CONVEX-91: water masses and circulation of the Northeast Atlantic subpolar gyre

Abstract

A description of the subpolar Northeast Atlantic is provided by a survey that took place in August 1991. A circulation scheme is determined from geostrophic calculations with a reference level that minimises the volume flux through the survey area. The circulation is weak but the pattern is the same as that found by other methods. The water masses observed during the survey are analysed with reference to the circulation scheme. The spreading rate of Labrador Sea Water is re-assessed but shows no evidence of the rapid rates postulated recently. The upper layers are analysed in detail. Regional studies and locally identified water masses are compared with the gyre-scale circulation of subpolar mode waters. The in-flowing water of the North Atlantic Current is identified by its stratification and slight freshness, which distinguishes it from the majority of the warm saline water seen in the region. The latter originates in the inter-gyre region through deep winter mixing and is very weakly stratified forming a deep homogeneous water mass that spreads around the eastern side of the subpolar gyre. Distinguishing between well-stratified and weakly-stratified waters reveals a patchy distribution of mode waters that is much more complex than would be generated by just cooling and advection. Possible mechanisms are described to explain the processes in operation. The dominant processes are deep mixing and cooling but the region is one of upwelling and therefore the characteristics of the mode water are set by the deeper water. The eddy transports are similar in magnitude to the mean flow so eddy processes are likely to play an important role in the modification of the water masses.