The upper branch of the THC

Abstract

The major features of the spreading of deep and bottom water from their source regions can be derived from the distributions of oceanographic tracers, as was illustrated in the previous two chapters. Mass conservation constraints require a return flow at shallower depths to bring the deep and bottom water back to their formation regions. Deep upwelling brings these cold water masses from abyssal depths in the ocean basins to shallower levels, either in the Southern Ocean or in the subtropical and tropical ocean basins and the subarctic, including the Atlantic Ocean. The temperature and salinity will adapt to the shallow boundary conditions by diapycnal mixing with the overlying waters, determined by atmosphere-ocean interaction, and the density of the upwelling waters will decrease. The upwelling in the Southern Ocean brings water directly into the ACC. The ACC enables zonal exchanges of this water between the different basins in the Southern Ocean where AABW and AAIW is formed. In Chapter 5 it was already shown that aged deepwater from the Indian and Pacific oceans is exchanged zonally between the different oceans by the ACC (UCDW). Deepwater that enters the thermocline in the Indian and Pacific oceans by upwelling has to follow another route to reach the source regions of NADW in the North Atlantic Ocean (Gordon 1986). Different pathways have been proposed for the interocean exchange of thermocline water (Gordon 2001).