Goal 3: Understand the relation between the horizontal and vertical velocity measured by remote sensing at the surface and that within and just below the surface boundary layer

How does this goal help address the hypothesis?

Existing models and observations indicate the potential for substantial differences between the velocities measured by DopplerScatt, sampling the top few centimeters, and the boundary layer currents in the top tens of meters that feed submesoscale and mesoscale vertical velocity. These differences are due both to the near surface shear and to the average velocity gradients due to surface waves, i.e. the Stokes drift shear. Thus, interpretation of the DopplerScatt velocity measurements requires that we understand their relationship to deeper velocities.

What needs to be done to achieve this goal and why?

The centimeter-depth DopplerScatt measurements will be compared with in situ measurements of vertical and horizontal velocity at multiple depths to build a transfer function from the remote sensing measurements to the interior. Away from fronts, the differences should depend primarily on the surface wave properties, the wind stress and near-surface density stratification, with smaller effects due to internal waves. Within submesoscale fronts, additional strong near surface shears will occur, associated both with the geostrophic shear of the front and its likely strong vertical density gradient.

For horizontal velocities, surface drifters will measure average horizontal velocity in the top 60 cm. ADCP’s from Wave Gliders, Saildrones and the research vessel will measure horizontal velocities from depths of approximately 3m to at least 50m. For vertical velocities, estimates from DopplerScatt (see Science Goal 4) will be compared with direct measurements by subsurface Lagrangian floats.