Submesoscale bio-physical interactions observed with high-resolution airborne remote sensing

Submesocale eddies and fronts are thought to play a key role in biogeochemical processes. Due to the associated strong vertical velocities and horizontal straining, they can alter phytoplankton distribution and growth conditions. As traditional in situ and remote sensing measurements are unable to sample their rapid dynamics (hours to days) and small spatial scales (0.1-10 km), our knowledge about the bio-physical interactions at the submesoscale is mostly derived from numerical models. During the Submesoscale Experiments (SubEx), we used a multi-platform sampling scheme to obtain measurements of submesoscale features in Southern California Bight with a resolution of < 1m. Two cyclonic cold-core eddies (L < 1 km) entrained in a front were observed for several hours with airborne visible and infrared imagery. Optical data show enhanced surface chlorophyll concentrations (CHL) in the eddy cores. Vertical in situ profiles indicate that the temperatures found at the surface of the eddy core occur at the same depth as a subsurface CHL maximum. It is therefore likely that the eddy is formed by upwelling of subsurface waters, while the eddy peripheries seem to be dominated by horizontal advection. A close-by front acts as an advection barrier inducing a convergence region with high surface CHL, due to surface phytoplankton accumulation, but indistinct SST signature. These results exemplify how the three-dimensional circulation at the submesoscales contributes to the spatial heterogeneity of surface phytoplankton distribution.