Local forcing mechanisms challenge parameterizations of ocean thermal forcing for Greenland tidewater glaciers
Local forcing mechanisms challenge parameterizations of ocean thermal forcing for Greenland tidewater glaciers
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DOI：
10.5194/tc-18-911-2024
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发表时间：
2024-02
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影响因子：
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通讯作者：
A. Hager;D. Sutherland;D. Slater
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文献类型：
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作者：
A. Hager;D. Sutherland;D. Slater
抽象的。自1972年以来，锋面消融已造成格陵兰冰盖质量损失的32%-66%，尽管其在驱动终点变化方面的重要性，海洋热强迫仍然粗略地纳入大尺度冰盖模型。在格陵兰岛，当地峡湾尺度的过程改变了冰-海边界的热力强迫的大小，但尺度太小，无法在目前的全球气候模式中解决。例如，CMIP 6冰盖相互比较项目（ISMIP 6）中用于预测未来冰盖变化的模拟依赖于将区域海水特性外推到峡湾中以驱动终点消融。然而，这种方法的准确性以前没有经过测试，由于格陵兰峡湾的观测不足，以及峡湾规模的模型无法真实地纳入冰山。利用马萨诸塞州理工学院大气环流模式（MITgcm）中最近开发的IceBerg软件包，我们在这里评估海洋热力强迫参数化预测冰川末端热力强迫的能力。这是通过敏感性实验，使用一组理想化的格陵兰峡湾，每个被迫与等效的海洋边界条件，但不同的潮汐振幅，冰下流量，冰山覆盖，和测深。研究结果表明，外部水的水深阻碍是近冰川热强迫的主要控制因素，其次是冰山海底融化。尽管海洋边界条件相同，我们发现模拟的峡湾过程可以修改接地线热强迫多达3 °C，其大小在很大程度上是由水深底床的相对深度控制的极地水-大西洋水温跃层。然而，在我们的模拟中，使用峡湾水深测量的普通调整，我们仍然可以预测接地线热强迫在0.2 °C以内。最后，我们介绍了新的参数化，另外占冰山驱动的冷却，可以准确地预测内部峡湾的热强迫配置文件在冰山载货模拟和观测Kangiata Sullua（Ilulissat Icefjord）。
Abstract. Frontal ablation has caused 32 %–66 % of Greenland Ice Sheet mass loss since 1972, and despite its importance in driving terminus change, ocean thermal forcing remains crudely incorporated into large-scale ice sheet models. In Greenland, local fjord-scale processes modify the magnitude of thermal forcing at the ice–ocean boundary but are too small scale to be resolved in current global climate models. For example, simulations used in the Ice Sheet Intercomparison Project for CMIP6 (ISMIP6) to predict future ice sheet change rely on the extrapolation of regional ocean water properties into fjords to drive terminus ablation. However, the accuracy of this approach has not previously been tested due to the scarcity of observations in Greenland fjords, as well as the inability of fjord-scale models to realistically incorporate icebergs. By employing the recently developed IceBerg package within the Massachusetts Institute of Technology general circulation model (MITgcm), we here evaluate the ability of ocean thermal forcing parameterizations to predict thermal forcing at tidewater glacier termini. This is accomplished through sensitivity experiments using a set of idealized Greenland fjords, each forced with equivalent ocean boundary conditions but with varying tidal amplitudes, subglacial discharge, iceberg coverage, and bathymetry. Our results indicate that the bathymetric obstruction of external water is the primary control on near-glacier thermal forcing, followed by iceberg submarine melting. Despite identical ocean boundary conditions, we find that the simulated fjord processes can modify grounding line thermal forcing by as much as 3 °C, the magnitude of which is largely controlled by the relative depth of bathymetric sills to the Polar Water–Atlantic Water thermocline. However, using a common adjustment for fjord bathymetry we can still predict grounding line thermal forcing within 0.2 °C in our simulations. Finally, we introduce new parameterizations that additionally account for iceberg-driven cooling that can accurately predict interior fjord thermal forcing profiles both in iceberg-laden simulations and in observations from Kangiata Sullua (Ilulissat Icefjord).