Decadal variability in the composition of Faroe Shetland Channel bottom water

Two standard sections across the deep water channel separating the Faroese Plateau from the Scottish continental shelf have been surveyed regularly si nce the start of the 20th century. There have been significant changes in t he characteristics of surface, intermediate and deep water masses during th is period. At intermediate depths; the presence of Norwegian Sea Arctic Int ermediate Water (NSAIW) was evident as a salinity minimum during the first decade of the century. During the decades 1960-1980 this salinity minimum d isappeared, and only four water types were identified in the Channel. Since 1980 the salinity of the intermediate water has again decreased, due to ch anges in the atmospheric forcing over the Nordic Seas, and it is again evid ent on a theta S curve as a distinct minimum. The salinity of the bottom wa ter in the Channel has also decreased (0.01/decade) linearly since the mid- 1970s, although at a slower rate than the intermediate water (0.02/decade). The decline in salinity of the bottom water cannot be accounted for by cha nges in the salinity of upper Norwegian Sea Deep Water (NSDW), which Faroe Shetland Channel Bottom Water (FSCBW) has traditionally been assumed to be composed of. There is evidence that the upper level of NSDW has become deep er outside the Channel owing to a reduced supply from the Greenland Sea. Th is has resulted in a change in the composition of FSCBW, from being approxi mately 60% NSDW during the period 1970-1985 to 40% NSDW since 1990. Thus, t he thermohaline circulation of the Nordic Seas has lost its deep water conn ection. The associated freshening of FSCBW has propagated out through the C hannel into the North Atlantic and has resulted in a reduction of the salin ity (0.02/decade) and transport (1-7%/decade) of Iceland Scotland Overflow Water (ISOW) into the North Atlantic. (C) 1999 Elsevier Science Ltd. All ri ghts reserved.