Estimating Kinematic Quantities of Submesoscale Structures Along Mesoscale Fronts Using Large Drifter Data Sets

Much of the vertical velocity near the surface of the ocean is thought to be associated with ageostrophic submesoscale phenomena. However, most in-situ observations of submesoscales are made near coastal outflows, while far less is known about how mesoscales and submesoscales interact. The vertical exchange of water associated with these ageostrophic flows plays a critical role in the transport of dissolved nutrients and gases, thus the production and fate of organic matter. Vertical velocities are challenging not only to model accurately, but also to measure because they are very hard to locate in the surface of the ocean. Using unique massive drifter releases during the LAgrangian Submesoscale ExpeRiment (LASER) in the Gulf of Mexico, and two CALYPSO experiments in the Mediterranean Sea, we observe the generation of submesoscale structures along mesoscale fronts separating differing water masses. In order to quantitatively describe the flow sampled by these drifter data sets, we use a novel method to project Lagrangian trajectories to Eulerian velocity fields. This interpolation method uses a squared-exponential covariance function, which characterizes velocity correlations in two-dimensional horizontal space and time, in such a way that the scales of variation are determined by the data itself. From the interpolated velocity fields, we then calculate kinematic properties, including horizontal divergence and vertical relative vorticity, in order to infer the vertical transport occurring within these submesoscale structures. We also compare the drifter data to available SST fields from satellite data and CTD measurements recorded across these strong fronts using a moving vessel profiler. Within the submesoscales located and tracked by drifters, we find divergence and relative vorticity magnitudes to be on the order of 5f (with f being the planetary vorticity).