Subglacial Discharge Reflux and Buoyancy Forcing Drive Seasonality in a Silled Glacial Fjord
Subglacial Discharge Reflux and Buoyancy Forcing Drive Seasonality in a Silled Glacial Fjord
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DOI：
10.1029/2021jc018355
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发表时间：
2022-05-01
影响因子：
3.6
通讯作者：
Nash, Jonathan D.
中科院分区：
文献类型：
--
作者：
Hager, Alexander O.;Sutherland, David A.;Nash, Jonathan D.
峡湾是潮水冰川和沿海海洋之间进行热量和质量交换的管道，因此调节近冰川水性能和冰川海底融化。陷入冰川下排放羽流是季节性冰川峡湾循环的主要驱动力。然而，通过冰川淡水的凹痕驱动的混合和回收或回流，流出的羽流可能会继续影响中性浮力后，可能会继续影响循环。尽管它在非冰川峡湾中的重要性，但淡水回流如何影响冰川峡湾的循环，在那里也存在强大的浮力强迫。在这里，我们将整个在2016 - 2017年在阿拉斯加Leconte湾进行测量的水文观察套件与峡湾的三维数值模型，以量化冰川淡水的SILL驱动的回流，并确定其对冰川峡湾循环的影响。当与冰下排放羽流驱动的浮力强迫配对时，sill的混合驱动器会驱动不同的季节性循环方案，这些季节性循环方案在将热量运输到冰川末端的能力上有很大差异。在夏季，在峡湾的浅门门槛上回流了53％-72％的表面流出，随后将其重新入口到峡湾头部的冰川下排放羽流中。结果，近端水性质受到入口处的混合的很大影响，并改变了循环以在200 m深度下向冰川接地线绘制温暖的，改良的外地水。在淡水回流最少的冬季，这种循环细胞不存在。在阿拉斯加东南部，巴塔哥尼亚，格陵兰岛和其他地方的其他冰川峡湾也可能存在类似的季节性行为。
Fjords are conduits for heat and mass exchange between tidewater glaciers and the coastal ocean, and thus regulate near-glacier water properties and submarine melting of glaciers. Entrainment into subglacial discharge plumes is a primary driver of seasonal glacial fjord circulation; however, outflowing plumes may continue to influence circulation after reaching neutral buoyancy through the sill-driven mixing and recycling, or reflux, of glacial freshwater. Despite its importance in non-glacial fjords, no framework exists for how freshwater reflux may affect circulation in glacial fjords, where strong buoyancy forcing is also present. Here, we pair a suite of hydrographic observations measured throughout 2016-2017 in LeConte Bay, Alaska, with a three-dimensional numerical model of the fjord to quantify sill-driven reflux of glacial freshwater, and determine its influence on glacial fjord circulation. When paired with subglacial discharge plume-driven buoyancy forcing, sill-generated mixing drives distinct seasonal circulation regimes that differ greatly in their ability to transport heat to the glacier terminus. During the summer, 53%-72% of the surface outflow is refluxed at the fjord's shallow entrance sill and is subsequently re-entrained into the subglacial discharge plume at the fjord head. As a result, near-terminus water properties are heavily influenced by mixing at the entrance sill, and circulation is altered to draw warm, modified external surface water to the glacier grounding line at 200 m depth. This circulatory cell does not exist in the winter when freshwater reflux is minimal. Similar seasonal behavior may exist at other glacial fjords throughout Southeast Alaska, Patagonia, Greenland, and elsewhere.