The physical and chemical oceanography of the Rockall channel

Abstract

Between August, 1963, and February, 1968, a hydrographic section was worked between Rockall Bank and Malin Head on twelve occasions. In addition to the vertical distributions of temperature, salinity and their variabilities, the data allow the annual cycles of temperature and density to be described to the depth of seasonal mixing (500–600 m). Results from repeated chemical sampling at two stations indicate the scale of annual charges in dissolved oxygen, phosphate and silicate and their general distribution through the water column. A 23-year series of surface observations in the area provides data about year-to-year changes. The T-S characteristics of the upper waters represent a mixed water formed in the vicinity of the Biscay-Porcupine Bank continental shelf, and not North Atlantic Central Water, during the years of our surveys. (Earlier work shows that this is not always so.) Low oxygen content clearly indicated the presence of Gulf of Gibraltar Water between depths of 800–1200 m, and below this a salinity minimum characterized Labrador Sea Water. At the lowest levels a water-mass having a Norwegian Sea Deep Water component was observed and limited evidence points to a source in the northern Rockall Channel. The largest northeast-going transport was normally found over the western half of the section, but a smaller northeast-going flow occured at the Irish continental slope. A net southwesterly flow above 1800 m was found once (July, 1964), but net northeasterly transport was the general rule. The mean 0–500 m volume transport relative to 1800 m was 2·70 x 10 6 m 3 sec −1. At 500–1200 m a mean transport of 0·99 x 10 6 m 3sec −1 must have left the Channel by the northwestern gaps. Correlations significant at the 0·1% level were found between net transport in the upper layers and 5-day mean east-southeast wind components. Estimates made from these suggest that the mean annual northeast-going 0–500 m transport in the years 1964 and 1965 was about 3 x 10 6 m 3sec −1.