Turbulence Observations Beneath Larsen C Ice Shelf, Antarctica
Turbulence Observations Beneath Larsen C Ice Shelf, Antarctica
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DOI：
10.1029/2019jc015164
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发表时间：
2019-08-01
影响因子：
3.6
通讯作者：
Nicholls, Keith W.
中科院分区：
文献类型：
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作者：
Davis, Peter E. D.;Nicholls, Keith W.
南极冰架下海洋驱动的基底融化增加导致接地冰加速流入海洋，对全球海平面产生影响。通过冰架-海洋边界层的湍流热量传递对于设定基础融化速率至关重要，但控制这种传递的过程人们知之甚少，并且在全球气候模型中没有得到充分体现。这给未来海平面上升的预测带来了很大的不确定性。 2011 年 12 月，使用热水钻孔，在拉森 C 冰架下方 2.5 和 13.5 m 处部署了两个湍流仪器组 (TIC)。这两个仪器返回了长达一年的湍流速度波动记录，为探索冰架-海洋边界层内的湍流过程提供了独特的机会。尽管湍流动能 (TKE) 耗散率和平均流速之间的比例随距冰架底部的距离而变化，但在两个 TIC 处，TKE 耗散率完全由剪切产生速率平衡。基底融化释放的淡水对 TKE 平衡没有影响。当上部TIC位于测井层内时，我们得出冰下阻力系数为0.0022，粗糙度长度为0.44 mm，表明冰基光滑。最后，我们证明，虽然规范的三方程融化速率参数化可以准确地预测这个例子的融化速率，这个例子是由冷潮汐强制边界层覆盖的光滑冰，但参数化所采用的壁假设定律在低流速下并不成立。 简单语言摘要 南极冰架是南极冰盖的浮动延伸。它们通过限制接地冰流入海洋来帮助控制南极洲未来对全球海平面上升的影响。许多冰架越来越多地被相对温暖的海水从下方融化，导致冰从冰盖内部流入海洋的速度加快。然而，控制基底融化速度的小规模湍流过程在全球气候模型中却很少体现，导致未来海平面上升的预测存在很大的不确定性。在这里，我们展示了长达一年的推动南极冰架下基底融化的湍流过程的记录。我们表明，在我们的测量地点，平均流产生的湍流完全通过小规模分子过程消散。基底融化释放的淡水不会显着影响湍流环境。然而，最重要的是，我们直接从我们的观察中证明，全球气候模型中现有的基础融化表示可以准确预测当湍流混合足够强烈以克服分层时的融化速率。由于冰架之间的湍流环境差异很大，因此仍有许多工作要做。
Increased ocean-driven basal melting beneath Antarctic ice shelves causes grounded ice to flow into the ocean at an accelerated rate, with consequences for global sea level. The turbulent transfer of heat through the ice shelf-ocean boundary layer is critical in setting the basal melt rate, yet the processes controlling this transfer are poorly understood and inadequately represented in global climate models. This creates large uncertainties in predictions of future sea level rise. Using a hot-water drilled access hole, two turbulence instrument clusters (TICs) were deployed 2.5 and 13.5 m beneath Larsen C ice shelf in December 2011. Both instruments returned a yearlong record of turbulent velocity fluctuations, providing a unique opportunity to explore the turbulent processes within the ice shelf-ocean boundary layer. Although the scaling between the turbulent kinetic energy (TKE) dissipation rate and mean flow speed varies with distance from the ice shelf base, at both TICs the TKE dissipation rate is balanced entirely by the rate of shear production. The freshwater released by basal melting plays no role in the TKE balance. When the upper TIC is within the log-layer, we derive an under-ice drag coefficient of 0.0022 and a roughness length of 0.44 mm, indicating that the ice base is smooth. Finally, we demonstrate that although the canonical three-equation melt rate parameterization can accurately predict the melt rate for this example of smooth ice underlain by a cold, tidally forced boundary layer, the law of the wall assumption employed by the parameterization does not hold at low flow speeds.Plain Language Summary Antarctic ice shelves are the floating extensions of the Antarctic ice sheet. They help control the future contribution of Antarctica to global sea level rise by restricting the flow of grounded ice into the ocean. Many ice shelves are increasingly being melted from beneath by relatively warm ocean waters, leading to an acceleration in the flow of ice from the ice sheet interior into the ocean. The very small-scale turbulent processes that control the rate of basal melting, however, are poorly represented in global climate models, resulting in large uncertainties in projections of future sea level rise. Here we present a yearlong record of the turbulent processes that drive basal melting beneath an Antarctic ice shelf. We show that at our measurement site the turbulence generated by the mean flow is dissipated entirely by small-scale molecular processes. The freshwater released by basal melting does not significantly influence the turbulent environment. Most importantly, however, we demonstrate directly from our observations that existing representations of basal melting in global climate models can accurately predict the melt rate when turbulent mixing is sufficiently intense to overcome stratification. Much work remains to be done, as the turbulent environment varies widely between ice shelves.