New hypotheses concerning the circulation of the northern North Sea and its relation to North Sea fish stock recruitment

Abstract

The routes of the major inflows into the northern North Sea have been the subject of much debate for some time. The three principal routes have been identified as being eastwards through the Fair Isle Passage, southwards along the western edge of the Norwegian Trench, and southwards along the east coast of Shetland. Dooley (1974), describing some preliminary results using data derived from recording current meters, deduced that little persistent and organized inflow occurred east of Shetland. More recent observations have called this hypothesis into doubt, and have revealed a significant and confined current flowing into trie North Sea along the Shetland coast. Biological evidence has suggested that this inflow may indeed have been weaker or entirely absent during the latter half of the 1970s, coinciding with the general lowering of North Atlantic salinities. The causative mechanisms that may link these two events are discussed, as well as their implications for recruitment to fish stocks. While the transport of oceanic water along the edge of the continental shelf is partly dependent upon north/south density-generated pressure gradients, and the cyclonic circulation within the northern North Sea basin itself is dependent upon the inshore/offshore density differences (at least during the summer months), the fact that the oceanographic changes that occurred during the years of the salinity anomaly did not significantly alter oceanic densities meant that these were not responsible for the suggested changes in the North Sea circulation. The most likely cause of decreased Atlantic inflow was the coincidence of a minimum in Gulf Stream transport, resulting from a decline in the formation of 18° mode water in the Sargasso Sea, with the lowest occurrence of westerly winds that has occurred over the British Isles this century.