Bottom boundary layers are a critical component of oceanic dynamics, as a means of drag on the overlying flow, as regions of enhanced vertical mixing, and as conduits for cross-isobath transport. In the presence of density stratification and a sloping bottom, cross-isobath Ekman transport in the bottom boundary layer moves water up or down the slope, creating lateral density gradients. These gradients, in turn, through a thermal wind balance, reduce the along-isobath bottom stress and hence the cross-isobath Ekman transport, possibly to zero. This process of "buoyancy arrest" is well understood for an idealized initial value problem with a constant interior flow, but much less is known about the response of a time dependent interior flow (tidal or lower frequencies). The influence of a time dependent flow is unclear because mixing is irreversible and the responses to upslope and downslope flows in the bottom boundary layer are asymmetric. Since oceanic flows are fundamentally time dependent, understanding buoyancy arrest of a time dependent flow is a critical step in applying these ideas to the ocean. Preliminary results show that adding time-dependence may enhance mixing, change the boundary layer structure, modify the cross-isobath transport and generate residual flows. In this study, scientists at Woods Hole Oceanographic Institution plan to examine how buoyancy arrest will operate in a more realistic ocean that includes a wide spectrum of temporal variability. The questions they will address include: How do superimposed tidal currents affect buoyancy arrest? How does the buoyancy arrest response to time-dependent forcing affect boundary layer structure and flow (both mean and fluctuating)? Does buoyancy arrest in an oscillating flow generate a residual mean flow and, if so, under what conditions? To address these questions, they will carry out a hierarchy of one- and two-dimensional numerical experiments. Results from these experiments will be compared to theory and to field observations where possible.
A better understanding of cross-isobath transport and vertical mixing within the bottom boundary layer is relevant to a wide range of interdisciplinary oceanographic processes including coastal upwelling of nutrients, benthic larval transport, and motion of suspended sediments. In the final year of the project, the PIs will host a 2-3 day workshop on bottom boundary layer processes which will broaden the impact of their work.
