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Internal waves impinging on sloping topography can generate mixing through the formation of near-bottom bores and overturns in what has been called the “internal swash” zone. Here, we investigate the mixing generated during these breaking events and the subsequent ventilation of the bottom boundary layer across a realistic nondimensional parameter space for the ocean using three-dimensional large-eddy simulations. Waves overturn and break at two points during a wave period: when the downslope velocity is strongest and during the rapid onset of a dense, upslope bore. From the first overturning bore to the expulsion of fluid into the interior, there is a strong dependence on the effective wave height, a length scale defined by the ratio of wave velocity over the background buoyancy frequency, an upper bound on the vertical parcel displacement an internal wave can cause. While a similar energetically motivated vertical length scale is often seen in the context of lee-wave generation over topography, the results discussed here suggest this readily measurable parameter can be used to estimate the size of near-boundary overturns, the strength of the ensuing turbulent mixing, and the vertical scale of the along-isopycnal intrusions of fluid ejected from the boundary layer. Examining a volume budget of the near-boundary region highlights spatial and temporal variability that must be considered when determining the water mass transformation during this process. 

Abstract The Chesapeake Bay is the largest estuary in the continental United States. Extreme temperature events, termed marine heatwaves, are impacting this ecologically important zone with increasing frequency. Although marine heatwaves evolve across space and time, a complete spatial picture of marine heatwaves in the Bay is missing. Here, we use satellite sea surface temperature to characterize marine heatwaves in the Chesapeake Bay. We consider three products: NASA MUR, NOAA Geo-Polar, and Copernicus Marine OSTIA, and validate their effectiveness using in situ data from the Chesapeake Bay Program. We find that Geo-Polar SST is the most suitable dataset for marine heatwave analysis in this location, with a root mean squared error of 1.6$$^\circ $$ $_{}^{\circ }$C. Marine heatwaves occur on average of 2.3 times per year and last 10.8 days per event. A north-south (along estuary) gradient is identified as a common pattern of spatial variability. Seasonally, summer marine heatwaves are shorter, more frequent, and have a more consistent duration, with an inter-quartile range of 6–11 days (median=8 days). December marine heatwaves have a much larger inter-quartile range of 6–28 days (median=13 days). Marine heatwaves are increasing at a rate of 4 events/year in the upper Bay and 2 events/year in the main stem of the lower Bay. Our analysis suggests that the major observed spatial patterns are a result of long-term warming, not shifts in the spread of the temperature distribution. Overall, the qualitative character of marine heatwaves in the Chesapeake Bay is not changing but they are becoming more frequent. 

Abstract Submesoscale processes provide a pathway for energy to transfer from the balanced circulation to turbulent dissipation. One class of submesoscale phenomena that has been shown to be particularly effective at removing energy from the balanced flow is centrifugal–symmetric instabilities (CSIs), which grow via geostrophic shear production. CSIs have been observed to generate significant mixing in both the surface boundary layer and bottom boundary layer flows along bathymetry, where they have been implicated in the mixing and water mass transformation of Antarctic Bottom Water. However, the mixing efficiency (i.e., the fraction of the energy extracted from the flow used to irreversibly mix the fluid) of these instabilities remains uncertain, making estimates of mixing and energy dissipation due to CSI difficult. In this work we use large-eddy simulations to investigate the mixing efficiency of CSIs in the submesoscale range. We find that centrifugally dominated CSIs (i.e., CSI mostly driven by horizontal shear production) tend to have a higher mixing efficiency than symmetrically dominated ones (i.e., driven by vertical shear production). The mixing efficiency associated with CSIs can therefore alternately be significantly higher or significantly lower than the canonical value used by most studies. These results can be understood in light of recent work on stratified turbulence, whereby CSIs control the background state of the flow in which smaller-scale secondary overturning instabilities develop, thus actively modifying the characteristics of mixing by Kelvin–Helmholtz instabilities. Our results also suggest that it may be possible to predict the mixing efficiency with more readily measurable parameters (viz., the Richardson and Rossby numbers), which would allow for parameterization of this effect. 

Abstract Bottom drag is believed to be one of the key mechanisms that remove kinetic energy from the ocean's general circulation. However, large uncertainty still remains in global estimates of bottom drag dissipation. One significant source of uncertainty comes from the velocity structures near the bottom where the combination of sloping topography and stratification can reduce the mean flow magnitude, and thus the bottom drag dissipation. Using high‐resolution numerical simulations, we demonstrate that previous estimates of bottom drag dissipation are biased high because they neglect velocity shear in the bottom boundary layer. The estimated bottom drag dissipation associated with geostrophic flows over the continental slopes is at least 56% smaller compared with prior estimates made using total velocities outside the near‐bottom layer. The diagnostics suggest the necessity of resolving the bottom boundary layer structures in coarse‐resolution ocean models and observations in order to close the global kinetic energy budget.