Buoyancy Arrest: Time Dependent and Residual Flows

底边界层是海洋动力学的一个关键组成部分，是上覆流的一种拖曳手段，是增强垂直混合的区域，也是跨等深线输运的通道。在存在密度分层和倾斜底部的情况下，底边界层中的跨等深线埃克曼输运使水沿倾斜向上或向下移动，造成横向密度梯度。这些梯度，反过来，通过热风平衡，减少沿等深线的海底应力，从而减少跨等深线的埃克曼运输，可能为零。这个过程的“浮力逮捕”是一个理想化的初始值问题，一个恒定的内部流，但少得多isknown的响应时间相关的内部流（潮汐或较低的频率）。由于混合是不可逆的，且底边界层中斜坡流和下坡流的响应是不对称的，因此随时间变化的流动的影响还不清楚。由于海洋流动基本上是随时间变化的，理解随时间变化的流动的浮力停止是将这些想法应用于海洋的关键一步。初步结果表明，增加随时间变化的流动可能会增强混合，改变边界层结构，改变跨等深线输运并产生剩余流。在这项研究中，伍兹霍尔海洋研究所的科学家计划研究浮力制动如何在更现实的海洋中运作，包括广泛的时间变化。他们将解决的问题包括：如何叠加潮流影响浮力逮捕？浮力制动对随时间变化的强迫的响应如何影响边界层结构和流动（平均和波动）？在振荡流中浮力停止是否产生剩余平均流，如果是，在什么条件下？为了解决这些问题，他们将进行一维和二维数值实验的层次结构。这些实验的结果将与理论进行比较，并在可能的情况下与实地观察结果进行比较。更好地了解海底边界层内的跨等深线输送和垂直混合与广泛的跨学科海洋学过程有关，包括沿海营养物上涌、底栖幼虫输送和悬浮沉积物运动。在该项目的最后一年，PI将举办为期2-3天的底边界层过程研讨会，这将扩大他们工作的影响。

OCE-0647050Bottom boundary layers are a critical component of oceanic dynamics, as a means of drag on theoverlying 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 thebottom boundary layer moves water up or down the slope, creating lateral density gradients. Thesegradients, in turn, through a thermal wind balance, reduce the along-isobath bottom stress and hencethe cross-isobath Ekman transport, possibly to zero. This process of "buoyancy arrest" is wellunderstood for an idealized initial value problem with a constant interior flow, but much less isknown about the response of a time dependent interior flow (tidal or lower frequencies). Theinfluence of a time dependent flow is unclear because mixing is irreversible and the responses toupslope and downslope flows in the bottom boundary layer are asymmetric. Since oceanic flows arefundamentally time dependent, understanding buoyancy arrest of a time dependent flow is a criticalstep 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 thebottom 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.