Abstract
The dynamical pathways of subduction, by which water from the oceanic
surface mixed layer makes its way into the pycnocline, are influenced by
both mesoscale (geostrophic) frontogenesis and submesoscale
(ageostrophic and vertical) frontogenesis in the mixed layer. In frontal
zones, subducted water masses that are tens of kilometers in extent can
be identified in the pycnocline for days to months. Here, we explore the
pathways and mechanisms for subduction with only weak surface forcing
using a submesoscale-resolving numerical model of a mesoscale front. We
use particle tracking to identify Lagrangian trajectories that exit the
mixed layer. By identifying the subducting water parcels, we study the
evolution of their dynamical properties from a statistical standpoint.
The velocity and buoyancy gradients increase as water parcels experience
frontogenesis and subduct beneath the mixed layer into the stratified
pycnocline. We find that water parcels subduct within coherent regions
along the front. These coherent subduction regions set the length scales
of the subducted features. As a result, the vertical transport rate of a
tracer has a spectrum that is flatter than the spectrum of vertical
velocity. An examination of specific subduction events reveals a range
of submesoscale features and frontogenesis processes that support
subduction. Contrary to the forced submesoscale processes that sequester
low PV anomalies in the interior, we find that PV can be elevated in
subducting water masses. The rate of subduction that we estimate is of
similar magnitude to previous studies (~100 m per 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.