Freshening of the Alaska Coastal Current recorded by coralline algal Ba/Ca ratios

Arctic Ocean freshening can exert a controlling influence on global climate, triggering strong feedbacks on ocean-atmospheric processes and affecting the global cycling of the world's oceans. Glacier-fed ocean currents such as the Alaska Coastal Current are important sources of freshwater for the Bering Sea shelf, and may also influence the Arctic Ocean freshwater budget. Instrumental data indicate a multiyear freshening episode of the Alaska Coastal Current in the early 21st century. It is uncertain whether this freshening is part of natural multidecadal climate variability or a unique feature of anthropogenically induced warming. In order to answer this, a better understanding of past variations in the Alaska Coastal Current is needed. However, continuous long-term high-resolution observations of the Alaska Coastal Current have only been available for the last 2 decades. In this study, specimens of the long-lived crustose coralline alga Clathromorphum nereostratum were collected within the pathway of the Alaska Coastal Current and utilized as archives of past temperature and salinity. Results indicate that coralline algal Mg/Ca ratios provide a 60 year record of sea surface temperatures and track changes of the Pacific Decadal Oscillation, a pattern of decadal-to-multidecadal ocean-atmosphere climate variability centered over the North Pacific. Algal Ba/Ca ratios (used as indicators of coastal freshwater runoff) are inversely correlated to instrumentally measured Alaska Coastal Current salinity and record the period of freshening from 2001 to 2006. Similar multiyear freshening events are not evident in the earlier portion of the 60 year Ba/Ca record. This suggests that the 21st century freshening of the Alaska Coastal Current is a unique feature related to increasing glacial melt and precipitation on mainland Alaska.

北冰洋变淡会对全球气候产生控制性影响，引发海洋 - 大气过程的强烈反馈，并影响全球海洋的循环。冰川补给的洋流，比如阿拉斯加沿岸流，是白令海陆架淡水的重要来源，也可能影响北冰洋的淡水收支。仪器数据显示，21世纪初阿拉斯加沿岸流出现了多年的变淡现象。不确定这种变淡是自然的几十年气候变率的一部分，还是人为导致的变暖的一个独特特征。为了回答这个问题，需要更好地了解阿拉斯加沿岸流过去的变化情况。然而，对阿拉斯加沿岸流连续的长期高分辨率观测只有过去20年的数据。在这项研究中，在阿拉斯加沿岸流流经区域采集了长寿的壳状珊瑚藻（Clathromorphum nereostratum）样本，并将其用作过去温度和盐度的记录。结果表明，珊瑚藻的Mg/Ca比值提供了60年的海面温度记录，并追踪了太平洋年代际振荡的变化，太平洋年代际振荡是一种以北太平洋为中心的十年到几十年的海洋 - 大气气候变化模式。藻类的Ba/Ca比值（用作沿海淡水径流的指标）与仪器测量的阿拉斯加沿岸流盐度呈负相关，并记录了2001年到2006年的变淡时期。在60年的Ba/Ca记录的早期部分，没有明显类似的多年变淡事件。这表明21世纪阿拉斯加沿岸流的变淡是一个与阿拉斯加大陆冰川融化和降水增加有关的独特特征。