Anomalous water mass distributions at 55W in the North Atlantic in 1977

Abstract

A hydrographic section made in July 1977 from the research vessel KNORR revealed a large-scale meridional distortion of the normal water mass distributions at 55W in the North Atlantic. Cells of pure Labrador Sea Water were found within both the Gulf Stream and the westward recirculation of the gyre. A large cell of Mediterranean Water was found in the Slope Water, in contact with a cell of Subarctic Intermediate Water. Water at 11°C to 13°C within both the Gulf Stream and the Slope Water was anomalously saline. Throughout the Slope Water, Gulf Stream, and northern Sargasso Sea there was very little standard Western North Atlantic Water in the temperature ranges 3.4° to 9.0°C and 11° to 13°C. It is suggested that these meridional distortions are due in part to an increase in the amount of rotation of the horizontal velocity vector with depth during 1977 that was observed with current meters in the northern Sargasso Sea. An increase in the westward return flow strength may also have contributed. The ultimate cause of the anomalous property distributions and currents may be changes in the production rate and strength of the source waters for North Atlantic Deep Water and western North Atlantic Water such as Labrador Sea Water, Mediterranean Water, and Eighteen Degree Water. The first and the last are known to have undergone convective formation events, in March 1976, and March 1977, respectively, in the period preceding the 1977 survey. The July 1977 section shows evidence of the recirculation of the new convectively formed Eighteen Degree Water.