Institute of Marine and Antarctic Studies, University of Tasmania, Hobart, TAS 7004, Australia;
Department of Mechanical Engineering, University of Melbourne, Melbourne, VIC 3010, Australia.
Proc Natl Acad Sci U S A. 2021 Feb 9;118(6). doi: 10.1073/pnas.2007541118.
The Antarctic Ice Sheet loses about half its mass through ocean-driven melting of its fringing ice shelves. However, the ocean processes governing ice shelf melting are not well understood, contributing to uncertainty in projections of Antarctica's contribution to global sea level. We use high-resolution large-eddy simulation to examine ocean-driven melt, in a geophysical-scale model of the turbulent ice shelf-ocean boundary layer, focusing on the ocean conditions observed beneath the Ross Ice Shelf. We quantify the role of double-diffusive convection in determining ice shelf melt rates and oceanic mixed layer properties in relatively warm and low-velocity cavity environments. We demonstrate that double-diffusive convection is the first-order process controlling the melt rate and mixed layer evolution at these flow conditions, even more important than vertical shear due to a mean flow, and is responsible for the step-like temperature and salinity structure, or thermohaline staircase, observed beneath the ice. A robust feature of the multiday simulations is a growing saline diffusive sublayer that drives a time-dependent melt rate. This melt rate is lower than current ice-ocean parameterizations, which consider only shear-controlled turbulent melting, would predict. Our main finding is that double-diffusive convection is an important process beneath ice shelves, yet is currently neglected in ocean-climate models.
南极冰盖通过其边缘冰架的海洋驱动融化失去约一半的质量。然而，控制冰架融化的海洋过程尚未得到很好的理解，这导致对南极洲对全球海平面贡献的预测存在不确定性。我们使用高分辨率的大涡模拟，在一个考虑了罗斯冰架-海洋边界层的地球物理尺度的模型中，研究海洋驱动的融化，重点关注在罗斯冰架下观察到的海洋条件。我们量化了双扩散对流在确定冰架融化率和海洋混合层特性方面的作用，这些特性适用于相对温暖和低速度的空腔环境。我们证明，在这些流动条件下，双扩散对流是控制融化率和混合层演化的一级过程，甚至比由于平均流引起的垂直剪切更为重要，并且是导致在冰下观察到的阶梯状温度和盐度结构或热盐级联的原因。多日模拟的一个显著特征是不断增长的盐扩散亚层，它驱动着随时间变化的融化率。与仅考虑剪切控制的湍流融化的当前冰-海洋参数化相比，这个融化率更低。我们的主要发现是，双扩散对流是冰架下的一个重要过程，但目前在海洋气候模型中被忽略了。
