Tracing Wyville Thomson Ridge Overflow Water in the Rockall Trough
Although it has long been known that cold dense waters from the Nordic Seas overflow the Wyville Thomson Ridge, the water masses' subsequent pathways and fate have been uncertain. This study conclusively places Wyville Thomson Ridge Overflow Water (WTOW) as an important water mass in the eastern subpolar North Atlantic for the first time. Using a variety of chemical tracer s (chlorofluorocarbons, oxygen, nutrients and aluminium) in conjunction with temperature and salinity, WTOW is traced southwards into the northern and central Rockall Trough as well as into the channels between the western banks. The overflow water has a clear temperature, salinity and chlorofluorocarbon (CFC-11 and CFC-12) signature. Additionally, levels of aluminium are elevated in WTOW suggesting that this element is potentially a useful and novel water mass tracer. The lower oxygen layer complicates the use of dissolved oxygen and nitrate as tracers in the mid water column. However, higher and lower concentrations respectively in the western trough reveal the presence of WTOW in this area. The overflow water does not appear to have a silicate or phosphate signature. Two branches of WTOW exist in the Rockall Trough: a slow-moving indistinct intermediate branch (600-1200 m) f ound in both the east and west of the basin; and a coherent deep branch (> 1200 m) that flows southward along the western banks of the trough. As well as having a large spatial footprint within the Rockall Trough, intermediate a nd deep WTOW are temporally persistent being present 65-75 % of the time between 1975 and 2008. The signature of WTOW at intermediate depths is absent from the Ellett Line record in the mid-1980s and early-1990s. As deep WTOW is still observed during these periods flow over the Wyville Thomson Ridge cannot have ceased. Instead, it is proposed that the strength of the Subpolar Gyre is an important driver in the temporal distribution of intermediate WTOW within the Rockall Trough. When the gyre is strong, such as in the mid-1980s and early-1990s, the mid water column is dominated by waters originating from the west which block the southward flow of intermediate WTOW. In contrast, when the gyre is weak, such as in the late-1990s and 2000s, subpolar waters lie further west enabling intermediate waters within the Rockall Trough to be dominated by the southern orig inating Mediterranean Overflow Water and the northern water mass of WTOW.
Networked Digital Library of Theses and Dissertations