TRACING THE CONVEYOR BELT IN THE HAMBURG LARGE-SCALE GEOSTROPHIC OCEAN GENERAL-CIRCULATION MODEL

The flow which constitutes the conveyor belt in the Hamburg large-scal e geostrophic ocean general circulation model has been investigated wi th the help of a particle tracking method. In the region of North Atla ntic Deep Water formation a thousand trajectories were calculated back ward in time to the point where they upwell from the deep ocean. Both the three-dimensional velocity field and convective overturning have b een used for this calculation. Together, the trajectories form a repre sentative picture of the upper branch of the conveyor belt in the mode l. In the Atlantic Ocean the path and strength (17 Sv) of the conveyor belt in the model are found to be consistent with observations. All t rajectories enter the South Atlantic via Drake Passage, as the model d oes not simulate any Agulhas leakage. Large changes in water masses oc cur in the South Atlantic midlatitudes and subtropical North Atlantic. Along its path in the Atlantic the water in the conveyor belt is tran sformed from Antarctic Intermediate Water to Central North Atlantic Wa ter. On the average the timescale on which the water mass characterist ics are approximately conserved Is only a few years compared to the ti mescale of 70 years for the conveyor belt to cross the Atlantic. The v entilation of thermocline waters in the South Atlantic midlatitudes is overestimated in the model due to too much convective deepening of th e winter mixed layer. As a result the fraction of the conveyor belt wa ter flowing in the surface layer is also overestimated, along with int egrated effects of atmospheric forcing. The abnormally strong water ma ss transformation in the South Atlantic might be related to the absenc e of Agulhas leakage in the model.