Formation of salinity maximum water and its contribution to the overturning circulation in the North Atlantic as revealed by a global general circulation model

Abstract

The formation of salinity maximum water in the North Atlantic is investigated using a simulated passive tracer and its adjoint. The results reveal that most salinity maximum water in the North Atlantic comes from the northwestern part of the subtropical gyre, and direct contribution from the evaporation-precipitation maximum region via the surface Ekman current is minor. Water originating from the evaporation-precipitation maximum region has to recirculate in the subtropical gyre before entering the sea surface salinity maximum region from the northwest. Once subducted, some portion (∼10%) of the salinity maximum water enters the equatorial region in the shallow subtropical cell, but most (~70%) of it appears to turn northward to join the North Atlantic Deep Water. The latter pathway involves a three-dimensional circulation. When the warm, fresh surface water flows northward along the western boundary, it turns eastward in the northern subtropical gyre. As a result of the large excess of evaporation over precipitation, this water gradually gains its salinity on the route, until it reaches the sea surface salinity maximum region in the central subtropical gyre. From there, the salinity maximum water is subducted and flows back to the western boundary in the depth range of the thermocline. With its high-salinity nature, a major portion of this water penetrates into the subpolar region and directly contributes to the deep thermohaline circulation. Key Points Most SSS-max water comes from the northwestern part of the subtropical gyre. Only ~10% of the subducted SSS-max water enters the equatorial region via STC. Most SSS-max water joins the deep thermohaline circulation as part of NADW. ©2013. American Geophysical Union. All Rights Reserved.