Structure and circulation of the Upper Atlantic Ocean northeast of the Azores

Abstract

Historical data, primarily from the IGY, but spanning the years 1914–1983, have been re-examined in an attempt to define the structure and circulation of the upper ocean (0–800 m) in the eastern north Atlantic (30–50°N 10–40°W). The saline (greater than 35.7% anticyclonic surface gyre shallows to the north like a wedge, being 300–500 m deep at 36°N (east of 30°W) and outcropping at about 45°N. It lies above the Mediterranean water, separated from it by a salinity minimum and by a pycnostad of subpolar mode water (MCCARTNEY and TALLEY, 1982). Eastward flow between 36°N and 46°N is primarily confined to the saline wedge, with speeds of 2–4 cm s −1. The flow converges gradually towards the east, resulting in fronts and strong meridional property gradients (e.g. mixed layer depth, ROBINSON, BAUER and SCHROEDER, 1979) east of 15°W. The majority of the supolar mode water is formed by deep winter convection north of the saline wedge and subducts to the south under it. Southward advection and ventilation rates in the northeast Atlantic (the main ventilation region for the North Atlantic, SARMIENTO, ROOTH and ROETHER, 1982) are similar, estimated to be 0.2 cm s −1. The freshening of the surface layer between 1967 and 1976 that has been found in widely separated regions of the North Atlantic (Rockall trough, Sargasso Sea, Labrador Sea) is also present northeast of the Azores. Salinities down to 500 m have decreased by 0.1%, equivalent to an addition of a 1.4 m column of fresh water at the surface. The freshening on σ T = 27.1 decreases to negligible values southwards of 40°N. It is suggested that changes in the balance of evaporation and precipitation must have occured.