Water mass properties and fluxes in the Rockall Trough, 1975-1998

A time series of a standard hydrographic section in the northern Rockall Tr ough spanning 23 yr is examined for changes in water mass properties and tr ansport levels. The Rockall Trough is situated west of the British Isles an d separated from the Iceland Basin by the Hatton and Rockall Banks and From the Nordic Seas by the shallow (500 m) Wyville-Thompson ridge. it is one p athway by which warm North Atlantic upper water reaches the Norwegian Sea a nd is converted into cold dense overflow water as part of the thermohaline overturning in the northern North Atlantic and Nordic Seas. The upper water column is characterised by poleward moving Eastern North Atlantic Water (E NAW), which is warmer and saltier than the subpolar mode waters of the Icel and Basin, which also contribute to the Nordic Sea inflow. Below 1200 m the deep Labrador Sea Water (LSW) is trapped by the shallowing topography to t he north, which prevents through flow but allows recirculation within the b asin. The Rockall Trough experiences a strong seasonal signal in temperatur e and salinity with deep convective winter mixing to typically 600 m or mor e and the formation of a warm fresh summer surface layer. The time series r eveals interannual changes in salinity of +/-0.05 in the ENAW and +/- 0.04 in the LSW. The deep water freshening events are of a magnitude greater tha n that expected from changes in source characteristics of the LSW, and are shown to represent periodic pulses of newer LSW into a recirculating reserv ior. The mean poleward transport of ENAW is 3.7 Sv above 1200 dbar (of whic h 3.0 Sv is carried by the shelf edge current) but shows a high-level inter annual variability, ranging from 0 to 8 Sv over the 23 yr period. The shelf edge current is shown to have a changing thermohaline structure and a baro clinic transport that varies from 0 to 8 Sv. The interannual signal in the total transport dominates the observations, and no evidence is found of a s easonal signal. (C) 2000 Elsevier Science Ltd. All rights reserved.