Exceptional freshening and cooling in the eastern subpolar North Atlantic caused by reduced Labrador Sea surface heat loss
Exceptional freshening and cooling in the eastern subpolar North Atlantic caused by reduced Labrador Sea surface heat loss
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DOI：
10.5194/os-18-1507-2022
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发表时间：
2022-10-21
期刊：
影响因子：
3.2
通讯作者：
Yashayaev, Igor
中科院分区：
文献类型：
--
作者：
Fox, Alan D.;Handmann, Patricia;Yashayaev, Igor
2010年代对北大西洋亚极地东部的观测显示，上层海洋异常地变新鲜和变冷，并在2016年达到峰值，创下120年来最低盐度记录。已发表的驱动淡水变新鲜机制的理论包括：减少了从亚热带向北输送的更咸、更暖的地表水，这与减少经向翻转有关；来自拉布拉多海的更新鲜、更凉爽的地表水由于风应力模式的变化而改变；以及亚极环流的东扩。利用高分辨率后推模式模拟的输出，我们提出，拉布拉多海异常变冷和变冷的主要原因是减少了表面热损失。从1990年到2020年，从北大西洋亚极东部回溯模型中的虚拟流体粒子，结果表明，变冷和变冷的主要原因是来自拉布拉多海的相对新鲜和寒冷的地表水流出量增加；从副热带向北输送的较暖、较咸的地表水的减少也起到了很小的作用。由亚极地和亚热带水域的这些变化比例所产生的冷却，而不是新鲜，由于北大西洋洋流减少了沿轨道向大气的热量损失而得到缓解。我们分析了拉布拉多海的模拟边界交换和水团转换，结果表明，自2000年以来，虽然较轻的地表水流入保持稳定，但这些水的输出增加是由于从21世纪初开始，拉布拉多海的表面热损失减少。进一步向上游跟踪粒子表明，从拉布拉多海输送出来的体积增加的较轻的水的主要来源是水的再循环增加，因此在次极环流的500-1000米的上层停留时间更长。
Observations of the eastern subpolar North Atlantic in the 2010s show exceptional freshening and cooling of the upper ocean, peaking in 2016 with the lowest salinities recorded for 120 years. Published theories for the mechanisms driving the freshening include: reduced transport of saltier, warmer surface waters northwards from the subtropics associated with reduced meridional overturning; shifts in the pathways of fresher, cooler surface water from the Labrador Sea driven by changing patterns of wind stress; and the eastward expansion of the subpolar gyre. Using output from a high-resolution hindcast model simulation, we propose that the primary cause of the exceptional freshening and cooling is reduced surface heat loss in the Labrador Sea. Tracking virtual fluid particles in the model backwards from the eastern subpolar North Atlantic between 1990 and 2020 shows the major cause of the freshening and cooling to be an increased outflow of relatively fresh and cold surface waters from the Labrador Sea; with a minor contribution from reduced transport of warmer, saltier surface water northward from the subtropics. The cooling, but not the freshening, produced by these changing proportions of waters of subpolar and subtropical origin is mitigated by reduced along-track heat loss to the atmosphere in the North Atlantic Current. We analyse modelled boundary exchanges and water mass transformation in the Labrador Sea to show that since 2000, while inflows of lighter surface waters remain steady, the increasing output of these waters is due to reduced surface heat loss in the Labrador Sea beginning in the early 2000s. Tracking particles further upstream reveals that the primary source of the increased volume of lighter water transported out of the Labrador Sea is increased recirculation of water, and therefore longer residence times, in the upper 500-1000 m of the subpolar gyre.