Simulating UNder ice Shelf Extreme Topography (SUNSET)

全球平均海平面正在以越来越快的速度上升。鉴于这一变化的巨大经济和社会影响，迫切需要准确预测海平面，以便为考虑缓解和适应战略的政策制定者提供信息。南极和格陵兰冰盖的融化目前约占海平面上升的三分之一。这种融化的未来非常不确定，最糟糕的情况是冰盖对危险的海平面上升有很大的贡献。冰盖对海平面的最大贡献发生在海洋融化冰架底部(地面冰盖的浮动延伸部分)时，增加了地面冰流入海洋的流量。冰架的融化速度取决于热量从海洋向冰层的转移。最近，在快速融化的冰架底部，发现了包括阶梯状梯田和1-10公里宽的水道在内的极端地形特征。这些特征显著改变了融化的模式和速度，因此对预测海平面至关重要。然而，这些特征通常太小，无法在气候模型中解决，必须了解它们的影响，并明确地将其纳入这些模型。我们将研究冰架底部的极端地形如何改变洋流和融化。在冰架下，冰架的底部是平坦的，逐渐倾斜的，融化可以被视为一个垂直的过程，这就是目前气候模型中所表示的。然而，观察表明，极端冰川地形陡峭斜坡上的融化实际上是水平的，而且比光滑的冰基融化快得多。此外，极端地形特征产生的湍流海洋涡流会将温暖的水混合到冰面上，进一步促进融化。我们将观察南极冰架下极端冰层地形的影响，使用加压热水钻穿超过一公里的冰，使其能够进入下面的海洋洞穴。我们将使用一套有针对性的最新观测测量来研究对融化的控制：跟踪冰地形和融化的雷达和声纳，以及声学洋流剖面图和一系列温度传感器，以监测海洋热量向冰层混合的情况。这将为海洋混合和冰川融化之间的密切相互作用提供一个独特的数据集。然后，我们将把这些观测与计算机模拟的层次结构结合起来，开发出极端冰川地形在气候模型中影响的新表示。我们将首先使用高分辨率大涡模拟来模拟极端地形地物周围的流动，其中解决了海洋湍流。这将提供对温水与冰基的混合以及它与融化的相互作用的洞察。然后，我们将使用海洋模型来研究冰道几何形状对融化速度和流动特性的影响。利用这些模拟，我们将开发出数学公式来表示极端冰川地形的影响。我们将把这些公式应用到海洋模式中，并测试其在气候模式中表示极端冰川地形影响的能力。

Global average sea level is rising at an ever-accelerating rate. Given the huge economic and societal impacts of this change, accurate forecasts of sea level are urgently needed to inform policymakers considering mitigation and adaptation strategies. Melting of the Antarctic and Greenland ice sheets currently contributes about one third of sea-level rise. The future of this melting is highly uncertain, and the worst-case scenario involves a substantial ice-sheet contribution to dangerous sea-level rise. The largest ice-sheet contribution to sea level occurs when the ocean melts the base of ice shelves (floating extensions of the grounded ice sheet), increasing the flow of grounded ice into the ocean. The melt rate of ice shelves is determined by the transfer of heat from the ocean towards the ice. Recently, extreme topographic features, including step-like terraces and 1-10 km wide channels, have been discovered to be ubiquitous on the underside of rapidly melting ice shelves. These features significantly modify the patterns and rates of melting, and so are crucial to predicting sea level. However, such features are generally too small to be resolved in climate models and their effect must be understood and explicitly built into these models. We will investigate how extreme topography on the underside of ice shelves changes ocean currents and melting. Beneath ice shelves with a smooth, gradually sloping base, melting can be viewed as a vertical process, and this is how it is currently represented in climate models. However, observations show that melting on the steeply sloping sides of extreme ice topography is actually horizontal, and much faster than the melting of a smooth ice base. In addition, turbulent ocean eddies generated by extreme topographic features will mix warm water up towards the ice, further enhancing the melting. We will observe the influence of extreme ice topography beneath an Antarctic ice shelf using pressurised hot water to drill through more than a kilometre of ice, enabling access to the ocean cavity beneath. We will study the controls on melting using a targeted suite of the latest observational measurements: radar and sonar to track the ice topography and melting, and acoustic ocean current profiling and a string of temperature sensors to monitor the mixing of ocean heat towards the ice. This will provide a unique dataset of the close interaction between ocean mixing and ice melting.We will then combine these observations with a hierarchy of computer simulations to develop a new representation of the effect of extreme ice topography in climate models. We will first simulate the flow around extreme topographic features using high-resolution large-eddy simulations, which resolve the ocean turbulence. This will provide insight into the mixing of warm water to the ice base, and its interaction with melting. We will then use an ocean model to study the role of ice channel geometry on the melt rate and flow properties. Using these simulations, we will develop mathematical formulae to represent the influence of extreme ice topography. We will implement these formulae into the ocean model and test its ability to represent the influence of extreme ice topography in climate models.