Abstract
In frontal zones, water masses that are tens of kilometers in extent
with origins in the mixed layer can be identified in the pycnocline for
days to months. Here, we explore the pathways and mechanisms of
subduction, the process by which water from the surface mixed layer
makes its way into the pycnocline, using a submesoscale-resolving
numerical model of a mesoscale front. By identifying Lagrangian
trajectories of water parcels that exit the mixed layer, we study the
evolution of dynamical properties from a statistical standpoint.
Velocity and buoyancy gradients increase as water parcels experience
both mesoscale (geostrophic) and submesoscale (ageostrophic)
frontogenesis and subduct beneath the mixed layer into the stratified
pycnocline along isopycnals that outcrop in the mixed layer. Subduction
is transient and occurs in coherent regions along the front, the spatial
and temporal scales of which set the scales of the subducted water
masses in the pycnocline. As a result, the tracer-derived vertical
transport rate spectrum is flatter than the vertical velocity spectrum.
An examination of specific subduction events reveals a range of
submesoscale features that support subduction. Contrary to the forced
submesoscale processes that sequester low potential vorticity (PV)
anomalies in the interior, we find that PV can be elevated in subducting
water masses. The rate of subduction is of similar magnitude to previous
studies (~100 m/year), but the pathways that are
unraveled in this study along with the Lagrangian evolution of
properties on water parcels, emphasize the role of submesoscale dynamics
coupled with mesoscale frontogenesis.