Variability of the Denmark Strait Overflow

丹麦海峡溢流的变化

• 批准号: NE/F013329/1
• 金额: $ 8.34万
• 依托单位: University of East Anglia
• 依托单位国家: 英国
• 项目类别: Training Grant
• 财政年份: 2008
• 资助国家: 英国
• 起止时间: 2008 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FF013329%2F1
• 关键词: Variability; Denmark; Strait; Overflow

This project will address a significant part of the global climate system. The overflow of cold, dense water from the Nordic Seas to the Atlantic Ocean across the Greenland-Scotland Ridge is an important component of the global thermohaline circulation (THC). About half of the overflow passes through the Denmark Strait, and half passes east of Iceland mainly through the Faroe Bank Channel. Intensive mixing immediately downstream of the Ridge induces strong entrainment of ambient Atlantic waters into the overflow plumes. This reduces the density of the overflow water but is believed to enhance the volume flux by a factor of about two. The system of overflow and entrainment is the main contributor to North Atlantic Deep Water, the deep limb of the global THC. The 4th IPCC Assessment reports that it is very likely that some degree of slowdown will occur in the THC during the next century. The models ranged between a slowing of 0% and 50%, and on average suggested slowing by 25%. However, confidence in these models remains weak and some of the greatest challenges for climate models lie in the processes associated with the formation, transport and fate of the overflows. They remain weak at reproducing deep convection, mixing and entrainment and boundary currents. Model improvement requires their examination and testing compared with direct, sustained observations in the region where these processes take place. Equally, understanding the reasons behind variability of observations at geographically isolated locations requires hypothesis testing through models. Since 1986 Cefas with colleagues in Germany and Finland have instrumented the core of the overflow south of the Denmark Strait at Angmassalik. Near-continuous temperature in the core of the overflow since 1986 and from SBE-37 salinity sensors deployed across the array since 1998 have provided clear evidence of interannual change in both temperature and salinity on the Angmagssalik line and have provided clues as to the likely upstream sources and downstream impacts of these changes. Subdecadal variability in temperature of the overflow has been attributed to changes in atmospheric conditions near Fram Strait. Others associated Denmark Strait overflow short term freshening, identified in annual hydrographic surveys at Cape Farewell, with anomalies in wind forcing local to Denmark Strait. The source water that becomes Denmark Strait overflow remains an open scientific debate. Evidence for the East Greenland current as the prime source is seemingly at odds with other evidence that suggests the intermediate waters of the Iceland Sea feed the overflow. This studentship will investigate the variability of the Denmark Strait overflow and examine the hypotheses regarding its sources and fate. There will be three complementary aspects to the study. Initially they will analyse the long-term observations taken by Cefas and colleagues at the Angmassalik array. The second resource available to the student will be output from a series of runs of the OCCAM ocean-ice model at 1, 1/4 and 1/12 degree resolution, undertaken by Martin Wadley at UEA. Tracer has been inserted into the surface of a number of boxes in the Arctic and Nordic Seas, and the model integrated in a variety of ways. These model runs, together with the original OCCAM 1/12 model output in Denmark Strait, will be used to investigate the hypotheses concerning the causes of the variability detected in the moored data time series. A further aspect of the project will be a study of any changes in the freshwater components of the water masses in the region. To this end, in summer 2009 the student will make an oxygen isotope survey of the overflow on a 30-day research cruise in collaboration with German colleagues. The student will construct timeseries from the oxygen isotope measurements taken over the last 15 years by UEA scientists on a variety of cruises to the East Greenland shelf/slope.

该项目将解决全球气候系统的重要组成部分。寒冷、稠密的海水从北欧海穿过格陵兰-苏格兰海岭流入大西洋，是全球温盐环流（THC）的重要组成部分。大约一半的溢流流经丹麦海峡，一半主要通过法罗浅滩海峡流经冰岛以东。紧邻海脊下游的强烈混合导致周围大西洋水域强烈夹带到溢流羽流中。这降低了溢流水的密度，但据信可将体积通量提高约两倍。溢流和夹带系统是北大西洋深水（全球 THC 的深水部分）的主要贡献者。第四次IPCC评估报告称，下个世纪THC很可能会出现某种程度的放缓。这些模型的放缓范围在 0% 到 50% 之间，平均建议放缓 25%。然而，人们对这些模型的信心仍然薄弱，气候模型面临的一些最大挑战在于与溢流的形成、运输和命运相关的过程。它们在再现深层对流、混合、夹带和边界流方面仍然很弱。模型的改进需要对这些过程发生的地区进行直接、持续的观察进行比较和测试。同样，了解地理位置偏远地区观测结果变异性背后的原因需要通过模型进行假设检验。自 1986 年以来，Cefas 与德国和芬兰的同事一起在丹麦海峡以南的 Angmassalik 溢流核心安装了仪器。自 1986 年以来溢流核心的近乎连续的温度以及自 1998 年以来在整个阵列中部署的 SBE-37 盐度传感器提供了 Angmagssalik 线上温度和盐度年际变化的明确证据，并为这些变化可能的上游来源和下游影响提供了线索。溢流温度的年代际变化归因于弗拉姆海峡附近大气条件的变化。其他人将丹麦海峡溢流短期清新联系起来，这是在告别角的年度水文调查中发现的，与丹麦海峡局部风力异常有关。丹麦海峡溢流的源水仍然是一个公开的科学争论。东格陵兰洋流作为主要来源的证据似乎与其他表明冰岛海中间水域为溢流提供水源的证据不一致。该奖学金将调查丹麦海峡溢流的变化，并检验有关其来源和命运的假设。该研究将包括三个互补的方面。最初，他们将分析 Cefas 及其同事在 Angmassalik 阵列进行的长期观测。学生可用的第二个资源将是 OCCAM 海冰模型以 1、1/4 和 1/12 度分辨率运行的一系列输出，由东安格利亚大学的 Martin Wadley 进行。 Tracer已被插入北冰洋和北欧海域的多个盒子的表面，并且该模型以多种方式集成。这些模型运行以及丹麦海峡的原始 OCCAM 1/12 模型输出将用于研究有关系泊数据时间序列中检测到的变异性原因的假设。该项目的另一个方面将是研究该地区水团淡水成分的任何变化。为此，2009 年夏天，该学生将与德国同事合作，在为期 30 天的研究巡航中对溢流进行氧同位素调查。学生将根据过去 15 年东格陵兰大陆架/斜坡的各种航行中东英吉利大学科学家进行的氧同位素测量数据构建时间序列。