Direct observations of Submesoscale Subduction

Although recent models and theories predict vertical transport from the surface into the interior at submesoscale fronts, very few direct observations of this process have been made. Furthermore, estimates of the magnitude of the vertical velocity vary widely and are highly dependent on the horizontal scale of the estimate. Here, direct observations of subduction at several submesoscale fronts are summarized. The measurements use Lagrangian floats equipped with ADCPs, measuring the stratification, three-dimensional velocity profile and, in many cases, turbulence levels along the subducting trajectories. Velocities are surprisingly high, often 1 cm/s (1000 m/day), compared to estimates from mesoscale inversions. However, these do not necessarily imply large vertical displacements, as the trajectories often return to the surface. The observations suggest several different mechanisms for subduction often with a significant role for boundary layer turbulence. The simplest lesson is that vertical velocity, like most small-scale oceanographic properties, is complex so that many measurements and a combined statistical and dynamical approach will likely be required for a fuller understanding.