An eastern Atlantic section from Iceland southward across the equator

Abstract

A long CTD/hydrographic section with closely-spaced stations was occupied in July–August 1988 from Iceland southward to 3°S along a nominal longitude of 20°W. The section extends from the surface down to the bottom, and spans the entire mid-ocean circulation regime of the North Atlantic from subpolar gyre through the subtropical gyre and the equatorial currents. Vertical sections of potential temperature, salinity and potential density from CTD measurements and of oxygen, silica, phosphate and nitrate, based on discrete water-sample measurements are presented and discussed in the context of the large-scale circulation of the North Atlantic Ocean. The close spacing of high-quality stations reveals some features not described previously. The more important findings include: (1) possible recirculation of the lightest Subpolar Mode Water into the tropics; (2) a thermostad at temperatures of 8–9°C, lying below that of the Equatorial 13°C Water; (3) the nutrient distribution in the low-salinity water above the Mediterranean Outflow Water that supports the previous conjecture of northern influence of the Antactic Intermediate Water; (4) a great deal of lateral structure of the Mediterranean Outflow Water, with a number of lobes of high salinity; (5) an abrupt southern boundary of the Labrador Sea Water at the Azores-Biscay Rise and a vertically well-mixed region to its south; (6) a sharp demarcation in the central Iceland Basin between the newest Iceland-Scotland Overflow Water and older bottom water, which has a significant component of southern water; (7) evidence that the Northeast Atlantic Deep Water is a mixture of the Mediterranean Outflow Water and the Northwest Atlantic Bottom Water with very little input from the Iceland-Scotland Overflow Water; (8) an isolated core of the high-salinity, low-silica Upper North Atlantic Deep Water at the equator; (9) a core of the high-oxygen, low-nutrient Lower North Atlantic Deep Water pressed against the southern flank of the Mid-Atlantic Ridge just south of the equator; (10) a weak minimum of salinity, and well-defined maxima of nutrients associated with the oxygen minimum that separates the Middle and Lower North Atlantic Deep Waters south of the equator; (11) a large body of nearly homogeneous water beneath the Middle North Atlantic Deep Water between 20°N and the Azores-Biscay Rise; and (12) a deep westward boundary undercurrent on the southern slope of the Rockall Plateau.