Variability of the Denmark Strait Overflow

该项目将涉及全球气候系统的一个重要部分。从北欧海经格陵兰-苏格兰海脊进入大西洋的稠密冷水溢流是全球温盐环流的重要组成部分。大约一半的溢出通过丹麦海峡，一半通过冰岛东部主要通过法罗银行海峡。洋脊下游的强烈混合导致周围的大西洋沃茨强烈夹带到溢流羽流中。这降低了溢流水的密度，但据信将体积通量提高了约两倍。溢流和卷吸系统是北大西洋深层水的主要贡献者，全球THC的深翼。IPCC第四次评估报告指出，在下一个世纪，THC很可能会出现某种程度的放缓。这些模型的减速范围在0%到50%之间，平均建议减速25%。然而，对这些模型的信心仍然很弱，气候模型面临的一些最大挑战在于与溢流的形成、运输和命运有关的过程。它们在再现深对流、混合、卷吸和边界流方面仍然很弱。模型的改进需要对这些过程进行检查和测试，并与这些过程发生的区域的直接持续观测进行比较。同样，要理解地理上孤立地点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Angmassalik的丹麦海峡南部的溢流核心进行了仪器测量。自1986年以来，溢流核心的近连续温度以及自1998年以来在阵列上部署的SBE-37盐度传感器提供了Angmagssalik线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。近十年来溢出温度的变化归因于弗拉姆海峡附近大气条件的变化。其他相关的丹麦海峡溢出短期清新，在告别角的年度水文调查中确定，与异常风迫使当地丹麦海峡。成为丹麦海峡溢出的水源仍然是一个公开的科学辩论。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沃茨为溢出提供了食物。本研究将探讨丹麦海峡溢水的变异性，并探讨其来源和命运的假设。这项研究将有三个相辅相成的方面。最初，他们将分析Cefas及其同事在Angmassalik阵列上进行的长期观测。学生可利用的第二个资源将是由UEA的Martin瓦德利在1，1/4和1/12度分辨率下运行的OCCAM海洋冰模型的一系列输出。示踪剂已被插入到北极和北欧海的一些箱子的表面，并以各种方式集成的模型。这些模型运行，连同丹麦海峡的原始OCCAM 1/12模型输出，将用于调查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区域水团淡水组成部分的任何变化。为此，2009年夏天，该学生将与德国同事合作，在为期30天的研究航行中对溢流进行氧同位素调查。学生将从过去15年来UEA科学家在东格陵兰大陆架/斜坡的各种巡航中进行的氧同位素测量中构建时间序列。

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.