Wind‐induced motion in the vicinity of a bottom density front: Response to forcing frequency

Abstract

A numerical model is used to examine the influence of nonlinear processes in the vicinity of a shallow sea bottom density front upon the depth of penetration of the wind's momentum at both superinertial and subinertial frequencies. Calculations show that nonlinear effects associated with differences in vorticity across the frontal jet significantly influence the depth of penetration of the wind's momentum and internal wave propagation. Differences in wind‐induced current amplitude and phase at depth due to nonlinear effects give rise to substantial vertical velocity in the frontal region, which drives near‐bed currents and forces internal waves. At superinertial frequencies, internal waves propagate away from the positive vorticity side of the front. However, at depth, internal waves are generated and current amplitude increases in the region of sloping density surfaces associated with the front. On the negative vorticity side, due to an effective inertial frequency below inertial, the wind's energy can propagate deeper into the water column. Wind forcing at subinertial frequencies does not lead to internal wave propagation. On the positive vorticity side of the front the wind's momentum cannot diffuse to depth due to turbulence suppression by stable stratification. The ability of the wind's momentum to penetrate to a greater depth in a frontal region through nonlinear effects suggests that these may be important processes influencing wind mixing in frontal regions.

On the Loss of Wind-Induced Near-Inertial Energy to Turbulent Mixing in the Upper Ocean

“…We use here forcing starting on 1 January 2001. Note that, even though our wind forcing has relatively high spatial resolution, the use of daily wind stress is likely to underestimate the wind near-inertial energy input to the ocean (e.g., Niwa and Hibiya 1999;Davies and Xing 2004;Furuichi et al 2008). Nevertheless, the overall pattern and magnitude of near-inertial energy input in our model is consistent with those obtained in the previous studies (see section 4a).…”

On the Loss of Wind-Induced Near-Inertial Energy to Turbulent Mixing in the Upper Ocean

“…We use here forcing starting on 1 January 2001. Note that, even though our wind forcing has relatively high spatial resolution, the use of daily wind stress is likely to underestimate the wind near-inertial energy input to the ocean (e.g., Niwa and Hibiya 1999;Davies and Xing 2004;Furuichi et al 2008). Nevertheless, the overall pattern and magnitude of near-inertial energy input in our model is consistent with those obtained in the previous studies (see section 4a).…”

Influence of a cold water bottom dome on internal wave trapping

“…The distribution in a “short dome”, L comparable to λ is dominated by frontal effects. At the dome's edges, horizontally propagating internal waves are reflected by the bottom fronts, and vertically propagating internal waves are trapped by the sloping isotherms [ Davies and Xing , 2004]. In the case of sub‐inertial forcing internal waves are trapped in the generation region, namely the frontal regions associated with the dome.…”

Observations of near-inertial surface currents off Oregon: Decorrelation time and length scales

“…Near‐inertial internal motions in the coastal regions can be generated by spatially uniform and periodic wind stress, associated with shelf circulation, as well as translating wind fronts [e.g., Gill , ; Crawford and Large , ; Davies and Xing , ]. In contrast to the behavior of inertial energy in the open ocean, subsurface near‐inertial oscillations are expected to be amplified offshore and decay in coastal regions due to “coastal inhibition”, the downward energy flux from the surface‐coast corner, that is, upward and offshore phase propagations [e.g., Kundu , ; Millot and Crepon , ; Pettigrew , ; Flagg , ; Kundu et al ., ; Kundu , ].…”

On the Loss of Wind-Induced Near-Inertial Energy to Turbulent Mixing in the Upper Ocean

“…We use here forcing starting on 1 January 2001. Note that, even though our wind forcing has relatively high spatial resolution, the use of daily wind stress is likely to underestimate the wind near-inertial energy input to the ocean (e.g., Niwa and Hibiya 1999;Davies and Xing 2004;Furuichi et al 2008). Nevertheless, the overall pattern and magnitude of near-inertial energy input in our model is consistent with those obtained in the previous studies (see section 4a).…”

Influence of a cold water bottom dome on internal wave trapping

“…The distribution in a “short dome”, L comparable to λ is dominated by frontal effects. At the dome's edges, horizontally propagating internal waves are reflected by the bottom fronts, and vertically propagating internal waves are trapped by the sloping isotherms [ Davies and Xing , 2004]. In the case of sub‐inertial forcing internal waves are trapped in the generation region, namely the frontal regions associated with the dome.…”

Observations of near-inertial surface currents off Oregon: Decorrelation time and length scales

“…Near‐inertial internal motions in the coastal regions can be generated by spatially uniform and periodic wind stress, associated with shelf circulation, as well as translating wind fronts [e.g., Gill , ; Crawford and Large , ; Davies and Xing , ]. In contrast to the behavior of inertial energy in the open ocean, subsurface near‐inertial oscillations are expected to be amplified offshore and decay in coastal regions due to “coastal inhibition”, the downward energy flux from the surface‐coast corner, that is, upward and offshore phase propagations [e.g., Kundu , ; Millot and Crepon , ; Pettigrew , ; Flagg , ; Kundu et al ., ; Kundu , ].…”

On the Loss of Wind-Induced Near-Inertial Energy to Turbulent Mixing in the Upper Ocean

“…We use here forcing starting on 1 January 2001. Note that, even though our wind forcing has relatively high spatial resolution, the use of daily wind stress is likely to underestimate the wind near-inertial energy input to the ocean (e.g., Niwa and Hibiya 1999;Davies and Xing 2004;Furuichi et al 2008). Nevertheless, the overall pattern and magnitude of near-inertial energy input in our model is consistent with those obtained in the previous studies (see section 4a).…”

Influence of a cold water bottom dome on internal wave trapping

“…The distribution in a “short dome”, L comparable to λ is dominated by frontal effects. At the dome's edges, horizontally propagating internal waves are reflected by the bottom fronts, and vertically propagating internal waves are trapped by the sloping isotherms [ Davies and Xing , 2004]. In the case of sub‐inertial forcing internal waves are trapped in the generation region, namely the frontal regions associated with the dome.…”

Observations of near-inertial surface currents off Oregon: Decorrelation time and length scales

“…Near‐inertial internal motions in the coastal regions can be generated by spatially uniform and periodic wind stress, associated with shelf circulation, as well as translating wind fronts [e.g., Gill , ; Crawford and Large , ; Davies and Xing , ]. In contrast to the behavior of inertial energy in the open ocean, subsurface near‐inertial oscillations are expected to be amplified offshore and decay in coastal regions due to “coastal inhibition”, the downward energy flux from the surface‐coast corner, that is, upward and offshore phase propagations [e.g., Kundu , ; Millot and Crepon , ; Pettigrew , ; Flagg , ; Kundu et al ., ; Kundu , ].…”