Buoyancy Arrest: Time Dependent and Residual Flows

底部边界层是海洋动力学的一个关键组成部分，作为上覆流的阻力手段，作为增强垂直混合的区域，以及作为跨等深线运输的管道。在存在密度分层和倾斜底部的情况下，底部边界层的跨等深线埃克曼输送将水向上或向下移动，形成横向密度梯度。这些梯度，反过来，通过热风平衡，减少沿等深线的底部应力，从而减少跨等深线的Ekman输运，可能为零。对于具有恒定内部流的理想初始值问题，这种“浮力抑制”过程被很好地理解，但对于随时间变化的内部流（潮汐或更低频率）的响应则知之甚少。时间相关流动的影响尚不清楚，因为混合是不可逆的，并且底部边界层上、下坡流动的响应是不对称的。由于海洋流动基本上是时间相关的，理解时间相关流动的浮力抑制是将这些想法应用于海洋的关键一步。初步结果表明，加入时间依赖性可以增强混合，改变边界层结构，改变跨等深线输送并产生残余流动。在这项研究中，伍兹霍尔海洋研究所的科学家们计划研究浮力抑制如何在一个更现实的海洋中起作用，其中包括广泛的时间变化。他们将解决的问题包括：叠加的潮汐流如何影响浮力抑制？浮力抑制对时间依赖性强迫的响应如何影响边界层结构和流动（平均和波动）？振荡流中的浮力停止是否会产生残余平均流量，如果是，在什么条件下？为了解决这些问题，他们将进行一系列的一维和二维数值实验。在可能的情况下，这些实验的结果将与理论和实地观察结果进行比较。更好地了解跨等深线运输和底部边界层内的垂直混合与广泛的跨学科海洋学过程有关，包括营养物质的沿海上涌、底栖幼虫运输和悬浮沉积物的运动。在项目的最后一年，pi将举办为期2-3天的关于底层边界层过程的研讨会，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.