15 km x 15 km region of the northeast Atlantic Ocean to identify and classify surface mixed layer conditions that suggest the presence of various hydrodynamic instabilities: gravitational, baroclinic, and symmetric. We use existing parameterizations to estimate vertical velocities near the surface of the ocean and the vertical structure of these motions. We then combine these estimates with Seaglider-based in situ optical measurements to infer carbon export out of the mixed layer due to submesoscale instabilities over a full seasonal cycle. Baroclinic and symmetric instabilities are found to have a substantial impact both on the timing of the springtime restratification and on the carbon export during the spring bloom. Motivated by these observations, we develop an idealized 1D model of carbon export along Lagrangian trajectories that parameterizes the influence of submesoscale subduction. We modify the Price-Weller-Pinkel (PWP) model, which is forced at the surface by reanalysis products and compared against in situ measurements from Argo floats, to include gravitational and baroclinic instabilities estimated using horizontal gradients in remotely-sensed sea surface temperature data. This approach improves upon existing parameterizations that assume a specific vertical structure for submesoscale motions and thus provides an more realistic representation of vertical transport at submesoscales and their impact on carbon export.