The deep silica tongue in the North Atlantic

Abstract

The distribution of dissolved silica and the variation of the temperature-salinity relationship are important tracers of the large-scale mid-depth flow of water in the Atlantic. A prominent feature of the silica distribution near 24°N is a westward-pointing silica tongue, analogous to the salty Mediterranean tongue concentrated at shallower levels. Earlier advection-diffusion studies showed how the structure of the salt tongue could be consistent with the expected general circulation of the subtropical gyre. The expected circulation pattern of the mid-depth levels is that of the Stommel and Arons model, with the deep interior flow to the north and east; however, an advection-diffusion model is unable to reproduce the deeper silica tongue for any value of horizontal diffusion with eastward flow, nor, it is argued, can reasonable spatial variations in horizontal diffusivity be responsible. Thus, while the Stommel and Arons circulation would have the deep interior flow to the east and north, evidence is presented here for a westward flow of water in this depth range (2000–3500 m). Such interior flow would tend to increase the deep western boundary current transport as it flows south. The explanation for this is suggested to be related to the existence of deep-water formation in the northern North Atlantic, which entrains mid-depth water and forces the area-averaged vertical velocity to decrease with height.