Variability of the Denmark Strait Overflow

这个项目将解决全球气候系统的一个重要部分。从北欧海向大西洋溢出的寒冷、稠密的水穿过格陵兰-苏格兰海脊是全球温盐环流（THC）的重要组成部分。大约一半的溢流通过丹麦海峡，另一半主要通过法罗海峡流向冰岛以东。紧接在海脊下游的强烈混合导致周围大西洋的水被强烈的夹带到溢出的羽流中。这降低了溢出水的密度，但据信将体积通量提高了约两倍。溢出和夹带系统是北大西洋深水的主要贡献者，是全球THC的深水分支。IPCC第四次评估报告指出，在下个世纪，THC极有可能出现某种程度的减缓。这些模型预测的经济放缓幅度在0%到50%之间，平均预测为25%。然而，对这些模式的信心仍然很弱，气候模式面临的一些最大挑战在于与溢流的形成、转移和命运有关的过程。它们在再现深层对流、混合、夹带和边界流方面仍然很弱。模式改进需要将其与在这些过程发生的地区进行的直接、持续的观测进行检查和检验。同样，理解地理上孤立地点的观测结果变化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们在丹麦海峡以南的Angmassalik用仪器测量了溢出的核心。自1986年以来，溢流核心的近连续温度和自1998年以来部署在阵列上的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，并提供了这些变化可能的上游来源和下游影响的线索。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。其他人则将丹麦海峡溢水的短期新生（在弗莱尔角的年度水文调查中发现）与风力异常迫使当地的丹麦海峡联系起来。丹麦海峡溢流的水源仍然是一个公开的科学争论。东格陵兰洋流是主要来源的证据似乎与其他证据不一致，这些证据表明冰岛海的中间水域是溢出物的来源。本研究将研究丹麦海峡溢流的变异性，并研究有关其来源和命运的假设。这项研究将有三个互补的方面。最初，他们将分析Cefas及其同事在Angmassalik阵列进行的长期观测。第二个可供学生使用的资源是由东安格利亚大学的Martin Wadley以1,1 /4和1/12度分辨率进行的OCCAM海冰模型的一系列运行输出。示踪剂已被插入到北极和北欧海的一些盒子表面，并以多种方式与模型集成。这些模型运行，连同丹麦海峡原始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率原因的假设。该项目的另一个方面将是研究该区域水团淡水成分的任何变化。为此，2009年夏季，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。该学生将根据东安格利亚大学科学家在过去15年中对东格陵兰大陆架/斜坡的各种巡航中所进行的氧同位素测量构建时间序列。

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.