Apparent changes in the climatic state of the deep North Atlantic Ocean

Abstract

Determination of any long-term changes in the large-scale characteristics of the deep ocean circulation would be an important clue in understanding the climatic interactions of the ocean and atmosphere. In the summer of 1981, the RV Atlantis II reoccupied two transatlantic sections at nominal latitudes of 24°30′ N and 36°16′ N with a conductivity–temperature-depth instrument (CTD). One purpose of the work was to make a comparison with previous surveys conducted during the International Geophysical Year (IGY)¹, when sections were obtained in October 1957 and April–May 1959. We report here that significant warming occurred in an ocean-wide band from 700 m to 3,000 m with a maximum temperature difference of 0.2 °C. These changes are sufficient to expand the water column by several centimetres. The historical temperature–salinity curve was apparently unchanged. Interannual changes in local water mass characteristics have been proposed previously^2,3. Perhaps it would be most surprising if no changes were seen to occur. What remains obscure is the significance of these changes and the extent to which they represent long-term climate trends, or merely the minor and random fluctuations to be expected in any complex fluid system.