Sonya Legg

Recent measurements in a region of continental slope characterized by ridges and valleys running up and down the slope reveal interesting high mode structure in the tidal band velocity signals, with enhanced mixing above the corrugations. In order to understand these observations, numerical simulations of the internal tide generation in this region of topography were performed. Here the focus is on the response of the flow to along;chslope barotropic tidal forcing. For small-amplitude barotropic forcing, internal waves are generated over the continental slope that propagate toward the ocean surface and toward shallower water. When higher-amplitude forcing is combined with large-amplitude corrugations, the flow is locally supercritical downstream of ridges, and transient internal hydraulic jumps result. As the flow relaxes each half tidal period, these jumps are released as internal wave packets, which propagate up into the thermocline. The internal hydraulic jumps are a source of mixing in the va... Abstract Recent measurements in a region of continental slope characterized by ridges and valleys running up and down the slope reveal interesting high mode structure in the tidal band velocity signals, with enhanced mixing above the corrugations. In order to understand these observations, numerical simulations of the internal tide generation in this region of topography were performed. Here the focus is on the response of the flow to along;chslope barotropic tidal forcing. For small-amplitude barotropic forcing, internal waves are generated over the continental slope that propagate toward the ocean surface and toward shallower water. When higher-amplitude forcing is combined with large-amplitude corrugations, the flow is locally supercritical downstream of ridges, and transient internal hydraulic jumps result. As the flow relaxes each half tidal period, these jumps are released as internal wave packets, which propagate up into the thermocline. The internal hydraulic jumps are a source of mixing in the va...

Recent measurements in a region of continental slope characterized by ridges and valleys running up and down the slope reveal interesting high-mode structure in the tidal band velocity signals and enhanced mixing over the corrugations. To deduce the processes responsible for the observed phenomena, numerical simulations of the internal tide generation in this region of topography were performed, focused on the response of the flow to cross-slope barotropic tidal forcing. The flow response is characterized by an internal tide generated at the shelf break that subsequently reflects from the corrugated slope. Above the corrugated slope, a high-mode structure may be created, but only if the Coriolis force is included. It is proposed that interference between the primary internal tide and secondary internal waves forced by the Coriolis-driven along-slope component of primary wave flow field is the cause of the high-mode structure in the simulations. Under suitable conditions of forcing, topography, an... Abstract Recent measurements in a region of continental slope characterized by ridges and valleys running up and down the slope reveal interesting high-mode structure in the tidal band velocity signals and enhanced mixing over the corrugations. To deduce the processes responsible for the observed phenomena, numerical simulations of the internal tide generation in this region of topography were performed, focused on the response of the flow to cross-slope barotropic tidal forcing. The flow response is characterized by an internal tide generated at the shelf break that subsequently reflects from the corrugated slope. Above the corrugated slope, a high-mode structure may be created, but only if the Coriolis force is included. It is proposed that interference between the primary internal tide and secondary internal waves forced by the Coriolis-driven along-slope component of primary wave flow field is the cause of the high-mode structure in the simulations. Under suitable conditions of forcing, topography, an...

Internal wave reflection from a sloping topographic boundary may lead to enhanced shear if the topographic angle to the horizontal is close to that of the internal wave group velocity vector. Previous analytic studies have suggested that shear enhancement is reduced at concave slopes as compared with convex and planar slopes near the critical angle. Here the internal wave reflection from concave and convex slopes that pass through the critical angle is investigated numerically using the nonhydrostatic Massachusetts Institute of Technology General Circulation Model (MITgcm). Overturning, shear instability, and resultant mixing are examined. Results are compared with simulations of wave reflection from planar slopes with angles greater than, less than, and equal to the critical angle. In contrast to the analytic predictions, no reduction in mixing is found for the concave slope as compared with the other slopes. In all cases, stratification is eroded in a band above the slope, bounded at its outer edge by the internal wave characteristic. The difference between numerical and analytic results is caused by the nonlinearity of the numerical calculations, where the finite-amplitude flow leads to generation of upslope-propagating bores for a wide range of topographic slopes around the critical angle.

Internal wave reflection from a sloping topographic boundary may lead to enhanced shear if the topographic angle to the horizontal is close to that of the internal wave group velocity vector. Previous analytic studies have suggested that shear enhancement is reduced at concave slopes as compared with convex and planar slopes near the critical angle. Here the internal wave reflection from concave and convex slopes that pass through the critical angle is investigated numerically using the nonhydrostatic Massachusetts Institute of Technology General Circulation Model (MITgcm). Overturning, shear instability, and resultant mixing are examined. Results are compared with simulations of wave reflection from planar slopes with angles greater than, less than, and equal to the critical angle. In contrast to the analytic predictions, no reduction in mixing is found for the concave slope as compared with the other slopes. In all cases, stratification is eroded in a band above the slope, bounded at its outer ... Abstract Internal wave reflection from a sloping topographic boundary may lead to enhanced shear if the topographic angle to the horizontal is close to that of the internal wave group velocity vector. Previous analytic studies have suggested that shear enhancement is reduced at concave slopes as compared with convex and planar slopes near the critical angle. Here the internal wave reflection from concave and convex slopes that pass through the critical angle is investigated numerically using the nonhydrostatic Massachusetts Institute of Technology General Circulation Model (MITgcm). Overturning, shear instability, and resultant mixing are examined. Results are compared with simulations of wave reflection from planar slopes with angles greater than, less than, and equal to the critical angle. In contrast to the analytic predictions, no reduction in mixing is found for the concave slope as compared with the other slopes. In all cases, stratification is eroded in a band above the slope, bounded at its outer ...

