A Lagrangian numerical investigation of the origins and fates of the salinity maximum water in the Atlantic

Abstract

The origins and fates of the Atlantic salinity maximum water (SMW), formed through excess evaporation in the tropics and subtropics of both hemispheres, are studied using monthly mean outputs of a numerical simulation of the world ocean climatological circulation. After defining formation domains from the surface salinity field and the vertical stratification, a Lagrangian technique is used to estimate the formation rates and main pathways in each hemisphere and the role of this water in the framework of the warm water return flow of the meridional overturning cell. Formation rates around 9 and 11 Sv are found in the Southern and Northern Hemispheres, respectively. While the export of the southern SMW from its formation area is realized by the western boundary currents, that of the northern SMW mainly results from interior subduction. Equatorward of the formation regions, a fraction of each SMW variety is entrained in the subtropical cells that connect the subtropics to the equatorial region. Poleward of them, both varieties are seen to feed the regions of subtropical mode water (STMW) formation around 35° of latitude in both hemispheres. The bulk of the transport associated with each variety eventually turns northward: This amounts to ∼6 Sv of southern SMW gathered in the North Brazil Undercurrent (NBUC), and ∼10 Sv of northern SMW found in the Gulf Stream at 35°N, of which 8 Sv have gone through the Caribbean Sea. Of the 13.4‐Sv northward transport of the meridional overturning cell estimated by the model at 47°N, more than 50% (6.9 Sv) is found to have transited through at least one of the SMW regions. This gives an indication of the likely important role of SMW formation in the observed northward salinity increase of the upper Atlantic Ocean.