Variability of the 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.