Frazil ice growth and production during katabatic wind events in the Ross Sea, Antarctica
Frazil ice growth and production during katabatic wind events in the Ross Sea, Antarctica
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DOI：
10.5194/tc-14-3329-2020
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发表时间：
2020-10-06
期刊：
影响因子：
5.2
通讯作者：
Loose, Brice
中科院分区：
文献类型：
--
作者：
Thompson, Lisa;Smith, Madison;Loose, Brice
沿海冰间湖的下降风使海洋面临极大的热量散失，导致整个秋季和冬季南极洲周围海冰大量生成且形成高密度海水。不断推进的海冰群，加上强风与低温，限制了冬季对冰间湖海洋表面的观测，进而阻碍了人们在南极黑暗的月份里对这些“造冰工厂”演变的新认知。在此，我们阐述了2017年5月在特拉诺瓦湾和罗斯海冰间湖多次下降风事件期间的海洋观测情况。风速经常超过20米/秒，气温低于零下25摄氏度，海洋混合层延伸至600米。在这些事件中，电导率 - 温度 - 深度（CTD）剖面显示，温暖、含盐的水体凸起直接位于海洋表面之下，并向下延伸数十米。这些剖面反映了在大气热量散失驱动下，未固结的碎冰生成过程中潜热和盐分的释放，这一过程在实验室之外极少（如果有的话）被观测到。一个简单的盐量收支分析表明，这些异常反映出碎冰的原位浓度范围为13至266×10⁻³千克/立方米。同时进行的垂直混合估算显示，在这些不稳定的密度剖面中存在快速对流，混合寿命为7至12分钟。根据盐量收支对冰生成量的单独估算揭示了短期冰生成的强度，在风最大的事件中，每日冰生成量高达110厘米，季节平均为每日29厘米。我们进一步发现，碎冰生成速率与风速以及沿冰间湖上下游的位置相关。这些测量结果表明，通过测量上层海洋水柱剖面，有可能间接观测和估算冰间湖中未固结冰的生成过程。这些强劲的冰生成速率表明，碎冰可能是冰间湖总冰生成量中的一个重要组成部分。
Katabatic winds in coastal polynyas expose the ocean to extreme heat loss, causing intense sea ice production and dense water formation around Antarctica throughout autumn and winter. The advancing sea ice pack, combined with high winds and low temperatures, has limited surface ocean observations of polynyas in winter, thereby impeding new insights into the evolution of these ice factories through the dark austral months. Here, we describe oceanic observations during multiple katabatic wind events during May 2017 in the Terra Nova Bay and Ross Sea polynyas. Wind speeds regularly exceeded 20m s(-1), air temperatures were below -25 degrees C, and the oceanic mixed layer extended to 600 m. During these events, conductivity-temperature-depth (CTD) profiles revealed bulges of warm, salty water directly beneath the ocean surface and extending downwards tens of meters. These profiles reflect latent heat and salt release during unconsolidated frazil ice production, driven by atmospheric heat loss, a process that has rarely if ever been observed outside the laboratory. A simple salt budget suggests these anomalies reflect in situ frazil ice concentration that ranges from 13 to 266 x 10(-3) kg m(-3). Contemporaneous estimates of vertical mixing reveal rapid convection in these unstable density profiles and mixing lifetimes from 7 to 12 min. The individual estimates of ice production from the salt budget reveal the intensity of short-term ice production, up to 110 cm d(-1) during the windiest events, and a seasonal average of 29 cm d(-1). We further found that frazil ice production rates covary with wind speed and with location along the upstream-downstream length of the polynya. These measurements reveal that it is possible to indirectly observe and estimate the process of unconsolidated ice production in polynyas by measuring upper-ocean water column profiles. These vigorous ice production rates suggest frazil ice may be an important component in total polynya ice production.