The oceanic boundary layer beneath ice shelves

南极冰架是南极冰盖的一部分，漂浮在南极洲海岸。冰架限制了冰从内陆流入海洋，因此控制了南极冰盖对全球海平面的影响。卫星数据显示，冰盖的一部分正在缩小，这表明流入海洋的冰量增加。南极洲这一部分的大陆架被相对温暖的海水淹没，导致冰架底部的融化速度很高。最近的模拟工作强烈地暗示，涌入大陆架的暖水的数量与风的条件密切相关。我们需要能够预测这些冰架对不断变化的海洋条件的反应，以便预测南极洲对海平面变化的贡献将如何受到未来气候变化的影响。这些预测最终将使用海洋的数值模型，包括浮动冰架下面的空腔。水在洞穴中循环的关键驱动力是当冰融化时在冰架底部释放出有浮力的融水。因此，关键的过程是热量从海洋通过冰-海洋边界层到达冰底的过程，也就是说，大约10米厚的水层，受到冰底存在的影响。冰架下的边界层是独特的：在某些重要方面与海冰不同。快速融化的冰架下的边界层的物理学特别是知之甚少，也没有充分的数值模型表示。其中一个问题是，融化本身增加了倾斜冰架底部的浮力流动，增加的速度增加了向冰的热量传递。与此同时，冰底附近浮力的增加使得密度更大的温水更难通过边界层被提升。竞争效应之间微妙的相互作用导致了一个复杂但迷人的地球物理问题。该项目的目的是钻一个进入孔，通过快速融化的冰架和缓慢融化的冰架，并在边界层进行测量，这将使我们能够改善它们在模型中的表现方式。冰层厚度约为350米，仪器经过专门设计，能够通过25厘米的钻孔工作。测量将非常详细，使我们能够探测到直径小到毫米的湍流漩涡。仪器将悬挂在冰架下，这样他们就可以监测边界层的流动速度，温度和基底融化的速度至少一年。一部分数据将被传输到英国。使用卫星数据链路，这样我们就不需要等到下一个野外季节从数据记录仪中恢复完整的数据集。这些数据将首次用于提供快速融化的冰架下边界层的全面视图，与缓慢融化的对应物进行对比，为我们理解独特环境的物理学提供了一个进步。这些数据将用于计算通过边界层的垂直热传输，并且将首次使用观测数据对模型中计算热传输的不同方法进行测试和校准。该项目的另一个组成部分是与一位对海洋湍流进行直接数值模拟（DNS）的同事合作。大多数模型需要粗略地近似湍流的影响，但DNS方法可以直接计算它们。结合现场观测与这种建模方法有效地使我们能够扩展的条件范围内，本模型可以进行测试。

Antarctic ice shelves are the part of the Antarctic ice sheet that goes afloat at the coast of Antarctica. The ice shelves act as a restraint on the flow of ice from the interior into the ocean, and as such act as a control on the Antarctic ice sheet's contribution to global sea level. Satellite data have shown one part of the ice sheet to be reducing in size, indicating increased ice flow into the ocean. The continental shelf in that sector of Antarctica is flooded with relatively warm water, resulting in high melt rates at the base of the ice shelves. Recent modelling work has strongly hinted that the amount of warm water flooding on to the continental shelf is closely related to the wind conditions. We need to be able to predict the response of these ice shelves to the changing ocean conditions in order to predict how the Antarctica's contribution to sea level change will be affected by possible future changes in climate. These predictions will ultimately be made using numerical models of the ocean that include the cavities beneath the floating ice shelves. The key driver for the circulation of water in the cavities is the release of buoyant meltwater at the base of the ice shelves as the ice melts. So the crucial process is the one by which the heat gets from the ocean up to the ice base through the ice-ocean boundary layer, that is, the layer of water, some 10's of metres thick that is affected by the presence of the ice base. The boundary layer beneath an ice shelf is unique: different to sea ice in some important respects. The physics of the boundary layer beneath rapidly melting ice shelves is particularly poorly understood, and also inadequately represented in numerical models. One of the problems is that the melting itself increases the buoyant flow up inclined the ice shelf base, and the increased speed increases the transfer of heat towards the ice. At the same time, the increased buoyancy near the ice base makes it more difficult for the denser, warm water to be lifted through the boundary layer. The subtle interplay between competing effects results in a complicated, but fascinating, geophysical problem. The aim of this project is to drill an access hole through a rapidly melting ice shelf and a slowly melting ice shelf and make measurements in the boundary layers that will enable us to improve the way they are represented in models. The ice will be around 350 m thick, and the instruments specially designed to be able to work through a 25-cm borehole. The measurements will be very detailed, enabling us to detect turbulent eddies right down to millimetres in diameter. Instruments will be left suspended beneath the ice shelf so that they can monitor the speed of flow of the boundary layer, its temperature, and the rate of basal melting for at least one year. A subset of the data will be transmitted to the U.K. using a satellite data link so that we don't need to wait until the full dataset is recovered from the data loggers during the following field season. The data will be used to provide, for the first time, a comprehensive view of the boundary layer beneath a rapidly melting ice shelf, to be contrasted with the slowly melting counterpart, providing a step forward in our understanding of the physics of a unique environment. The data will be used to calculate the vertical heat transport through the boundary layers, and, for the first time, the different ways in which the heat transport is calculated in models will be tested and calibrated using observations. An additional component of the project is to collaborate with a colleague who makes direct numerical simulations (DNS) of oceanic turbulence. Most models need to make crude approximations of the effects of turbulence, but DNS methods can calculate them directly. Combining field observations with this modelling approach effectively allows us to extend the range of conditions over which the present models can be tested.