Water-mass formation and distribution in the Nordic Seas during the 1990s

Abstract

Abstract Hydrographic, oxygen and nutrient data collected in the Nordic Seas during the 1990s are presented. During the decade, deep waters originating from the Arctic Ocean, identified by salinities in excess of 34.9, spread into the Greenland Basin. In 1991, these waters extended westward from the mid-ocean ridge to about 2°E. This process continued over time and by 1993 there was a layer with salinities above 34.9 along the entire section, between 7.6°W and the Barents Sea Slope, and probably across the whole basin. In 2000 the basin had these high salinities at depths greater than 1400 m. At 1500 m in the central basin the salinity increase during the decade was 0.012 units, decreasing to 0.006 at 3000 m, and associated temperatures increased by 0.28 and 0.09°C, respectively. This warming more than compensated for the salinity increase so that the density of the deep water decreased during the decade, σ3 decreasing by 0.027 kg m−3 at 1500 m and by 0.006 kg m−3 at 3000 m. Decreasing oxygen content and increasing concentrations of silicate further indicated the increasing influence of Arctic Ocean Deep Water. Interaction with the atmosphere is decisive for the conditions in the area. In the central Greenland Sea there is close correlation between wind forcing and upper-layer salinity. Significant deep-water formation occurs only during cold winters, or rather, in periods with several succeeding cold winters and the 1960s were the first period in which these conditions occurred since 1920. This is shown by meteorological observations at Jan Mayen since 1921, and at Stykkisholmur, Iceland, since 1823. Relatively high salinities were observed near the bottom over the Iceland Plateau. These waters seem to be derived from Arctic Ocean deep waters that have been diverted from the East Greenland Current, into the East Icelandic Current. While flowing through the Iceland Sea their nutrient concentration increases considerably. This water flows into the Norwegian Basin where it forms a slight salinity maximum around 1500 m, which is associated with a minimum in oxygen content. At greater depths the water masses are from the Greenland Sea. The salinity decreases and the oxygen increases toward approximately 2500 m, from where the trends are reversed toward a slight salinity maximum around 3000 m, where there also is a minimum in oxygen as well as in CFC-11. These characteristics seem to derive from Arctic Ocean Deep Water, floating above waters more characterized by Greenland Sea Bottom water nearest to the bottom as suggested by decreasing salinity and an increase in both oxygen and CFC-11 concentration. This shows that even the very homogeneous Norwegian Sea Deep Water is stratified. There are also slight differences between the deep waters of the basins in the Norwegian Sea. In the Norwegian Basin the deep water has slightly higher salinity, lower dissolved oxygen and higher silicates than the deep water in the Lofoten Basin, and even more so compared with the area west of Bear Island. This shows that the Lofoten Basin and the northern Norwegian Sea are more directly influenced by waters from the Greenland Sea than the Norwegian Basin.