During the recent Labrador Sea Deep Convection Experiment, numerous isobaric floats were deployed. Interpretation of the quasi-Lagrangian measurements from these floats requires an understanding of any biases that may be introduced by the response of the floats to the flow in which they are embedded. To investigate the float measurement biases in convecting flow numerical simulations of isobaric floats in a domain containing several mesoscale eddies have been performed. When a surface heat loss is applied, spatially variable convective mixing and baroclinic instability result. The authors find that without surface cooling, probability density functions of Eulerian and isobaric float measurements of tracers and velocities are very similar, given an initial distribution of isobaric floats that is random with respect to the initial features of the tracer field. However, with cooling isobaric statistics are biased compared to the Eulerian statistics. In particular, in near-surface regions isobaric floats appear to oversample regions of dense downwelling fluid. Since in near-surface layers downwelling dense fluid is associated with convergence, a probable explanation of the isobaric float biases is a tendency for floats to concentrate in regions of horizontal convergence. Also, with cooling floats may be more easily exchanged between eddies and the ambient fluid. The escape of a float from an eddy can be identified from changes in the values of material tracers. The authors identify a positive skewness in the time derivative of the buoyancy measured by the individual near-surface floats as a indicator of convection in the presence of mesoscale eddies.

Ocean convection often occurs in regions of mesoscale eddy activity, where convective mixing and geostrophic eddy dynamics interact. The authors examine the interactions between a group of geostrophic eddies and convective mixing induced by surface buoyancy loss through a series of numerical simulations using a nonhydrostatic Boussinesq model. The eddies are initially baroclinic, with a surface-intensified density anomaly and sheared flow, but they are stable to baroclinic instability because of their small size. In the absence of buoyancy loss, eddy mergers occur much as in previous studies of geostrophic turbulence. With the addition of surface buoyancy loss, the surface stratification is eroded by small-scale convection. The convective mixing is highly heterogeneous, being deeper in regions of weaker initial stratification and shallower in more strongly stratified regions. The deformation radius is reduced in mixing regions and the weakly stratified eddies become baroclinically unstable. The b... Abstract Ocean convection often occurs in regions of mesoscale eddy activity, where convective mixing and geostrophic eddy dynamics interact. The authors examine the interactions between a group of geostrophic eddies and convective mixing induced by surface buoyancy loss through a series of numerical simulations using a nonhydrostatic Boussinesq model. The eddies are initially baroclinic, with a surface-intensified density anomaly and sheared flow, but they are stable to baroclinic instability because of their small size. In the absence of buoyancy loss, eddy mergers occur much as in previous studies of geostrophic turbulence. With the addition of surface buoyancy loss, the surface stratification is eroded by small-scale convection. The convective mixing is highly heterogeneous, being deeper in regions of weaker initial stratification and shallower in more strongly stratified regions. The deformation radius is reduced in mixing regions and the weakly stratified eddies become baroclinically unstable. The b...

In regions of active oceanic convection, such as the Labrador Sea, small- and mesoscale spatial variability is observed in the temperature and salinity fields (T and S). Often T and S structures are “density-compensated,” with the density contribution of the S anomaly nearly equal and opposite to the contribution from the T anomaly;this is manifest as variability in the “spice” field, τ = αT + βS, where α and −β are the local expansion coefficients for T and S. Here the mechanisms for generating T and S variability by convection around a preexisting mesoscale eddy, with particular attention to τ variability, are investigated. The authors perform several numerical experiments with identical density stratification, mesoscale circulation, and surface buoyancy forcing, but with different combinations of T and S in the stratification and surface flux. In all cases with both T and S variations present, it is found that spice variability exceeds that of density. In particular, there are substantial heat... Abstract In regions of active oceanic convection, such as the Labrador Sea, small- and mesoscale spatial variability is observed in the temperature and salinity fields (T and S). Often T and S structures are “density-compensated,” with the density contribution of the S anomaly nearly equal and opposite to the contribution from the T anomaly;this is manifest as variability in the “spice” field, τ = αT + βS, where α and −β are the local expansion coefficients for T and S. Here the mechanisms for generating T and S variability by convection around a preexisting mesoscale eddy, with particular attention to τ variability, are investigated. The authors perform several numerical experiments with identical density stratification, mesoscale circulation, and surface buoyancy forcing, but with different combinations of T and S in the stratification and surface flux. In all cases with both T and S variations present, it is found that spice variability exceeds that of density. In particular, there are substantial heat...

Our long-term objective is to understand how deep convection, induced by strong buoyancy forcing at the ocean surface, influences the ocean circulation through convective plumes and geostrophic eddies.