Cold Filament Frontogenesis and Arrest by Ocean Boundary Layer Turbulence
Cold Filament Frontogenesis and Arrest by Ocean Boundary Layer Turbulence
Abstract:
The spatial and temporal state of the upper ocean boundary layer
is determined by a set of poorly understood complex interactions
between submesoscale turbulence, (e.g., fronts, dense filaments and
coherent vortices) and small-scale boundary-layer turbulence. Of
particular interest here is the life-cycle of a cold dense filament
undergoing frontogenesis in the presence of wind-wave generated
Langmuir turbulence. Cold filaments generate secondary circulations
that are frontogenetic with super-exponential sharpening of the
cross-filament buoyancy and horizontal velocity gradients. Within
less than a day, the frontogenesis is arrested at a very small
width, < 100 m, primarily by boundary layer turbulence, with a
subsequent slow decay by further turbulent mixing. This phenomenon
is examined in Large-Eddy Simulations (LESs) with resolved turbulent
motions in large-horizontal domains using 10^9 gridpoints. Winds
and waves are oriented in directions both perpendicular and parallel
to the cold filaments in the LES. The LES solutions show that the
boundary layer turbulence is strikingly inhomogeneous in relation
to the submesoscale filamentary currents and density stratification,
and there is large horizontal transport of cold water at the base
of the mixed layer. Also, the spatial and temporal evolution of
frontogenesis is dependent on the orientation of the winds and
waves, and for some wind-wave combinations the sharp filament
exhibits an unexpected submesoscale lateral shear instability that
facilitates the frontogenetic arrest.
is determined by a set of poorly understood complex interactions
between submesoscale turbulence, (e.g., fronts, dense filaments and
coherent vortices) and small-scale boundary-layer turbulence. Of
particular interest here is the life-cycle of a cold dense filament
undergoing frontogenesis in the presence of wind-wave generated
Langmuir turbulence. Cold filaments generate secondary circulations
that are frontogenetic with super-exponential sharpening of the
cross-filament buoyancy and horizontal velocity gradients. Within
less than a day, the frontogenesis is arrested at a very small
width, < 100 m, primarily by boundary layer turbulence, with a
subsequent slow decay by further turbulent mixing. This phenomenon
is examined in Large-Eddy Simulations (LESs) with resolved turbulent
motions in large-horizontal domains using 10^9 gridpoints. Winds
and waves are oriented in directions both perpendicular and parallel
to the cold filaments in the LES. The LES solutions show that the
boundary layer turbulence is strikingly inhomogeneous in relation
to the submesoscale filamentary currents and density stratification,
and there is large horizontal transport of cold water at the base
of the mixed layer. Also, the spatial and temporal evolution of
frontogenesis is dependent on the orientation of the winds and
waves, and for some wind-wave combinations the sharp filament
exhibits an unexpected submesoscale lateral shear instability that
facilitates the frontogenetic arrest.