The influence of the mid-Atlantic ridge upon the circulation and the properties of the Mediterranean water southwest of the Azores

Abstract

The warm, salty water influenced by the Mediterranean outfow can be observed at mid-depth throughout the Central North Atlantic Ocean. Katz (1970) first noted that rather than a gradual salinity decrease away from the source, large changes were observable over major topographic features such as the mid-Atlantic Ridge, despite the fact that the topography presented no direct physical barrier to the core layer. Two mechanisms are considered which can account for this sub-surface frontal transition: variable eddy diffusivity and horiontal shear induced by submarine topography. The structure of the actual geostrophic currents and water masses southwest of the Azores is explored with CTD, XBT, and float data collected in June 1977 aboard the R.V. Knorr. A CTD section normal to and crossing the ridge axis near 35N, 35W shows the thermocline to be domed up 200 m over the ridge axis compared with stations 200 km to either side. At 1000 meters depth a change in salinity of nearly 0.15% in the Mediterranean Water is observed to occur over a horizontal distance of 100-150 km, and is located west of the ridge near 38N and over the ridge axis near 34N. Near this transition two neutrally buoyant floats were tracked for a period of a day. CTD stations around and over a float at 810 meters depth showed the temperature and salinity intrusions to persist with time and horiontal space scales in excess of 15 hours and 4 km. If the currents observed SW of the Azores are representative of the general circulation of the region, they not only explain the water mass structure in the thermocline and Mediterranean Water but also point out that the historical data base of zonal hydrographic sections does not adequately resolve the baroclinic structure.